Still have significant Coma

[saysme]

Hi guys this is a stack of 4 images taken last night to test my 10" GSO I have added felt to the tube from Prostar and collmination springs from Bobs nobs. I have a GSO Coma corrector insitu and the scope was collminated with the Coma corrector in. The image was taken prime focus with a 600D focused on the moon. Could someone tell me the sequence of hardware they are using and if this amount of coma is typical for my setup, and how to correct it please.

My hardware setup is, Focuser, 35mm extension (required for focusing), coma corrector, Coma corrector to eos adapter, eos adapter, Camera. Thedistance from the coma corrector to the camera chip is 60mm +/-.

Also there is what I think is reflection at 2 oclock from Altair, which featured in all photos from this run which did not appear in my other photos, would appreciate any thoughts on this to.

Thanks heaps

[tlgerdes]

Your coma doesnt appear concentric, I think you may have image plane tilt.

[Nico13]

I thought the same.

[pmrid]

I assume your scope is a newtonian, not a RC or something else.

Two things:
1. not all coma correctors are created equal. Is the GSO one you're using suitable for your scope? Can you borrow a Baader MPCC from someone to try it out?
2. Do you have the backfocus for your coma corrector set correctly - i.e. distance to the image sensor. This varies from one coma corrector to another. Since you're using an EOS camera with a T-Ring, yours will be working at about 55mm (i.e. 45mm in the camera and 10mm in the T-Ring adapter). This is right for an MPCC but may not be right for a GSO, for example. Can you check this?

And yes, there is uneven distortion in that image so you may have more than one factor working there. If your scope is a newtonian, is it properly collimated?

Peter

[saysme]

According to the specs the back focus distance is 70 mm, so an additional 10mm spacer might help?

The scope is collminated using a cats eye collinator and checked using a straight laser collminator, I see the point about focal plane. 2 possibilities really, secondary mirror not square, or the coma corrector itself.

Thanks for help happy to hear more, like can you get a 10-15mm t-piece spacer?

Whats the best way to Make sure the secondary is square?

[tlgerdes]

If it is a GSO focuser, it could be that as you tighten the screws everything is shifting slighly as well (I had this problem).

[tlgerdes]

When you figure that one out, can you tell me? :lol:

[multiweb]

When looking into the focuser with a cheshire or draw tube it should look like a circle and the clips of the primary should be at the edge of your secondary circular shape.

[saysme]

Thanks just centered the primary better in the secondary as below, I had to bring the secondary up the tube a little. Focuser looks more central in the secondary now to. Found spacers for t ring too. Suppose I should order a set!

Now let's get another clear night to check it all out.

[pmrid]

One thing at a time. You can't fully judge the image plane problem while there is so much coma. Your 70mm will be made up of the camera (there is a mark on the body on the top left side showing where the image plane is and I'm pretty sure it's 45mm to the front of the camera body). The T-ring will take up between 10-12 mm when fitted. measure that.
The you'll need an adapter of 15-17mm. There are often adapters sold as part of things like the Orion Off-axis Guider, as an example, and they are of varying lengths. Ask around. The other options are to get one made by Precise Parts in Florida (expensive) or by Peter Tan in Hong Kong (much cheaper) - Google him at "Available Astro Gears" and drop him a line. he's very helpful. He made an adapter for me that is able to be screwed in and out, with a locking ring so as to provide between 20-45mm spacing. Give him the measurements you want - say 10-20mm adjustment and tell him a male T-thread (T24 I think it is) on one end a female the other.

Peter

[saysme]

Thanks i still intend to get the spacing corrected. Orion produce a set of spacers specifically or this purpose a set contain 5, 10, & 20 mm spacers. At $29 it's ok value I think.

The adjustments made today may not have corrected the whole problem but its easy to check, even without the spacer the coma should be even. Well at least not obviously lop sided.

Ta.

[Peter.M]

As far as I am aware you dont do this. If you are using the cats eye autocollimator I would have assumed that the coma corrector would deform the reflections. Also an autocollimator and cheshire will be far more accurate than any laser you use to check it with in my opinion.

[saysme]

My understanding is, as there could be a slight variation in the actual centre of view between with and without the coma corrector, if it's collminated with it in then the visual centre (path of light) will be correct, while ever it is being used. Which is really the point of collmination, making sure the light is directed to a single point. A coma corrector added at a later date has the potential to shift that point if its optical centre is different. Well at least that how I read it 'some where'. Happy to go with a popular consensus on this one though.