Coma in Orion ST80-A

[dunc66]

Hi

I have an ED80 and it focuses stars to a pinpoint through an extension tube and 40mm eyepiece.

I used the same tube and eyepiece on my newly purchased Orion ST80-A guider and the coma is very bad right across the FOV. I originally bought it for guiding but then read articles about tweaking it. Would be nice to have it as a portable wide-field scope. How bad should the coma be?

In my attempts to find the problem I have removed the lenses and focusing tube and reassembled them. I adjusted one focuser grub screw to reduce the slop in the rack and pinion mechanism. While I was at it I shortened a screw in the focuser tube and blackened a few bright areas. I found no problems and everything seated back nicely. There is no way to correct the squareness of the lenses to the tube or the focuser to the tube, easily.

I made a round target out of paper with a compass and fitted it snugly and centrally into the objective lens cap which I fitted to the scope. When I insert my laser collimator (which I have checked), the laser dot hits the centre of the target (roughly, given that the laser dot is oblong!).

When I look down an illuminated Cheshire collimator, I see a centred green ring.

I will now start rotating one objective lens with respect to the other to see if the coma goes away.

What do I do next? Insert shims behind the rear objective lens to try and square things up? If this is the way to go, do I shim on the fan side of the coma or the point side when viewing through the eyepiece?

If I am still unable to fix it. Is there anyone in Brisbane /Ipswich who could help out.

Cheers

[Merlin66 (Ken)]

It's more usual to see mis-alignment issues, astigmatism or chromatic aberrations than true coma in small refractors.
Suiter recommends alignment of the optics with a Cheshire eyepiece.
If it was coma then tilting the element toward the brighter pint is the way to go....

Ken

[dunc66]

I am assuming it is called coma. It is a fan/wedge shaped flaring of the light from the target star or distant Xmas decoration. Can't find any clear picture of coma on the internet to confirm. I'm not talking airy discs or diffraction rings. I am seeing this with a low power eyepiece.

[Merlin66 (Ken)]

Certainly sounds like coma....
I have the ST-80 as a guide scope and never noticed anything like coma????

[TheDecepticon]

I have an ST80 also, but I never look through it, it is only worthy of guide scope status. What do you expect for the price?

[dunc66]

Hello TheDeception

Nice friendly and helpful reply, especially for a beginner .

However, I thank you for making me rethink my problem and any criticisms sent my way would have been well deserved as I have reassembled the lens elements incorrectly .

I used the article on Bill's Place (Orion Short Tube 80) website to arrange the glasses but I don't think it accurately depicts the correct orientations.

A better diagram for the beginner telescope tinkerer is attached. Hope it helps others.

I am curious to see how the scope works tonight , with the lenses arranged as ORION/SYNTA intended!!!

Got to

[Merlin66 (Ken)]

The illustration you show, doesn't give the necessary detail on an AIR SPACED achromatic doublet.
On my ST80 there are three small aluminium tags which separate the front element from the rear in the objective.
So, these two pieces of glass must be:
-Front element facing the correct way ( the curves may look similar but could be different!)
-Have the correct spacing between them
and be in the correct radial alignment to each other.

[dunc66]

The new ST80 that I have has only one black circular spacer between the elements.

[Merlin66 (Ken)]

The one I have is a definate Skywatcher model, probably 2 years old (and still work very well!) Sounds like they dumbed it down...

[notoriousnick (Nicholas)]

Originally Posted by dunc66

I am assuming it is called coma. It is a fan/wedge shaped flaring of the light from the target star or distant Xmas decoration. Can't find any clear picture of coma on the internet to confirm. I'm not talking airy discs or diffraction rings. I am seeing this with a low power eyepiece.

http://starizona.com/acb/basics/equi...s101_coma.aspx