Saturday, July 22, 2017

Low cost DIY solar filter for small/medium telescopes

Photo Details: 127SLT with SLR at prime focus, stacked from 1 minute of video

In preparation for the coming eclipse, I decided I wanted to get a solar filter for solar observation and photography. What I quickly found was that the actual filter material is not expensive, but buying a filter with a mount designed for your specific telescope can be. I decided to build a mount. Here's one approach that has worked for me.


There are relatively few ways to seriously injure yourself in amateur astronomy, but solar observation and photography is absolutely one of them. All it takes is a glance through an unfiltered telescope to destroy your eye, rendering yourself blind. You remember how you can set ants on fire with a magnifying glass? A telescope is a very large magnifying glass. Read the warnings that come with the solar filter film and follow them. Cover your finder scope! Be careful. Really careful. The information presented here is what worked for me, but your safety is your responsibility alone. If you are not confident in your ability to build a solar filter that will be securely attached to your telescope, don't undertake a project like this. This filter is for occasional use in dry conditions - it will not hold up with exposure to moisture.

This picture shows how the filter mounts - note that you must cover the finder scope before use!

I checked into the various types of solar film, and decided I like the yellow cast that the Thousand Oaks Optical solar film gives. Amazon sells sheets of it in various sizes. I decided that the easiest way to mount it was to buy a sheet larger than my scope's aperture, and sandwich it between two sheets of foam board. I'd then stack some layers of foam board on the back with a cylinder cut out, so that it had a snug friction fit over the telescope's tube. I bought the 8x8" sheet for my 5"/127 mm scope, which cost about $20.

You don't want the filter falling off while you're observing. A gust of wind must not be able to remove it, so I made it as snug a fit as I reasonably could.

Here's the steps I took.I started by cutting two pieces of foam that were a bit larger than my 8x8" solar film. Those two pieces will support the film and serve as the two layers of the foam/film/foam sandwich.

I then cut four more pieces that were a little smaller than 8x8" to serve as the friction mount on the tube. I used the telescope cap as a guide - remember that you want the inside diameter of the cap, though, not the inside diameter.  I traced the outside diameter and then conservatively freehanded the inside diameter. I cut to the inside diameter, leaving a small amount of material. Remember, we want a snug fit - we can't have this thing falling off and letting the sun burn a hole in our retina or camera sensor. Safety first!

The two sandwich pieces should have a hole cut that is smaller than the tube diameter, because you want the filter mount to slide over the tube and then stop. You want it to hit foam board before it hits film.

I stacked four of the smaller friction mount pieces and glued them together hot glue, and then carefully sanded the inner hole until it fit very snugly over the optical tube. I then hot glued the stack to one of the sandwich mount pieces.

I then sandwiched the film in between the two front sandwich mount pieces, and taped them together securely with electrical tape.  Here you see the finished filter face down, from the back/telescope side. Remember to observe the orientation of the film as specified in the film's instructions!

Your finder scope must be covered, or have a filter of its own. You can get the alignment close by watching the shadow cast by the scope. I usually remove the filter and put the protective cap on the telescope (to protect the optics and against mistakes) and then move the scope until the shadow is a round circle. That gets you pretty close. Then I remove the cap and quickly install the filter.

Go slow, and think through every move before you make it - your natural temptation is to look up at your target. If there are kids or adults who are unfamiliar with telescopes and solar observations with you, be cautious and communicate the hazards to them.

I really enjoy using the filter for both observing and photography. Here's the video that the first image was stacked from, just to give you an idea of what to expect. Be careful, and have fun!

Saturday, July 15, 2017

Lunar 100 Target 15 - The Straight Wall

Photo Details: Celestron 127SLT, ZWO ASI290NC, 1.5x Barlow, stacked from 1 minute video

View Full Size Image

Midway across the moon's southern hemisphere, just north of Tycho crater, is an odd sight. On a lunar surface pocked with round craters, a seemingly straight line cuts across one of the dark, smooth cooled lava plains. This is Rupes Recta, or the Straight Wall. It's the best example of a linear fault line to be seen on the moon with a small telescope.

A fault is a crack in an otherwise continuous in a section of rock. In this case, it is thought that the crack resulted from tension in the crust. The rock would have deformed at first, and then broken. One side drops, exposing a rock face called a scarp. The "wall" looks nearly vertical, but is known to have a slope ranging from 7-20 degrees. It is about 110 km/ 68 miles long and 2.5 km/1.5 miles wide. Estimates of its height range from 240m/800 ft to 500m/1640 ft.

