Trouble with Coma Abberation

[Bikeman55]

I have just gotten set up for the first time with astrophotography and have taken my first couple images. I have an Orion 8" Newtonian Astrograph F3.9. To the best of my knowledge I am collimated extremely well, I am still learning everything but I feel like my collimation is right on.

The problem I am having is with the coma I am getting in my images. I am using a HighPoint Scientific coma corrector but it doesn't seem to be doing much, I wanted to take some photos without the coma corrector to see how it would compare but I don't have the proper adapters to attach my camera and get it to focus without the coma corrector.

My question is, is my coma corrector insufficient and should I get a new one? My HighPoint Scientific coma corrector costs about $120, the description says that it is for newtonians with a focal range of F3 to F6 but optimized for F4.5. My telescope is definitely toward the lower end of that range and below what it is optimized for. 

I have attached a slightly edited image to give you an example of the issues I am having for your thoughts. You can see around the edges, and even when zoomed in on the stars in the center, they exhibit pretty bad coma. 

 

Just curious on what your thoughts might be. 

 

[Peter Drew]

The two suspects for this result are squaring on of the optical train and spacing of the coma corrector. If this was the best the cc could do then I think is belies its description.

[bobro]

It looks very similar to the GSO CC  http://agenaastro.com/gso-2-coma-corrector.html

Note the page on this corrector states : Additional spacers will be required (not included) between the corrector and your camera's T ring depending on the camera's CCD chip location to ensure that your CCD plane is set to the correct back-focus distance from the corrector.

 

 

[alacant]

Hi. There is a lot of tilt and/or the cc is at the wrong spacing. It needs 75mm from the shoulder of the m48 thread to the sensor, a 20mm extension will do it. So, collimate again with cross-hair Cheshire and have another go.

Note: it seems a pity to have such a fast telescope and then use a cc which makes it slower. A Baader or a SW cc maybe worth considering. The former leaves the fl unaltered whilst the cheaper variety of the latter would give you a 10% reduction and an addictive f3.6. HTH.

[laser_jock99]

On 22/05/2017 at 21:26, alacant said:

Note: it seems a pity to have such a fast telescope and then use a cc which makes it slower. A Baader or a SW cc maybe worth considering. The former leaves the fl unaltered whilst the cheaper variety of the latter would give you a 10% reduction and an addictive f3.6. HTH.

Or if you're really bonkers get the ASA Keller 0.7x coma correcter/reducer and turn the F4 scope into an F2.9 photon hoover!

Your collimation/tube stability needs to be sub milllimeter perfect though.

[Bikeman55]

My troubles have been solved, thank you all very much for your help. It was indeed an issue with spacing. My sensor was placed 100mm or so behind the coma corrector with the setup I was using and the attachment that came with the coma corrector, not knowing any better. I have now changed that to 70mm with great results, still have a little tinkering to do but it is much improved. Thank  you all for your help.

I will indeed upgrade my coma corrector though in the future.

 

Thanks again.