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• View topic - brightness control

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 Post subject: brightness control
PostPosted: Thu Apr 01, 2010 6:39 pm 
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galaxy
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I want to make a stop-mask for my cpc 1100,
I have used one on my dob and it works fine
when observing the moon, especially when
brightness increases with waxing.

Will a stop mask work ok with a CAT, I
am supposing it will. Anybody here use one ?

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PostPosted: Thu Apr 01, 2010 9:25 pm 
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Sure it'll work, Gene. You can stop down any scope type with good effect. At higher mags you may notice contrast reduction, since you will be increasing the ratio of central obstruction, but I think the overall effect will be more useful than a moon filter. I'm seldom a fan of throwing filters in the mix, since most of them will noticeably soften the sharpness of the view at higher powers. In addition, you'll probably increase the stability of seeing by cutting out some atmosphere.

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PostPosted: Thu Apr 01, 2010 9:55 pm 
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galaxy
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Thanks
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PostPosted: Fri Apr 02, 2010 12:58 am 
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Another option is to make an off axis mask. That would eliminate the contrast loss since you'd be essentially eliminating the central obstruction. With an 11" SCT, you should be able to make an off axis mask with a hole in the 3" range. 3" of aperture is plenty on the moon and it will reduce the brightness. :)

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PostPosted: Fri Apr 02, 2010 1:36 am 
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PostPosted: Fri Apr 02, 2010 1:38 am 
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galaxy
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I can sometimes spend hours
on the moon, but I have to cut
out some of that light. It's too bright. :mrgreen:

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PostPosted: Fri Apr 02, 2010 6:25 am 
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Another trick, if you are in your backyard, is to leave a porch light on. It keeps your eyes from fully adapting and can have the effect of reducing the apparent brightness of Luna.

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PostPosted: Fri Apr 02, 2010 9:16 am 
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galaxy
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oh yea, it's like, almost unbearable.

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PostPosted: Fri Apr 02, 2010 4:21 pm 
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With an 11" scope, you might consider investing in a variable polarizing filter. They're inexpensive and are great because you can set the level of dimming according to the phase (brightness) of the moon or for a different scope. I've also found the VPF useful on Jupiter and Venus.

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PostPosted: Fri Apr 02, 2010 4:49 pm 
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PostPosted: Fri Apr 02, 2010 10:53 pm 
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Sure, Gene- no problem. :)

The off axis mask is an easy solution. I used to have an 8" Orion dob and I went to the Orion store and ordered an extra plastic dust cap. They only charged me $10 for it and I cut a nice 3" off axis hole. Of course, making one out of cardboard is free but I liked the dust cap since it snapped on nicely to the front of the scope. Regarding the VPF, I most often just screw it to the eyepiece or adapter. Mine is 2" so once I have it set correctly, I can change magnifications without having to adjust or remove the filter.

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Homebuilt 16" Truss Dob
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos/Peterson PipeMount


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