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Why do Scope Caps have smaller ones in them?


Toymaster
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I'm curious on this one:


 


  • Is it to reduce the  light when looking at the Moon?
  • Is it to take off the smaller one when the scope is cooling down or being brought back inside when covered with condensation?
  • Or what?

 


At the risk of being stupid.......


 


Adrian 


 


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I've not heard of people trying it with jupiter; i'd have thought more aperture was key with planetary. I've certainly found as i go up in aperture, the inage gets better, and i've never had trounle with visual contrast. But i have nearly finished a bottle of pinot

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Thanks for that folks


 


I thought they were coffe cup holders for use while waiting for clear skies  :blush:


 


Does it matter they are off center - I suppose it doesn't as the center is the blind spot hence the 2nd Mirror holder etc ?


 


Am I Anywhere near ?


 


Thanks Ade for asking that I did wonder but thought it was perhaps for fitting a sun filter . Great question ! :thumbsup:

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Thanks Martyn - any disadvantage to Flextube Dobs as the cap fits the bottom half below the 2nd Mirror?


 


Does using this idea for Jupiter, as stated by Ron etc , depend on the main mirror size. Is it not worth it below a certain size as you are cutting down the amount of light.


 


Still I will try the latter out - when the skies ever clear again.


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You need extra aperture to gather light from fainter objects in the deep sky. But planets are bright enough and, depending on position relative to the sun, can appear brighter and more blinding in the eyepiece. So a cut in aperture using the cap often helps.


 


The plastic mound next to the offset hole is to store the small cap on. With flextubes you place it over the end of the top box for observing moon and bright planets. But when the scope is not in use the cap serves as a bottom box lid to protect the mirror. :)


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You need extra aperture to gather light from fainter objects in the deep sky. But planets are bright enough and, depending on position relative to the sun, can appear brighter and more blinding in the eyepiece. So a cut in aperture using the cap often helps.

 

The plastic mound next to the offset hole is to store the small cap on. With flextubes you place it over the end of the top box for observing moon and bright planets. But when the scope is not in use the cap serves as a bottom box lid to protect the mirror. :)

Thanks Kim - I would have left it on the bottom half  - TA

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Oh one other thing - when the moon isn't so bright (eg a thin crescent) or planets seem a bit dim - then they can be viewed with the lid totally off quite easily - you may well need full aperture to pick up extra detail and contrast. It's largely a question of judgement on the night. :)


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You will notice that Skywatcher refractor caps have the hole central, because there is no central obstruction. I dont think I have ever used the caps like that, I prefer to use filters, that way you get the arpeture/resolution you paid for in the scope and dim the image to what you want

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You will notice that Skywatcher refractor caps have the hole central, because there is no central obstruction. I dont think I have ever used the caps like that, I prefer to use filters, that way you get the arpeture/resolution you paid for in the scope and dim the image to what you want

 

I was wondering about this too, I figured that it was either a free way to reduce light or some weird way to help cooling without taking the whole cap off... crazy thought I know!

 

So if you're inhibiting the amount of light entering the objective, or reducing the aperture, aren't you losing something more than just the brightness? Sorry if this sounds silly, it's been about 10 years since I tried to study physics, and I just got in from work (plenty more excuses in the book!  :P )

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You're reducing the surface area of the mirror and therefore the ability to capture light which in time reduces the amount of light transmitted to the eye, so it makes bright objects less bright. I'm unsure what you mean by "something more" but I am sure someone will be along to help us out if you are correct :)

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Because you reduce the diameter of the aperture, it also stops down the focal ratio. See post #5. 


 


f-ratio = focal length / aperture


 


So a 200P with fl=1000mm is f-5. If you reduce the aperture to say 50mm then you get 1000 / 50 = f-20. and you end up with a much slower scope. Hth :)

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