The Straight Wall was first recorded in a drawing by Christiaan Huygens in 1686.

Friday, June 30, 2017

Lunar 100 Target 3 - Mare/highland dichotomy

Photo Details: Celestron 127SLT, ZWO ASI290NC, composite from video
03-Apr-2017, cropped from full disk.

A very large high resolution version of this image is available for download.

Note: As mentioned in the first post, these posts will not list the Lunar 100 features in order because the features are not always visible at different times. Item 2, moonshine, will be posted when viewing conditions allow.

The third target on the Lunar 100 list is the mare/highland dichotomy. When you look at the surface of the moon, you'll notice two distinctive surface types. Smoother dark areas, the maria, are so named because they were mistaken for actual seas by early astronomers. The cropped photo above highlights the Mare Crisium. It is approximately 550 km wide. The maria are relatively smooth plains of basaltic rock that formed from cooling lava produced by volcanic eruptions between 3 and 4 billion years ago. It is believe that deep impressions formed by impacts were filled in with magma, and then hardened to form the maria. Higher concentrations of titanium and iron make this rock significantly darker than the rest of the surface of the moon. They are the youngest lunar surfaces, and show significantly less crater activity from impacts than the lunar highlands.

The darker, smoother mare shows a lower density of impact craters than the surrounding lighter highlands

The lighter areas of the lunar surface are significantly older. They are believed to have formed between 4 and 4.5 billions years ago when the surface of the moon was still molten. They are composed primarily of anorthosite, which is an igneous rock. Anorthosite forms when molten rock cools more slowly than in the formation of basalts. This indicates that the highlands solidified under different conditions than the maria. The highlands formed very early in the formation of the solar system, which is itself estimated to be 4.6 billion years old.

Notably, the rocks from the lunar highlands are older than the oldest Earth rocks found this far. On earth, the igneous rocks formed at the beginning of the Earth's life have been predominately covered by tectonic activity and sedimentary rock formation. The moon has cooled sufficiently that it has no significant tectonic activity, and the lack of water and atmosphere make sedimentary rock formation impossible. The oldest rocks on the moon are still exposed.

Lunar 100 Target 1 - The Moon

Photo Details: Celestron 127SLT, ZWO ASI290NC, composite from video
07-May-2017 Larger version configured as 4k wallpaper is here

Welcome! This series of posts will document my efforts to explore, photograph and learn about the Lunar 100. This is a list of significant features on the moon that tell a story about the moon's geology, and history. This list was published by Charles A. Wood in a Sky and Telescope article and was his effort to produce a list similar to the deep sky object Messier List for lunar observers. 

The list is sorted by difficulty, with the easiest to find at the top. However, it is not possible to observe all the objects all the time, and certainly not optimal to photograph them. The best time to photograph a lunar feature is when it is near the terminator - the line separating dark and light - since that sets up less direct lighting. Much like a portrait of a person is less than flattering when shot with the pop-up flash on a camera, and far better with off-axis studio light, lunar features are much more prominent and aesthetically pleasing when shot near the terminator. As a result, I will likely present this list out of order, as I shoot them successfully. My goal will be to present a good photograph of each item and a few paragraphs describing their significance. The original list only gives a few words describing the significance of each, so I will learn a lot doing the research. I hope you find it interesting. 

The first entry in the list is simply the moon itself. (The list is sorted by order of difficulty from easiest to most difficult) The Lunar 100 list describes the significance of the moon as simply "large satellite".

The distance from the moon to earth varies through it's orbit, but averages 238,000 miles/383,000 km. By an odd coincidence, the disk of the full moon in the sky is the same size as the disk of the sun - this is why a solar eclipse can blot out all of the sun but the much wider corona. The moon has effectively no atmosphere, and a mass of 1/80th that of Earth.

The moon is tidally locked with the Earth. This means that it takes the same time to complete 1 revolution about it's axis of rotation (1 moon day) as it does to complete a revolution around the Earth. As a result, the same side of the moon is always facing towards Earth (with minor variance due to libration) An excellent animation showing how this works is here - I had trouble getting it conceptually, but it makes sense instantly when you see it in motion. The far side of the moon is thus never visible from Earth - our only imagery comes from orbiting spacecraft.

The moon's near side is pockmarked with craters from impacts with rocks from space. The far side is even more badly marked. It's enough to make one appreciate Earth's atmosphere even more than before.

There are two related theories as to how the moon was formed. The composition of lunar material is very similar to that found on Earth. The most generally accepted theory is that the moon formed from the ejected material from a glancing impact with an object about the size of mars four and a half billion years ago. The ejected material orbited the Earth and then slowly coalesced in to the moon.

