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.