Hello, long time reader first me poster!
I think I made an error with my DIY solar filter, but luckily not the kind that leads to blindness! I've got an Orion 8" Dobsonian reflector, I've had it since 2013 and have had some great viewing over the years. I've managed to view the last transit of Mercury with binoculars and the 2012 transit of Venus in the same way. I was very excited to try viewing this transit with my scope since mercury is so small. I set to work making a solar filter using the solar film i had left over after making a set for my binoculars.
The thin and delicate nature of the mylar has always made me a bit concerned about pinholes or scratches etc, so i thought i'd try to sandwich it between two pieces of clear acrylic (about 3mm thick) to protect it . My biggest mistake was haste, I was rushing to finish/assemble the filter this morning and by the time i got it finished and onto the scope i was in the final hour of the transit, but to my surprise and frustration, I was completely unable to bring the sun into focus, i tried with a 26mm 2" diameter eyepiece and then a 6mm 1.25" diameter eyepiece, i rolled completely through my focus wheel and just could not get it to resolve, I couldn't even make out the spot that was mercury. It was clear to me that I'd made a misstep and overestimated my understanding of the scope. I really needed to get to the office at this point, so I had not choice but to pack it in, and try to figure out what went wrong later. As a consolation I was able to see the speck that was mercury through the finder scope with my old binocular solar filter over the end.
So my question is: Where did i go wrong, and what messed up my focus?
1. I had assumed the acrylic would have little to no effect on the scope, maybe a slight softening, but overall thought it was probably going to be unnoticeable. Am I completely wrong in this assumption? did the acrylic somehow refract the light or it that it's just not optically pure enough for high levels of magnification? I now realize every tutorial i've ever seen for making a solar film filter uses the film "as is" with no protective layer of any kind, my first assumption was that this was the problem, had i skipped the acrylic layer and just used mylar alone stretched across the end of the scope, the filter would have worked and the sun would have come into focus.
2. The diameter of the cardboard opaque front-most ring on my filter "sandwich" was about an inch smaller than my scope diameter, did changing the effective opening of the end of my scope affect my aperture enough to alter the focus distance? I've never put any sort of filter over the end of my scope before, so i've never considered what might happen, i saw some off-axis solar filters for very large scopes that have a small circular opening in a mostly opaque disk, so it didn't occur to me that this might cause a focus issue.
3. Collimation could be off. Admittedly I haven't used the scope in 5-6 months, but i've never had it get this out of alignment after a spell of non-use before, so it's possible that i can't get anything in focus as all right now. I almost pulled the filter off this morning to test this, but i had used quite a bit of tape to secure and close any potential light leaks, and I didn't have any distant enough object to test the focus on from my backward. I'll try to test the focus on the moon this week to see if i can get it into focus, and check my collimation as well when i can, but this seems like the least likely explanation since it's never been an issue before.
Thanks for reading this rather lengthy post, if you have any ideas I would really appreciate them. Unfortunately I won't be viewing any more transits any time soon, but i'd still like to be able to use the scope for solar viewing on occasion. Thanks so much for all of the info and assistance you all provide to the community, it really goes a long way to help new folks like myself feel more confident with their telescopes!
-Chris