However, simulations of this type of event show that a configuration of moon and orbit only rarely result from such a large single impact. An alternative theory indicate that numerous smaller impacts ejected material into orbit, which formed rings that condensed into smaller moons, and then into the moon. An excellent article on these two theories was published by Sky and Telescope magazine.

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes) Part 4

Update: Based on feedback, I have broken the original large post into four smaller posts to make it easier to read.

Capture Software Settings and Exposure Determination

I'd recommend you select RAW16 as the format, which will generate SER files as the output. These preserve more color information than an 8-bit per channel AVI. The camera is capable of 12 bits per channel, and SER retains that. It is read by most astronomy image processing software directly.

If your software supports it, turn on WinJupos file naming conventions. This will be useful when you progress to the point of wanting to do software de-rotation.

While you are hunting for your target you'll want the widest view you can get, so set your resolution to max to start.

Gain is similar to the ISO setting on a film or digital camera. It sets the light sensitivity of the camera. Set your gain in the middle for planets and lunar work to start. My camera has a range of 0-600 for gain, and I have found that shooting in the 250-350 range works best because you get more frames, and are more likely to get sharp views as the flicker past. The tradeoff is that you get noisier images, but the stacking will compensate for that, and having more sharp frames to work with is good. Past about 350, the resulting images are too noisy for my taste.

Set your exposure time to about 300 ms while hunting for your target. This is comparable to shutter speed on a traditional camera. This ensures that even one of Jupiter's moons will be hard to miss as you scan around, and the glow from the planet will be visible when you get close, before it is visible in the frame.

At this exposure, moons are clearly visible and all the surface detail is lost, but you sure can see it.

Launch the histogram function and adjust the exposure until the histogram tops out at 65%-70%. This ensures that the frames will not be overexposed, which will lose all surface detail.

Center your target in the frame, and reduce your resolution. The only time I use full resolution is for lunar shots. Planets are fairly small in your view, and capturing the center 800x600 pixels is usually plenty. Remember, the data from the camera is uncompressed, and uncompressed video is HUGE. You'll use a lot less disk space, enabling you to capture longer.

Additionally, you can capture more frames per second if the target is bright enough, and more frames in a short period of time is the name of the game. The USB connection can pass a lot of data, but if your exposure time is short, you can max it out with larger frames.

Processing your video into images - Stacking

There are two very popular stacking programs in use by hobbyists at the time of writing. The first is Autostakkert, and the other is Registax. I personally consider the stacking in Autostakkert to be more robust and produce better results for me. However, Registax is superb for the next step in the processing chain, so I use both. I stack the video frames with Autostakkert and then sharpen the resulting image in Registax.

There is a very good Autostakkert tutorial in the documentation.

It takes some experimentation to figure out what stacking works best. On an average night, I find stacking the best 15-25% of frames gives me the best images. If you stack more, you start to include frames which are not optimal. If you stack less, your noise level increases. It's a balance.

On nights of superb clarity, you might be able to stack as many as the best 40-50% of frames.

Processing your video into images -  Wavelet Sharpening

As you see above, stacking images from video dramatically improves your signal to noise ratio, but the image is probably not as sharp as it could be. This is due to atmosphere and other variables. You can dramatically improve it with cautious wavelet sharpening in Registax. I have had the best results with the noise-trapping technique shown in this set of tutorials.

It is very easy to over-do the use of wavelet sharpening, which results in an image that looks artificial. It's a highly subjective process - it will take some time to determine what you like. I personally like to err on the side of under-sharpening.

Lunar Photography

The process for lunar photography is similar. I still focus on a nearby star with the Bahtinov mask, and then swing the scope over to the moon. Since the moon is so bright, your shutter speeds tend to be pretty fast, and shooting at full resolution uses a ton of disk space, so I usually only shoot a minute's worth of video on a particular target. I sometimes bracket the same shots doing one minute clips at slightly different exposures to see what I like best.

You will get the best results by far if you wait until your target is near the line between light and dark. Shooting a full moon is actually pretty boring, because the light makes everything look flat. It's similar to how a person photographed with on-camera flash will look harsh and flat. Side light is much nicer and brings out far more detail.

Here's an image from stacked video of the Apennine mountain range, near Copernicus crater.

It's also a ton of fun to take a video and pan across the surface of the whole moon, pausing for a few seconds over each area. You can then use Microsoft's Image Composite Editor to stitch a very high resolution image of the moon. It takes a fair amount of processing time, but the results are really good. It results in very large images.

