Distortion at higher magnifications

#1 05-01-2010, 09:23 AM

I'm unsure why this is but as I increase magnifacation with my other plossi eyepieces I now get distortion. Looking last night at the moon rise I noticed an orange planet left of the Moon. I'm assuming Mars. As I changed the EP from 30mm to 20, to 15, 10 the image got bigger, blurrier and not round. Sort of squashed. Now I think some of the problem is smoke in the upper atmosphere as the Moon was very Orange last night, But I also am starting to think there is a limit to how much you can magnify with a 203mm Reflector and a limit with the EP's.

Or am I still having other mirror issues even though Collimation has been sorted out.

Your advise & comment please.

astro744 · #2 05-01-2010, 09:59 AM

Collimation will have a huge impact on the quality of the image. However, once collimated poor seeing will also severly impact on what you see. Mars has a 97% phase at the moment and is slightly egg shaped.

High magnification should only be used in very good seeing otherwise no further information (detail) is visible even though the image is larger.

Now a 10mm on your 200mm telescope and assuming 1200mm focal length would give 120x which is quite within the range of your telescope. However, Mars will be quite small at this power. 160-200x would be a practical limit for nights of good seeing and 200-240x would be a limit on nights of very good seeing and 240-300x on nights of exceptional seeing (rare). To get 300x you would need an eyepiece of 4mm and this would give you an exit pupil of 0.75mm. When the exit pupil is 0.5mm the image starts to become a little too dim but OK. Lower than 0.5 and you need more aperture.

Exit pupil = focal length of eyepiece / focal ratio of telescope.

mental4astro (Alexander) · #3 05-01-2010, 10:16 AM

'Seeing' is governed by the atmosphere. Haze, humidity, high altitude turbulance all conspire against us,

. Usually the highest practical magnification allowed by the atmosphere is around 300X. Very rarely anything higher unless you chase down a specific observing site.

Your 8" scope should be able to go to a upper magnification range theoretically of 1000X (the rule of thumb being 50X per inch of apeture). However, rarely would this possible (see above). More typically the order of 100X to 150X before distortion can be seen. As an example, I've only twice been able to push my 17.5" dob to 360X in the last year with good, not excellent, quality image. Note that astro744 also stated a range of 200X - 240X on good seeing! 300X+ is rare.

Patience, is the only other suggestion I can give.

#4 05-01-2010, 10:34 AM

How do I calculate the f= number of my apature.

tube is 665mm long, 220mm opening, mirror 203mm

mental4astro (Alexander) · #5 05-01-2010, 10:51 AM

f/ratio is given by dividing the focal length of the telescope by its apeture.

So as an example, for a 203mm apeture scope with a focal length of 1000mm, the result is:

1000 / 203 = f/4.9 (rounded off to f/5).

It is exactlly the same calculation that gives the f/ratio in a camera. Make the diaphram smaller ( the apeture ) you increase the f/number.