Toymaster Posted January 29, 2014 Share Posted January 29, 2014 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 Link to comment Share on other sites More sharing options...
Guest Eddy Posted January 29, 2014 Share Posted January 29, 2014 Nothing is a stupid question You are right with the first one, to reduce aperture for the moon etc, some people mod them to put solar film in place of the gap instead Link to comment Share on other sites More sharing options...
Tibbz2 Posted January 29, 2014 Share Posted January 29, 2014 Yup very helpful for the Moon as an alternative to blinding yourself without a filter Link to comment Share on other sites More sharing options...
+RonC Posted January 29, 2014 Share Posted January 29, 2014 It's worth trying it with Jupiter, cuts down the glare! Link to comment Share on other sites More sharing options...
BAZ Posted January 29, 2014 Share Posted January 29, 2014 Not to mention it changes the focal ratio, as you have reduced the diameter of the aperture. This improves contrast no end. Link to comment Share on other sites More sharing options...
dawson Posted January 29, 2014 Share Posted January 29, 2014 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 Link to comment Share on other sites More sharing options...
stash Posted January 29, 2014 Share Posted January 29, 2014 Thanks for that folks I thought they were coffe cup holders for use while waiting for clear skies 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 ! Link to comment Share on other sites More sharing options...
BAZ Posted January 29, 2014 Share Posted January 29, 2014 Your'e right, they are off centre to avoid the secondary mirror, you just have to make sure that you don't put it over one of the spider arms as well. Link to comment Share on other sites More sharing options...
stash Posted January 29, 2014 Share Posted January 29, 2014 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. Link to comment Share on other sites More sharing options...
Toymaster Posted January 29, 2014 Author Share Posted January 29, 2014 Thanks all. I'll try a few experiments when we have clear skies again............... Ade Link to comment Share on other sites More sharing options...
Brantuk Posted January 29, 2014 Share Posted January 29, 2014 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. Link to comment Share on other sites More sharing options...
stash Posted January 29, 2014 Share Posted January 29, 2014 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 Link to comment Share on other sites More sharing options...
parklife2uk Posted January 29, 2014 Share Posted January 29, 2014 Great question - I always wondered what it was for!! Link to comment Share on other sites More sharing options...
glider Posted January 29, 2014 Share Posted January 29, 2014 Sorry guys, totally unfair. This one comes up every year and us Mods just can't help having a joke. 'Stash' was right all along. IT'S JUST A COFFEE CUP HOLDER. Link to comment Share on other sites More sharing options...
stash Posted January 29, 2014 Share Posted January 29, 2014 Just you rotten lot wait ! Revenge is best served up cold Link to comment Share on other sites More sharing options...
dawson Posted January 29, 2014 Share Posted January 29, 2014 He who laughs last... didn't hear the punch line Link to comment Share on other sites More sharing options...
Brantuk Posted January 29, 2014 Share Posted January 29, 2014 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. Link to comment Share on other sites More sharing options...
stash Posted January 30, 2014 Share Posted January 30, 2014 "May the clouds be with you" all members of EMS Staff /Mods Link to comment Share on other sites More sharing options...
philjay Posted January 30, 2014 Share Posted January 30, 2014 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 Link to comment Share on other sites More sharing options...
Guest Jmz Posted January 31, 2014 Share Posted January 31, 2014 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! ) Link to comment Share on other sites More sharing options...
catman161 Posted January 31, 2014 Share Posted January 31, 2014 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 Link to comment Share on other sites More sharing options...
catman161 Posted January 31, 2014 Share Posted January 31, 2014 *in turn not in time Link to comment Share on other sites More sharing options...
Brantuk Posted January 31, 2014 Share Posted January 31, 2014 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 Link to comment Share on other sites More sharing options...
catman161 Posted January 31, 2014 Share Posted January 31, 2014 Hmm I've never thought about it like that-would this help with planetary imaging? Link to comment Share on other sites More sharing options...
Daz Type-R Posted January 31, 2014 Share Posted January 31, 2014 Hmm I've never thought about it like that-would this help with planetary imaging? Neither have I. F24 for us. Link to comment Share on other sites More sharing options...
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