distorted stars on photos

[jeffmar]

Last year I bought a Meade 8" f/6.3 SCT to use for taking photos. The result was very good moon and nebula pictures. Loved the images as long as it was in the middle third of the frame.

The camera I use is a Sony A77m2 with an aps-c sensor and 24 megapixels. I am sure the pixel count does not cause the distortion but the sensor size might. I don't know much about ccd cameras but apparently sensor sizes in most of them are far smaller. Is it the telescope or the camera or both that is the problem?

[Ruud]

It might be the curved focal plane (field curvature) of the SCT meeting your flat sensor that is causing your problem.

My 6" f/10 Celestron SCT has a lot of field curvature. The 0.63 times fielf flattener / focal reducer cures the problem to a certain extent, but not completely.

Yours is a native f/6.3, right?

[ronin]

Usually it is the edges and the cause being the curvature from the scope, everyone draws the image as flat but it isn't. However you must have a long focal length, close to 3 meters I guess, and that would/should help. Not sure if you can get a flatener for the scope. Even aps sensors are "big".

Another is coma, SCT's do have it, some have a coma corrector built in. So find out if yours has. Was a post of someone having an coma corrected SCT and they just bought a coma corrector and fitted it  As it was already corrected the additional one simply added coma in.

All became apparent when the question asked was:- Why with a coma corrected SCT are you adding an additional one?

[jeffmar]

Usually it is the edges and the cause being the curvature from the scope, everyone draws the image as flat but it isn't. However you must have a long focal length, close to 3 meters I guess, and that would/should help. Not sure if you can get a flatener for the scope. Even aps sensors are "big".

Another is coma, SCT's do have it, some have a coma corrector built in. So find out if yours has. Was a post of someone having an coma corrected SCT and they just bought a coma corrector and fitted it  As it was already corrected the additional one simply added coma in.

All became apparent when the question asked was:- Why with a coma corrected SCT are you adding an additional one?

The scope has a 1260 mm focal length. It was manufactured with a larger secondary mirror that was set closer to the primary mirror. I do not believe it has any built in field flatteners like my celestron edge does.

[Thalestris24]

Last year I bought a Meade 8" f/6.3 SCT to use for taking photos. The result was very good moon and nebula pictures. Loved the images as long as it was in the middle third of the frame.

The camera I use is a Sony A77m2 with an aps-c sensor and 24 megapixels. I am sure the pixel count does not cause the distortion but the sensor size might. I don't know much about ccd cameras but apparently sensor sizes in most of them are far smaller. Is it the telescope or the camera or both that is the problem?

Hi

Maybe you could post an image which shows the problem (.png is best), reduced in size if necessary.

Louise

[jeffmar]

Thank you Louise. I will do that.

[Thalestris24]

Sorry, I can't really see - a much bigger image would be better! Collimation might be off?

Louise

Edit: ps also maybe a single sub might be good to see + exposure details. How are you guiding?

[jeffmar]

Louise, You are right. The collimation could be off. My old C11 was much easier to collimate than the LX200 f/6.3. If I don't have the donut in the exact center when I am collimating the image becomes very skewed. In many images I have taken with the Meade scope the outermost stars all point to the center of the image like boomerangs and the center stars are mostly round. The image I posted has been cropped a bit and does not show as many distorted stars. That same image is in the general gallery somewhere under my user name. 

[jeffmar]

Something I forgot to say. I was not guiding. The scope as aligned well and my exposures did not exceed 15 seconds. That still could be a factor as you say Louise. 

[Thalestris24]

Well you can't expect too much with an sct unguided!

[lensman57]

I have not heard of a Meade with F6.3 ratio, the new ACF range are all @ F8. Is this a custom scope?

A.G

[jeffmar]

I have not heard of a Meade with F6.3 ratio, the new ACF range are all @ F8. Is this a custom scope?

A.G

It was around 15 years ago, perhaps longer, Meade produced 8 inch and 10 inch f/6.3 SCT's. The were aimed at the astrophotography people. It also has quite a wide field for a Schmidt Cassegrain telescope. Meade did this by putting a larger secondary mirror closer to the primary mirror. It has a 43% obstruction. I do not know why it was discontinued but it does not have good flat field for photography from my experience. It seems to be nothing like the latest higher end SCT's from Meade and Celestron. I have had really good moon photos from it. The optics seem very sharp.