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Old 29-03-2013, 09:12 AM
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Ryderscope (Rodney)
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Meade ED80 - Spherical Aberration??

This post is a follow on from the flat field problem that I am having with my Meade Series 5000 ED80. It was recommended that I do a star test on the scope which I have done and it seems that the OTA may have some spherical aberration.

I tried the star test visually using an 8mm Ethos and Televue 2x barlow (had to also use a 50mm extension to get sufficient back focus). This showed a definite difference between the forward and back focus. I then hooked up the camera with the 2x barlow and took the attached images. The images were 1/5 second, ISO 800, zoomed to 100% and then cropped from the screen image. I made sure to centre the image in the field of view when taking the photos.

I was wondering if I could garner the views of those more informed than I am on these matters to see if this test is showing spherical aberration and if this is the case what, if anything, can be done about it.
Attached Thumbnails
Click for full-size image (Back Focus.JPG)
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Click for full-size image (Forward focus.JPG)
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Click for full-size image (In Focus.JPG)
10.2 KB56 views
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Old 29-03-2013, 09:34 AM
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multiweb (Marc)
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This is certainly surprising with a refractor. I'd expect that with an SCT. Having said that the in and out of focus airy pattern will always look sharper on the out as the air turbulence inside the tube will blur the disc.

When testing with a barlow you usually introduce aberrations that are not from the original telescope unless you have a very hig quality barlow like a Baader FFC.

The colour shift in your photos is obvious and I think it's your barlow.
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Old 29-03-2013, 10:40 AM
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Ryderscope (Rodney)
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Thanks Marc, the Barlow that I was using was the Televue 2" 'big barlow' hopefully this should be ok. I might try it again without the barlow and see how it looks.
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Old 02-04-2013, 05:06 PM
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Merlin66 (Ken)
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Rodney,
I have a very good SW ED80 (Pro "gold") which gives very good results - it comparison tests (close double stars etc) it actually gives my ol' Genesis a good run for it's money!
I've never felt the need to test the objective (Foucault/ Ronchi etc) but if it would help you I can set it up both with a webcam and on the bench.
BTW which magnitude star did you use for your test? Altitude of star?
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Old 03-04-2013, 06:52 AM
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Ryderscope (Rodney)
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Hi Ken, I was using Acrux as my test star.
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Old 03-04-2013, 07:27 AM
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Merlin66 (Ken)
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Hmmm
Maybe a poor choice....
Acrux is a bright 5" double star
A third mag single star image would be better.
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Old 03-04-2013, 08:38 AM
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Satchmo (Mark)
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If when running from inside focus outward the light appears to bunch in the centre and form a bright spot inside the expanded disc and then look like darker patch there when outside focus ( as it appears from your pics ) then you have under-corrected spherical aberration.
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