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Hi Everyone

Is Spherical aberration in a reflector a problem or normal to fast parabolic mirrors ? Can the SA be caused by a primary position being to far/ sort in adjustment in the OTA ? IE: over adjusted to far up the OTA.

SPACEBOY

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Hi spaceboy,

If I understand it rightly (no guarantee of that though :)), spherical aberration is only a problem of mirrors (and lenses) with spherical surfaces. Parallel rays of light are brought to focus at different points along the optical axis by different parts of the optical surface.

Both parabolic and hyperbolic mirrors resolve this problem, but whilst parabolic ones introduce comatic aberration, hyperbolics don't.

But there is an increasing difficulty and cost of manufacture with these : spherical --> parabolic --> hyperbolic.

Same is true for lenses, that's why aspherical ones are darn expensive - the complex surface profile is a pain to make and get right.

This link explains it a lot better than I can, whilst keeping it simple (just scroll down a bit). There's some nice, helpful diagrams too.

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Spherical aberration is a result of a poorly figured Mirror.

It's usually due to the Paraboloidal curve not reaching to the edge zone of the mirror. It was this very fault that plagued the Hubble mirror, and resulted in a repair mission to the telescope to correct the problem.

A mirror positioned too close to the secondary, is more likely to result in vignetting, meaning all the light reflected from the main mirror, will not be intercepted by the flat, and therefore a loss of brightness can occur.

Chromatic aberration is normally a defect in a lens, not a mirror, unless it is introduced by a poor eyepiece, or inferior multiplying lens in the system.

Ron.

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Thanks guys but it's looking like I need to chase up on this while I still have a guarantee. I usually rely on collimator's so rarely bother with star tests unless I'm sure there is a problem. I'm happy with the views from my explorer 200p so never bothered but by chance ended up aligning my GT mount the other night and while at it did a star test. I usually check ever thing is OK buy referring to this site Diffraction pattern and aberrations I will have to double check next time the skies are clear but I'm sure the result from my star test came up as FIG: 2 top row 2 - Spherical aberration, λ/8 (top) and λ/4 P-V, noticeably brightens the first bright ring; the inside pattern is larger and dimmer, with the intensity falling from the center out, opposite to the out-of-focus pattern's intensity distribution. The extra-focal patterns are reversed for the negative (under-corrected) aberration. OK on inward focus but incorrect on outward.

Grateful for any input as if it is an issue I will check again next time I have clear skies and then get in touch with the supplier.

SPACEBOY

Edited by spaceboy
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I've found you need near perfect seeing conditions to get a reliable star test so it's possible only rarely and even then interpreting the intra and extra focal star images is notoriously difficult.

In mass produced achromatic refractors (eg: Synta, Guang Sheng) a degree of spherical abberation is very common. Usually they are undercorrected a little. It's entirely possible that the same applies to mirrors from the same source.

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The problem is John I purchased the scope in Dec 1010 and the clocks ticking so if there is an issue I wanted to get in touch with the supplier sharpish and get it resolved. I was aware that there has been issues regards Chinese refractors but given how difficult optics are to get right I never gave it a thought the same applied to mirrors. TBH I have got some great views from the scope and have had no reason to question any problems but due to the recent down time of summer nights I decided to fine tune my collimation and while I was aligning bright stars I thought I may as well star test. The result being below. Seeing was reasonably good despite a little cloud and the skies never actually getting dark. I doubt there is an issue but didn't want to let it go unmentioned in case there is a mirror fault.

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I would say that the under correction in your scope is one of those things you'll have to live with and is often a consequence of buying mass produced optics. On most nights you may not even notice the difference between a better figured mirror however if you want top optics (eg 1/8 wave correction and high strel values) you need to pay extra for it, hence why Orion optics scopes are far more expensive than the ones you may get from sytna etc. IMHO i would be very suprised if a supplier would consider exchanging your scope based on the star test images you've supplied and once you go down the road of trying to quantify the quality of your optics you'll soon only be satisfied with the best, which comes at a large price!

HTH

Cheers

Adam

Edited by adam88
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I think you are right Adam. As I said before I don't actually see any problems with the images I get so it is probably that I'm secretly desiring some top quality OO optics and looking to pick faults :)

Thanks all.

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I don't actually see any problems with the images

The bottom line is if you are happy with the views. You have to

get some premium optics to obtain a near perfect star test.

On most nights, the state of the atmosphere will limit the quality

of the views, top end optics are usually only apparent on rare nights.

Regards, Ed.

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If the star images look like the ones in your picture I would think you've done pretty well for a mass produced telescope, you could easily have had worse errors than a slight correction issue :). As others have said, if you're pleased with the performance, job done.

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I think this is like the old adage about the 'funny noise' in the car engine that no mechanic can solve. the answer is simple, turn up the radio and never listen for it again. :)

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Ha Ha Ha....... Been there and done that. The wife has a car and I noticed a noise. Took it to 4 different dealers and they couldn't find a problem. Determined not to be palmed of as it was under warranty I went out with the mechanic and pointed out the problem suggesting it may be the gear box. A new gear box fitted and we were a noise and £1000 better off :) blumming mechanics! If there "£20k" computer doesn't tell them the problem they don't have a clue.

You are right Shane. On this occasion I think I'm just being overly fussy. Collimation OCD me thinks :(

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