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Planetary observing help


T A WOW

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My maths is dried up with "fuzzy head Sunday syndrome" lol. The exit pupil reduces and the secondary obstruction becomes larger - with three holes you can get focus  really good like with a bahtinov - here's a really good thread on CN about the subject:


 


http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5573991/Main/5571937


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Guest Ely Ellis

I think if you make two holes then you get a weird combination image.


That is how focus masks work, providing multiple images, which combine to make a diffraction pattern for focusing.


 


So, I maybe corrected on this because its only what I have experienced, you should only have ONE opening regardless of its position.


Position meaning that if its on a Mak, SCT or Newt, you don't have the opening behind the secondary mirror :)


 


Mine being a refractor, I have the opening directly in the centre, it not only gives the advantage of reducing image brightness, but because it uses the best part of the lens (ie misses the edges), you get a better quality image / view. (Halo reduction etc).


 


One point to be clear on, this is altering the speed (F) of the scope NOT the focal length and therefore does not alter the magnification factor.


 


It is similar to a normal camera lens, you stop down the lens (F-stops on the aperture) and it makes focusing much less critical but you need longer exposures. If you open the aperture, then focus becomes much more critical but you can use a faster shutter speed. It will also alter the depth of field. I'm sure Mark (H) would explain that one better.


 


Like all things in life, you have to trade one for the other, you can't have both. (Well, you probably could, but not on my budget ;)


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Blimey. Never thought this thread would get so lively.

Another little thing. If like people are saying, you remove the small cap on the end of the lens cap to reduce the appeture and therefore increase the F/ratio. Would that not impact on the image quality by reducing the scopes resolution?

Smaller aperture = less resolution = lower image quality

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  • 2 weeks later...

I can post the DVD tomorrow to whoever wants it first.

 

James

 

Edit - it's not a highly complex DVD, it's all quite basic stuff but useful to new comers like me; I'm not sure how useful it will be for advanced astronomers like you lot.

People who I look up to as "advanced astronomers" admit that they are still learning. Sometimes I find there's things I should know but don't. I find high focal ratios are good for doing webcam imaging of planets but for visual use I start off with low power and work my way up until the image becomes fuzzy. The amazing thing is that it varies night to night and some nights Mars looks best at about 100x magnification and other nights it is best around 200x, despite the viewing conditions appearing similar. I sometimes still use afocal projection to snap planets, especially midweek if I'm teaching the next day but prefer to use a webcam when I have time.

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Just to throw this into the mix as well


Seeing (not to be confused with transparency) is the most important thing when planetary viewing or imaging.


If the object is dancing around at x50 it will dance around 4 x as much at x200 mag


some of my best views of planets/moon have been when the sky is steady almost or even misty, if the stars are twinkling like mad forget high mag planetary viewing or imaging


 


 


The trick is to sit and wait for the good seeing conditions they may be  fleeting but when they come WOW


 


some advantage of using a barlow/powermate is with a lower powered eyepiece you get more eye relief, this with todays better designs is not too much of a problem but back when I started the eye relief on a say 6mm eyepiece was limited to say the least and was also a bit like looking through a straw


Edited by Ibbo
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Coming in a bit late on this one Im afraid but I shall add my 5 pennies worth


 


Yep you can use a barlow for viewing planets and the better the barlow, the better the eyepiece and the better the viewing conditions will give you good views, however :)


From experience you cannot beat a long focal length scope instead of barlows. Yep you can up the FL using a mask if you have the original aperture to be able to get away with it. On a 10" scope an aperture mask will work OK still giving bright images but a 4" scope with aperture mask will suffer on image brightness for example so you end up loosing out a little


 


I prefer long FL scopes because you can get good magnifications with nice comfy longer focal length eyepieces. IMHO you cant beat it, I very rarely use barlows when using my F15 on Jupiter and Saturn.  


 


Notice I haven't mentioned refractors once here :D That's because a good long F7 plus reflector will give good planetary views if well collimated however you will get better contrast with a comparable size refractor, (that's just physics)


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