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Help with a home made filter - Reducer size.

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#1 NerfWrangler

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Posted 16 July 2020 - 07:43 AM

Hi,
 
I have final managed to get my hands on some solar filter film to make a filter for my 8 inch (203mm) Dob. A lot of the literature I have gone though suggests reducing your aperture, but non provided a ratio by witch to reduce it?
 
I mad a couple of lunar reducers to take my scope to down to a 114mm aperture for full moon viewing (See Image). I was going to uses the template to make the solar one but again I am not sure of the aperture needed. 
 
The film in question is Thousand Oaks Optical SolarLite Solar Filter Film (ND 5).

 

Any advice is appreciated

 

Attached Thumbnails

  • Nerf's Jank as Frig reducer.jpg

Edited by NerfWrangler, 16 July 2020 - 07:45 AM.

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#2 MalVeauX

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Posted 16 July 2020 - 08:39 AM

Looks good!

 

Any aperture is fine, it depends on seeing conditions as to what aperture would be great for you as you're not going to be lacking light. The larger aperture if seeing supports it, will show convection cells more readily and will be even better with a large sunspot once the maximum returns in a few more years. You could do the full 8" aperture, or stop down for poor seeing conditions, totally up to you. There's no magic focal-ratio or aperture size, it all relates to seeing conditions. You can always dim light with ND filters or magnification visually.

 

Very best,


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#3 NerfWrangler

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Posted 16 July 2020 - 09:43 AM

Looks good!

 

Any aperture is fine, it depends on seeing conditions as to what aperture would be great for you as you're not going to be lacking light. The larger aperture if seeing supports it, will show convection cells more readily and will be even better with a large sunspot once the maximum returns in a few more years. You could do the full 8" aperture, or stop down for poor seeing conditions, totally up to you. There's no magic focal-ratio or aperture size, it all relates to seeing conditions. You can always dim light with ND filters or magnification visually.

 

Very best,

Thanks Marty,

 

Your point on the convection cells and bigger aperture is something I haven't considered. Thanks!

 

I think the best option then would be to mount the film in a full 8 inch mount, and then make a set of reducers to go over the mount at 1/2 and maybe 1/8 the diameter. Guess this weekend is going to be more arts and crafts than I anticipated. 

 

I will also need more fomecore....


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#4 DAVIDG

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Posted 16 July 2020 - 12:30 PM

 I would make the off axis aperture size such that the result is an unobstructed system. So for your 8" Dob  you must likely have an diagonal that has minor axis of about 2". So that works out to an off axis  aperture of 3" and hole placed in the mask so you have unobstructed light path.  If you go with  a larger off axis aperture, the diagonal is now in the light path and is  blocking some the light but it is also a larger percentage of the total obstruction so adding more diffraction into the image. 

  You could also make a mask with the largest off axis aperture you can with the size of the film you have then make  a mask  with multiple size apertures in  it that is placed over it  and fixed to it in the middle. Now you can rotate the different sizes over the largest opening and see what aperture gives you the best view for the seeing conditions. 

 

         - Dave 


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#5 UniversalMaster

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Posted 17 July 2020 - 08:09 AM

I made a full aperture Baader film filer for my 10" f4.7 Newton. It beats the hell out of my 80mm f6 triplet Apo with the same type of Baader film. If you already have the film, is say go full aperture and make some masks if you don't like the full aperture filter.

#6 briansalomon1

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Posted 19 July 2020 - 10:39 PM

Looks good!

 

Any aperture is fine, it depends on seeing conditions as to what aperture would be great for you as you're not going to be lacking light. The larger aperture if seeing supports it, will show convection cells more readily and will be even better with a large sunspot once the maximum returns in a few more years. You could do the full 8" aperture, or stop down for poor seeing conditions, totally up to you. There's no magic focal-ratio or aperture size, it all relates to seeing conditions. You can always dim light with ND filters or magnification visually.

 

Very best,

It sounds like 70 or 80mm will likely fit into the unobstructed area of your scope. I can tell you I observe with a 100mm aperture (rarely) and am happy with 60mm most of the time but I'm a very casual observer.




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