If lunar photography interests you, you might want to check out my blog dedicated to photographing the Lunar 100.

A link to a higher resolution version of this image, scaled down  for use as 4k wallpaper, is here.

Useful software for planning

A key part of astrophotography is planning. The following resources and programs are very helpful.

Stellarium is simply outstanding. There are versions for the PC and mobile devices. I use the PC version to see what will be in the sky, where it will be at a given time and date. You can even see where the moons of Jupiter will be and whether the great red spot will be facing you. If you photograph a moon of Jupiter, you can go back in time later in Stellarium and figure out which moon it was. You can even control a scope with it, but that's a topic for a later article.

Virtual Moon Atlas is terrific for planning your lunar photography session. It shows where the line between light and dark will be and helps you identify what you are seeing.

Weather apps and sites are very helpful for figuring out when the skies will be suitable for observing and photography. The best I've found are these:

Clear Dark Sky
The Clear Outside app for Android

Of special note is, which can visualize the jetstream. If you set the the altitude to 9000m for the wind, you can see it's path, which is surprisingly varied from day to day. Atmospheric seeing is best when you aren't under it, so if you see a day that is clear and the jetstream has moved off of you, get outside! Their cloud cover map is also great.

Sample workflow

Image capture:

1) Plan session using Stellarium and the weather applications. Look for a night with good to better than average seeing where your target is high in the sky. The higher up in the sky it is, the less atmosphere is in the way between your camera and it. Ensure laptop and telescope power battery is charged, and make sure you have > 50 GB available in disk space.

2) Carefully align your telescope as precisely as you can, according to the manufacturer's instructions.  I like to use an eyepiece that results in a fairly high magnification for this to ensure the alignment star is as centered as I can make it. An EP with a reticle would be handy too!

3) Install your camera and Barlow, as determined in the section on optimal magnification.

4) Install Bahtinov mask

5) Slew to a bright star near your target.

6) Use the focusing aid in your capture software to get the error as close to zero as you can. After this don't touch the focuser.

7) Remove Bahtinov mask. Really. It's easy to forget. :-)

8) Set camera gain to the middle and exposure time to 200-300 ms.

9) Set camera resolution/capture area to maximum.

9) Slew to target and center in the capture window. Using reduced motor controls helps here (motor speed 3-5 on SLT scopes)

10) Start histogram and adjust exposure such that the peak is between 65 and 70% to avoid overexposing.

11) Capture video. Limit videos to times appropriate for the rotation of your target. Optionally, capture a series of 4-6 two minute videos and combine them later using PIPP. This is handy for picking the periods of best seeing, and can also be used in derotation software later.

12) Sleep. Try to resist the urge to look at what you just captured, other than to back it up if desired.


1) Optionally, use PIPP to crop and center the planet. You can also use it to join multiple 2 minute segments into 1.

2) Stack the video.

3) Use Registax for wavelet sharpening

4) Use Photoshop/GIMP/etc for final level/contrast/saturation adjustments as desired. You can also correct orientation and scale the images. I had best results using the Lanczos algorithm.

Thank you! I hope you have found this series useful. If you have, I'd appreciate you sharing the link.

- Jason

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes) Part 3

Update: Based on feedback, I have broken the original large post into four smaller posts to make it easier to read.

Focus, Grasshopper....

Believe it or not, the trickiest part of the whole process is getting a sharp focus. You'd think it would be easy, watching the laptop screen, but because the atmosphere is moving, and because the scope mount shakes a bit when you touch it, it's actually kind of hard.

Trust me on this. As early in the process as you can, make or buy a Bahtinov mask. They are a wonderful focusing assist tool. You will save yourself a great deal of frustration. There is nothing quite like the feeling of rolling out of bed at 3:00 AM to shoot Saturn, and later processing the images and determining that you were a touch out of focus. My eyes don't work right at 3:00 AM.

Making one is easy, thanks to the Bahtinov mask generator at AstroJargon. Simply plug in your scope's numbers and print it black. Then have the paper laminated, and cut out the pattern with an exacto blade. I left 4 tabs evenly spaced around the circle that I folded over and added velcro tabs to secure it to the telescope. I also used a Sharpie to black out the back side to minimize reflections.

You can also 3d print one, if you have access to a fairly large printer. The laminated paper one is holding up just fine.

To use it, attach it over the front of the scope. Orientation doesn't matter. If you have a dew cap, you can attach it to the end of the dew cap with no concerns - the distance to the objective is not critical.

