Is this Coma?

[cuivenion]

Still taking baby steps when it comes to DSO's, and the camera I'm using isn't best suited to it. I've noticed rather dramitic star shapes in my images:

I'm using an EQ5 with a Skywatcher 130p (130mm F5), with a Altair GPCAM. I'm using a reducer and the reduction is about 0.33. They are unguided but the exposure length is only 10 seconds.

Is it coma thats causing the star shapes? The stars also appear quite bloated even though I'm using an IR filter, is this a result of too much gain?

[spaceman_spiff]

The FoV in that image is quite small, did you crop it? 

Im no expert but I haven't heard of a focal reducer being used with a fast Newtonian reflector. Maybe the reflector already has a flat field and the reducer is causing some over correction. 

It doesn't look like coma to me, coma tends to orientate out towards the edges of the image. 

Dan 

[DHEB]

It looks like coma to me, particularly with the brighter stars, even if everything is slightly ot of focus. Was the nebula centered on the field or is this a crop of a larger image? Coma is symetric with respect to the axis so, if collimation was good you might be more certain about it. I would also suggest you use a Bahtinov mask for focusing.

[Uranium235]

Yeah, the distortion seems to be rotational - meaning its probably a result of the reducer being used (ie: not optically matched to your telescope). The only reducing corrector that is suitable I can think of is the SW coma corrector (0.9x) - but that requires a 2" focuser.

[DHEB]

6 minutes ago, spaceman_spiff said:

The FoV in that image is quite small, did you crop it? 

Im no expert but I haven't heard of a focal reducer being used with a fast Newtonian reflector. Maybe the reflector already has a flat field and the reducer is causing some over correction. 

It doesn't look like coma to me, coma tends to orientate out towards the edges of the image. 

Dan 

Coma is always present outside a very small area around the optical axis. The faster the scope the smaller coma free disk will be. In all cases the aberration gets more pronounced towards the edges. Have a look at the Wikipedia article, for example:

https://en.wikipedia.org/wiki/Coma_%28optics%29?wprov=sfla1

[cuivenion]

Thanks for the replies. I've not cropped the image. This is the reducer:

http://www.altairastro.com/0.5x-reducer-for-autoguiding-video-astronomy.html

[cuivenion]

11 minutes ago, Cinco Sauces said:

It looks like coma to me, particularly with the brighter stars, even if everything is slightly ot of focus. Was the nebula centered on the field or is this a crop of a larger image? Coma is symetric with respect to the axis so, if collimation was good you might be more certain about it. I would also suggest you use a Bahtinov mask for focusing.

Does this mean my collimation was out?

[DHEB]

1 minute ago, cuivenion said:

Does this mean my collimation was out?

No, not necessarily. You now said that the image is not cropped. If it was centered on the nebula, I would then say it is definitely coma. Collimation seems fairly done, at lesst nothing that I can comment. Try with a coma corrector. I have a Baader Mark III and works nice. It needs a 2" focuser though. Good luck!

[spaceman_spiff]

5 minutes ago, Cinco Sauces said:

Coma is always present outside a very small area around the optical axis. The faster the scope the smaller coma free disk will be. In all cases the aberration gets more pronounced towards the edges. Have a look at the Wikipedia article, for example:

https://en.wikipedia.org/wiki/Coma_%28optics%29?wprov=sfla1

Thanks, 

I still don't think this looks like coma though, the stars would otherwise form comet like shapes pointing outwards.

Could the OP upload the uncropped image?

Dan. 

[cuivenion]

3 minutes ago, Cinco Sauces said:

No, not necessarily. You now said that the image is not cropped. If it was centered on the nebula, I would then say it is definitely coma. Collimation seems fairly done, at lesst nothing that I can comment. Try with a coma corrector. I have a Baader Mark III and works nice. It needs a 2" focuser though. Good luck!

Thats a relief. When I upgrade to a 130pds I'll get a coma corrector.

[DHEB]

Just now, spaceman_spiff said:

Thanks, 

I still don't think this looks like coma though, the stars would otherwise form comet like shapes pointing outwards.

Could the OP upload the uncropped image?

Dan. 

You are right with that. I am thinking that the aberration would not become so aberrant (pun ) so close to the axis, if we assume that the image was centered on the nebula. It seems also clear for me that aberration is radially symmetric. Perhaps some expert might clarify?

Cheers

[cuivenion]

3 minutes ago, spaceman_spiff said:

Thanks, 

I still don't think this looks like coma though, the stars would otherwise form comet like shapes pointing outwards.

Could the OP upload the uncropped image?

Dan. 

The image hasn't been cropped the cam has very small field of view:

Pixel size microns: 3.75x3.75 um

Resolution in pixels: 1280x960

[Peter Drew]

Incorrect spacing of the reducer can also introduce these effects.

[cuivenion]

4 minutes ago, Peter Drew said:

Incorrect spacing of the reducer can also introduce these effects.

As far as I knew the closer the reducer is to the chip the more powerful the reduction effect. I'll try a couple of targets without the reducer and see how I get on.

[Uranium235]

I've taken a look at that reducer and can confirm it is definitely not intended for serious imaging. Been there, tried it, failed

Its intended use is to increase the FOV of a guidescope - or as the product blurb states, video astronomy... where the region of interest is usually at the very centre of field and the periphary is of no interest or value. It is not designed to flatten or correct in any way at all.

It doesnt matter how much you fiddle with the spacing, it will never sort your field out.

[cuivenion]

7 minutes ago, Uranium235 said:

I've taken a look at that reducer and can confirm it is definitely not intended for serious imaging. Been there, tried it, failed

Its intended use it to increase the FOV of a guidescope - or as the product blurb states, video astronomy... where the region of interest is usually at the very centre of field and the periphary is of no interest or value. It is not designed to flatten or correct in any way at all.

It doesnt matter how much you fiddle with the spacing, it will never sort your field out.

Cool, I'll stick to targets that'll fit without the reducer then. For the more serious images.