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Mylar solar filter issue

solar collimation reflector
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#1 thehagfish

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Posted 11 November 2019 - 10:25 PM

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



#2 NorthernlatAK

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Posted 11 November 2019 - 11:18 PM

Acrylic is not an good optical window to look through. You'd have been better off with just the film. Definitely try to collimate the scope after a long period of no use. The diameter of your filter would only change your f ratio a little bit if any. That shouldn't affect focus like the acrylic window or possible miscollimation. If it's not your collimation then it's your acrylic window for sure.
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#3 Kipper-Feet

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Posted 12 November 2019 - 01:45 AM

Hello Chris, welcome to the CN forum and if you get a chance sometime soon, please check the Inbox of your Personal Messenger. You got mail.


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#4 sg6

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Posted 12 November 2019 - 03:25 AM

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.

Cannot think it would be collimation - would have expected a poor image but an image.

 

Smaller aperture at the front would also have made no real difference, just a bit less light gather.

 

So would come to one of 2 options - the 3mm acrylic and haste.

If somewhat "uneven" looking through the acrylic then the light passing through would be uneven on exiting and acrylic is not the best material. Optically not that good. It s more to just allow light through but not really view through.

Haste comes from you may have left something in the focuser that needed removing and so meant you could not get to the correct position. I was making a solar filter the night before and day of the transit. I was getting in a mess and abandoned the thing as it just wasn't going to come together easily - didn't matter at all as it turned out, the other bits worked fine.

 

I used 2 sheets of acrylic but the thin stuff and clear, about 0.5mm I suppose - lot easier to cut as well. As I also have a red sheet I have wondered how that would work, may try when I have the rest solved.



#5 Jon Isaacs

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Posted 12 November 2019 - 07:15 AM

I have to think it was the acrylic.  Next time you are out, try the scope with the acrylic window and see how things look.  The optics of a telescope are incredibly precise, even looking through a glass window will distort the image. 

 

Jon



#6 SeaBee1

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Posted 12 November 2019 - 08:46 AM

Chris - Two things stand out from your post... first bad thing = sandwiching your solar filter between two pieces of acrylic. Normal acrylic is not an optical quality medium, and will cause problems with your viewing. You mention 3mm and so I am thinking 2 sheets of 3mm acrylic for a total of 6mm of non optical quality "obstruction" over the front of your scope. Second bad thing = you say you stretched the solar filter. The mylar solar filters are not meant to be stretched across the aperture. Wrinkles don't matter, and are in fact how the filter works best.

 

My recommendation would be to start over, maybe with a fresh piece of filter film, unstretched and no acrylic, AFTER you check the other things mentioned by other responders.

 

Good hunting!

 

CB



#7 Great Attractor

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Posted 12 November 2019 - 12:26 PM

One more thing:

 

The thin and delicate nature of the mylar has always made me a bit concerned about pinholes or scratches etc

 

Is it the Baader solar film we're talking about here? Try to tear an unused piece sometime, you'll be surprised wink.gif I mean, of course, preferably the surfaces should not touched, but it's not really delicate/fragile.

 

Notwithstanding all of the above, a filter should be inspected before every use.


Edited by Great Attractor, 12 November 2019 - 12:28 PM.

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#8 thehagfish

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Posted 12 November 2019 - 05:30 PM

Thanks everyone for your thoughtful replies!  I was able to view the moon clearly last night, so it sounds like the culprit was the 3mm of clear acrylic.  I should have had some reassurance of the durability of the Mylar when it took several repeated cuts with an Exacto blade to extract the 8" section i needed in the first place, no need for reinforcement!

 

I'll try the plain acrylic for lunar viewing some night to see if it has the same effect, but it makes sense that it would distort the light entering the scope in a similar way to atmospheric turbulence.

 

Thanks again for helping me figure this out!

 

-Chris



#9 Phil Sherman

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Posted 13 November 2019 - 02:09 PM

When I made my filter for the 8", I used two rings to sandwich hold the Baader film. The film was glued with contact cement to a similar ring of cardboard first. Glue was only applied to the cardboard which kept all of it off of the film. The filter assembly is stored in a cookie tin which prevents damage to it.



#10 Don W

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Posted 13 November 2019 - 02:22 PM

Why use the acrylic at all? It's not meant to be used optically. Toss it!



#11 Eddgie

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Posted 13 November 2019 - 04:54 PM

Why use the acrylic at all? It's not meant to be used optically. Toss it!

From the OP (emphasis by me)

 

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



#12 Hugh Peck

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Posted 14 November 2019 - 09:50 AM

It's unlikely that either aluminized Mylar or Astrola (Baader) Safety Film will be damaged in normal use. I have made a bunch of solar filters over the years and never had one get damaged. FYI, the Astrola isn't Mylar. Aluminized Mylar gives you a blue image whereas Safety Film gives a natural white image. 




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