Point the telescope at a bright star as close to your target as is reasonable. You need a point light source, not a disk, so pointing it at the moon or a planet is not recommended. Adjust your gain/exposure until you can see three lines making up the diffraction pattern. Two will cross in an X, and the third will be off center when it crosses the other two. As you focus the telescope, that third line will move. When it's centered at the intersection of the other two lines, you're in focus. A gentle touch on the focuser helps, and you'll need to let the scope settle each time you touch it. One of these days I'm going to build an electric focuser, but that's a project for another article.

In the picture below, the middle line, closest to vertical, is the focus indicator. It's not centered between the other two lines, indicating the system is not in focus.

The capture software I'm using has a Bahtinov assist feature, which quantifies how many pixels off center you are. It doesn't always reliably identify all three lines, so it can take a little time for it to settle down, but if that number is oscillating around 0 or 1, you're in good focus. Here's what the sequence looks like.

Way out of focus.  Turn the focuser in the direction that makes this smaller.

Getting better, but still off by 4 pixels

Focus acheived. 

Once you are properly focused, remove the mask and slew the scope over to your actual target. Remember, the mask won't work pointing it at a disk - it needs bright point source. A bright star near your target is best. Slewing the scope can impact your focus, so minimize the movement.

"Lucky Imaging" - making cleaner still images from video

It may seem counter-intuitive at first, but unlike terrestrial photography, you'll get your best results not by shooting a single frame, but by shooting hundreds to thousands of frames. You then use stacking software like AutoStakkert to automatically select the best frames, align them, and combine them into a single image.

The advantage of this approach is that you can overcome some of the variability in atmospheric seeing that is causing the view to swirl and ripple. There are split seconds during which the image is clearer than average, and those frames get selected and combined. By combining many of these images, you can reduce noise, and dramatically improve detail. Essentially, you are improving the signal to noise ratio by integrating data over time into a single image - the details and noise don't happen in the same parts of the image each time, so you get better detail over the whole image as you add frames.

When you are shooting, use a gain in the middle of your camera's range. If you decrease the gain, you'll get cleaner individual frame, because noise increases with gain. However, your exposure time has to increase, and this means that you can't capture as many frames. If the camera exposes for 1/30 of a second for each from, you get 30 frames per second, maximum. Modern astro cameras are much faster than that - connected to a USB3 port you won't be bandwidth limited until 100 fps or more, if you aren't capturing the full view of the camera. More frames increases the odds of getting frames that capture those split seconds of clarity as the atmosphere shifts and ripples.

USB3 and a solid state disk (SSD) are preferred, but I'm currently using a laptop that is limited to USB2 with acceptable results. It does limit my maximum frame rate when shooting bright targets. You can reduce the impact by only capturing a small window of the camera's view - the planet will generally easily fit in the middle 800x600 or 640x480 pixels, so you really only need to capture that.

The resulting files are BIG. It is really easy to shoot 100 GB in a session. Fifty GB is about the minimum. The resulting video files are not compressed.

Start doing video captures of 2-6 minutes. I set the motor speed of my control handset to 3-4 for this, after the planet is centered, so that I can make very small adjustments to keep the planet centered. The tracking is good if you align the scope carefully, but not perfect.

During capture, don't move the scope more than you have to - remember that the stacking software can take care of even significant drifting from center as it aligns the image. Moving the scope more than needed will reduce the available pool of good frames the stacking software has available to it.

Raw video frame 


Best 20% Stack of Video Frames

Wavelet Sharpened

Effects of Planet Rotation and duration of video capture

After a couple tests where you capture a few minutes of video, it will occur to you that you can get more frames simply by recording longer videos. I tried 10 minutes at a time on Jupiter, and couldn't figure out why the results showed very little detail compared to the 4 minute captures.

It's pretty simple - everything's moving. While you are taking video of it, Jupiter is rotating, and it rotates FAST. It completes a full rotation in a bit over 9 hours! If you take a couple of sequential 5 minute captures and produce stacked images from each, you'll be stunned at how much it has moved during this time - you can actually see it rotating if you cycle through the resulting pictures.

Limit your exposures of Jupiter to 4 to 6 minutes, and your exposures of Saturn to perhaps 7 or 8. Repeat this several times if you can - the seeing varies substantially from minute to minute, and one of these will usually turn out better than the others in the same session. The time required to shoot another 6 minute sequence is small compared to the time required to set up and focus.

There is a way to use software to de-rotate the planet in longer exposures, but that is beyond the scope of this article. I will link to it when completed. If you are interested, check out the software package Winjupos and the tutorials online.

