Friday, June 30, 2017

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 windy.com, 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.

Processing

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











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