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes) Part 2

Update: Based on feedback, I have broken the original large post into four smaller posts to make it easier to read.


The amount of magnification you can use and get good results is limited by aperture and atmospheric conditions. The larger your aperture, the more light your scope gathers, and the more you can magnify an image. If you try to use too high a magnification for your aperture, your image will get dim and fuzzy, and you'll have a really hard time focusing, even with a Bahtinov mask.

For a given aperture and focal length telescope paired with a given camera, you can estimate the magnification at which you will get optimal detail. It is generally best to shoot at this magnification, and then rescale your resulting image in an image editing program. 

The focal ratio of a telescope (or camera lens) is defined as the focal length over the aperture. The use of a Barlow lens to increase magnification multiplies the focal length - the top number of the ratio - so it linearly changes your focal ratio.

focal ratio = focal length / aperture

The general guideline for determining magnification is this:

When seeing is average, use a focal ratio of 5 times your camera's pixel size in microns. 
If the night is excellent seeing, you may be able to push to 7 times pixel size in microns

The ASI290MC has a pixel size of 2.9 microns. My optimal focal ratios are therefore:

Average seeing: 5 x 2.9 = f/14.5
Exceptional seeing: 7 x 2.9 = f/20.3

With no Barlow or extension tube, the 127 SLT has the following focal ratio:

1500 mm focal length/127 mm aperture = F/11.8.

A Barlow effectively multiplies your focal length, so:

With 1.5x Barlow: 1.5 x 11.8 = F/17.7
With 2x Barlow: 2 x 11.8 = F/23.6
With 2.5x Barlow: 2.5 x 11.8 F/29.5

Therefore, the guidelines indicate that for good to exceptional seeing, I should run a magnification of 1.5x. Even on the best nights, 2x is not optimal for detail.

I tested it. The results are shown below. The night I used 2x was significantly better seeing than the night I used the 1.5x, but the image shot at 1.5x is significantly sharper, and you can see a detail that you can't make out at 2x.

I learned from a post on a forum that the inexpensive Celestron 2x Barlow is super flexible. If you use it as-is, you get 2x magnification. If you unscrew the lens portion and attach it to your camera using the filter threads, you get a 1.5x magnification.

If you have a different telescope, run the math above - you'll likely get a different answer because your starting focal length is different.

Image Orientation

Be aware that the view you see through the viewfinder or on the camera may be different than the orientation of your target. Refractors, reflectors, and Mak-Cass telescopes all behave differently because of the way the optics work.

A great summary of the different configurations and their effect on image orientation is available this article on star diagonals

This is especially useful when looking at the moon, because an upside-down right-left flipped image is hard to navigate.

Dew Control

If you have a refractor or Mak (like the 127SLT) you'll likely run into the problem of dew forming on your objective. As the temperature approaches the dew point, it will become more and more likely. You might think that as long as the temperature of the air is above the dew point, you don't need to worry about it, but it's more complicated than that. When pointed at a dark sky, your telescope actually radiates heat off into space, chilling it below the ambient temperature and allowing moisture to condense out of the air just as it does on the outside of a cold beverage glass.

The dew will form so gradually you won't immediately notice it - you will just notice that you are losing sharpness at first. You can't safely wipe the dew off without risking damaging the coatings on your objective - if it forms, you are done for the evening. Just let it air out inside with the covers off.

You can reduce the odds of dew forming be adding a simple foam shield that extends out past the end of your scope. It should be at least 1.5 times as long as your aperture is wide. Many people report good results with using part of a foam camping mat about 1/2" thick - that was a little too rich for my blood, so I used a couple sheets of 1/8" foam from a craft store laminated together with hot glue. I added velcro strips to attach it to the scope. This has pretty much eliminated the problem for me for sessions of up to three hours.

If you live in a area where dew formation is more of a problem, or you intend to have very long observing sessions, it's possible to buy or make small low power electrical heaters to prevent dew from forming on the glass surfaces of your telescope, including eyepieces and finders.

Monday, June 26, 2017

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes)

If you did any reading at all on forums before buying your telescope, you invariably ran across statements like this:

"An alt/azimuth scope is fine for visual observing, but you can't do astrophotography with them!"

What they mean, generally, is "You can't photograph nebula and galaxies using long exposures." And they are absolutely right. The Alt/Azimuth mount is a terrible choice for long exposures of the sky. You can, however, have an absolute blast taking pictures of some of the planets, and you can take terrific pictures of the moon. These objects are actually quite bright,and there is some very clever software available for free that make it possible to get much better planetary and lunar photos than you might suspect. There are limits, of course, and your results probably won't match what the guys with scopes and mounts costing thousands of dollars will get - but it's a lot of fun, and the results can be pretty respectable if you are careful.

Earlier this year I bought a Celestron 127SLT. I have been having a ton of fun shooting the moon, Jupiter, and Saturn from my front yard. I drove to a darker site on a good night and viewed and photographed a comet. The following documents the process I went through to learn how to get the best results I can from my very modest telescope. I hope others might find it useful. 

This is distilled from many hours of reading the excellent Cloudy Nights forums, and a lot of practice. I am far from an expert on any of this - I have been at it about 5 months now. This article is an attempt to save the reader some time by gathering pointers to the most useful information in one place. I welcome feedback and suggestions for improvement.

Saturn, photographed with my Celestron 127SLT/ZWO ASI290 MC/1.5x Barlow


Altitude/Azimuth (Alt/Az) mounts are the astronomy name for a pan/tilt mount. They let you rotate the scope around a base, parallel to the horizon, and point it up and down, just like a standard photo tripod does. This is easy to make, and very intuitive to use. The 127SLT (and other scopes in the SLT line) has a motorized Alt/Az Go-To mount, meaning that once you help the mount figure out where it is and which way it is pointed, it can automatically find objects in the sky, and track them for you.

The motion of an Alt/Az mount  does not match what's happening with sky objects, though. An Alt-Az mount will introduce a rotation of your frame. As a result, any single exposure picture of stars you take that is over 15 seconds will start to blur, and become comma-shaped. Stars are rather dim, so long exposures are required to capture them well.

This limitation isn't a problem for planets and the moon, though. They are plenty bright, so you can take short exposures. In fact, as we'll see, the best amateur planetary and lunar images are actually created by digitally combining the best frames from high speed video.

Taking good pictures of Jupiter, Saturn, and the moon are well within the reach of a modest Alt-Az telescope like the SLT line, or the smaller NexStars, Meade also makes comparable lines for introductory to mid-level scopes. I will use the 127SLT for this article, because it's what I have, but the approach and limitations should be similar for most 4-6" scopes of this type.

For capturing the data from the camera, I have used SharpCap 2.9 for this article since I prefer it. You will find similar functionality in the very popular capture software Firecapture.


The SLT series of scopes is reputed to be pretty rough on AA batteries. I read that they would start to slow after a single session of observing, so I chose to skip them entirely. I bought a portable 12V jump starter with an included 5V USB output and the 12V power cable for my telescope. I have not regretted it. I have run it in extremely cold weather and across 3 nights without recharging.


I did some initial experiments with taking pictures with a Raspberry Pi camera through the telescope, just to learn the basics and see if I liked it. I did, so I chose the ZWO ASI290MC. It has a USB3 port, allowing for higher frame rates on bright targets where you are not limited by the exposure time. As we will see, within limits, the faster you can take frames of the target, the less noise your final picture will contain.

I also added an IR cut filter which is installed on the back side of the camera's mounting tube. This keeps dust off the sensor, and drops IR light. IR light focuses a little differently than visible light, so IR light can cause slight blurring in your images. I installed mine on the inside end of the mounting tube, leaving the other end for attaching other filters or lenses. The IR cut filter is the slightly red glass shown below.

I'd recommend removing the 90-degree mirror diagonal if your scope has one. You don't need it, since you'll be looking at the laptop screen, and it flips the image right to left. You also lose a bit of light - most inexpensive diagonals are about 90% efficient optically. Installed this way, my scope/camera combination produces correctly oriented images.

Saturday, April 1, 2017

Astrophotography with the Raspberry Pi Camera - A Cheapskate's Guide to Solar System Photography

Update: I have been doing a lot of work with the 127SLT and the astro camera and have written an article that concisely explains the challenges a beginning planetary/lunar photographer will encounter, how to solve them, sample workflows, and images that give expectations for a scope this class. It also shows how to use a Bahtinov mask for easier, better focusing. Check it out!


I recently bought a Celestron 127SLT telescope with the goal of doing some solar system observation and photography. I had a Raspberry Pi camera and a B+ board kicking around from a previous robotics project, and decided to give it a try. It's not an ideal camera for this sort of thing, not by a long shot - but I already had it. The v1 camera can now be had for $13. It was worth a try.

The results were better than I expected. Here's some results. Details on how I did it are below.

Copernicus Crater, The Moon


 Edge of a Mare, The Moon

How the Camera Is Mounted

A friend printed this mount for me. It was easy to assemble and everything fits well. This allows you to plug the camera in like an eyepiece. If you just replace the eyepiece with your camera, you are doing prime focus photography, which uses the telescope as a giant camera lens and focuses the image directly on your camera's sensor. To do this, you remove the lense that came on the camera. The magnification you get depends on the sensor size and the telescope focal length. The Pi camera has a very small sensor, and the 127SLT has a long focal length of 1500mm, resulting in a fairly high magnification level. Shorter focal length scopes will result in less magnification.

I mounted the camera first to check fit, and then carefully unscrewed the lens to remove it. There is a small piece of glue securing the lens - remove that with an Xacto knife before unscrewing the lens. Grab the black square camera body with one pair of pliers and gently twist the len with hemostats. You don't want the camera body to transfer those forces to the PCB or it may be torn off the board. Alternatively, there are printable spanner sets for this purpose on Thingiverse.

I then mounted the camera assembly to a printed case with a RaspPi B+.

I powered the Pi off a USB battery pack intended for charging phones - even a small one provides several hours of juice.

Software on the Raspberry Pi

In order to frame the picture and to focus, you need to be able to see a preview of what the camera is seeing. The best way I found to do this was using RPi-Cam-Web-Interface. I set it to stream 640x480 previews at 40% quality, which was pretty snappy.  You can then take pictures and video over the web interface. It's really slick. The installation instructions are excellent.

In order to be able to use it away from my home access point, I also set up the Pi to act as a wireless access point. That's completely optional, but is convenient if you intend to operate the system away from home. Then you just connect to the Pi with your phone, tablet, or computer and control the camera from there. You can adjust contrast, exposure compensation, ISO, shutter speed, etc.

Capturing Data and Best Camera Settings

I experimented with both video and still capture. Without fail, images made from stacked video turned out much better than still frames. The noise from the camera is high enough that even lunar shots benefit from stacking frames to improve signal to noise ratio.

The capture mode you select matters a great deal. I had best results using modes that result in binning. This combines data from adjacent pixels into one, which reduces resolution but is more sensitive and gives you a better signal to noise ratio. For Jupiter I used 640x480, which results in 4x4 binning, and 1296x730, which results in 2x2 binning.

Autoexposure will not be ideal - it will almost certainly overexpose. Try using -2 EV compensation and manually dialing the contrast in.


I used AutoStakkert or Registax 6 to stack frames, and did wavelet sharpening in Registax 6. I first ran the video through PIPP to crop the video and make alignment easier.

For comparison, stacked frames resulted in the picture on the left, and wavelet processing resulted in the final image on the right.The right image is also flipped right to left, since my telescope does that with a diagonal installed.

I have also gotten good results stitching lunar panoramas from video shot on the Pi and my SLR with Microsoft's Image Composite Editor, which is flat out awesome.

I usually do finishing touches on the lunar pictures with Silver Efex Pro, part of the now-free Nik Collection.

Attaching to the Telescope - Prime Focus

For lunar photography this is the way to go, unless you want extreme closeups. Just plug the camera in just like you would an eyepiece. You could probably gain a small amount of efficiency by removing the mirror diagonal, but if a 3D printed component broke I wanted it falling into my diagonal, not my telescope.

Eyepiece Projection for Planetary Imaging

For higher magnfication you can use Eyepiece Projection (EP). An eyepiece is used to project the image onto the sensor. An adjustable tube allows you to increase the magnification by increasing the distance of the eyepiece to the camera sensor. I used a 25 mm eyepiece and the extension tubes shown belowm along with this adapter. The same hardware can accept an SLR with T-ring, or a dedicated astro camera. Note: with EP, you focus by adjusting the telescope's focuser, and adjust magnification by sliding the camera in and out. I didn't actually see that written down anywhere and had to determine it experimentally. :-)

Update: Using my new ZWO astro camera, I have gotten MUCH better results with a 2x Barlow (Celestron Omni) I have not tried it with the RaspPi cam, but you may well get better results. I tried the ZWO cam with both and the Barlow was far superior. This shot of Jupiter at opposition was taken with a 2X barlow. 


The low light sensitivity, extremely small sensor, and high noise combine to make the RPi cam a far from ideal solution, but it does work and is a very fun way to dip your toe into astrophotography. You are limited to 6 second exposures, so you won't be imaging nebulas, but the moon and the planets are fun.

It was enough to convince me I wanted to spend the money on a dedicated astronomy camera, so I bought a ZWO ASI290MC. I will keep the Pi scope camera though - it might be fun to play with streaming video from the scope over the internet.