Field rotation or poor alignment

[Lordspace]

Hi, so I managed to get some subs and what not of of the Orion nebula a few days ago and notice that the stars Inthe outer edges and towards the centre were trailing. 

When I set up my mount which is a skywatcher alt az synscan, (not ideal but it's all I have)

I used all 4 alignment arrows when fine tuning the stars in the eye piece,

I've seen and read that it's best to only use the up and right arrows when doing the final adjustments??

Does anyone know if this is true or is this more a case of field rotation.

 

The neb was south in the sky not sure on the exact hight but it wasn't very high. 

I have attached the very poorly edited image.

Thanks.

[david_taurus83]

What kit are you using? That looks fairly close up so a long focal length?

[Lordspace]

58 minutes ago, david_taurus83 said:

What kit are you using? That looks fairly close up so a long focal length?

 

I'm using the skywatcher 114p but the mirror has been moved up the type to get focus with the dlsr. Also the image was cropped when editing

[michael.h.f.wilkinson]

It looks a bit like coma to me (perhaps combined with field rotation). Did you collimate the scope beforehand? The comet-like stars seem to be radiating away from a point in the upper right-hand side of the image. That seems more like coma than field rotation. I assume this is a 4.5" F=500mm instrument, which is pretty fast for a Newtonian, and a coma corrector might be needed.

[vlaiv]

I also vote coma. Scope needs collimation after mirror adjustment.

Unfortunately there is no Coma corrector in 1.25" format.

[Lordspace]

It has been fully committed before I went out, everything looked spot on with the laser. Also it a skywatcher 114p but the mirror is about 2 inches further up the tube then normal.

[vlaiv]

10 minutes ago, Lordspace said:

It has been fully committed before I went out, everything looked spot on with the laser. Also it a skywatcher 114p but the mirror is about 2 inches further up the tube then normal.

Maybe above image is a crop? Do you have single unedited sub from your camera?

Coma is symmetric aberration - and it should be minimal in center of the image for well collimated telescope. It is not central in above image - if you take lines going thru the stars and their tail (coma tail) - they converge to roughly the same point in the image and it is not center of the image:

Either your scope needs to be collimated properly or you have cropped the image. In second case - it will help that you put target in the center of the FOV and then crop to it. Don't let it be to one side because it will lead to coma being asymmetric in the final image.

Of course, way around this is to use Coma corrector - that way you will get good definition across whole field, but like I mentioned - there is no coma corrector in 1.25" format commercially available. You can DIY one - do a search for 1.25" coma corrector and you will find instructions in ATM sections of astronomy forums for DIY 1.25" two element coma corrector - you can try that.

If you can't be bothered with that, maybe best thing to do is move to a bit larger but scope that is actually suited for AP - 130PDS. That one has 2" focuser and properly positioned mirrors so you can reach focus with camera.

 

 

[Lordspace]

9 minutes ago, vlaiv said:

Maybe above image is a crop? Do you have single unedited sub from your camera?

Coma is symmetric aberration - and it should be minimal in center of the image for well collimated telescope. It is not central in above image - if you take lines going thru the stars and their tail (coma tail) - they converge to roughly the same point in the image and it is not center of the image:

Either your scope needs to be collimated properly or you have cropped the image. In second case - it will help that you put target in the center of the FOV and then crop to it. Don't let it be to one side because it will lead to coma being asymmetric in the final image.

Of course, way around this is to use Coma corrector - that way you will get good definition across whole field, but like I mentioned - there is no coma corrector in 1.25" format commercially available. You can DIY one - do a search for 1.25" coma corrector and you will find instructions in ATM sections of astronomy forums for DIY 1.25" two element coma corrector - you can try that.

If you can't be bothered with that, maybe best thing to do is move to a bit larger but scope that is actually suited for AP - 130PDS. That one has 2" focuser and properly positioned mirrors so you can reach focus with camera.

 

 

I did crop the image but I don't have a raw sub on my phone. 

I had a scan over one of the subs on my camera and it looks like 99% of the stars have a trail so would that more likely be collimation than coma??

[vlaiv]

17 minutes ago, Lordspace said:

I did crop the image but I don't have a raw sub on my phone. 

I had a scan over one of the subs on my camera and it looks like 99% of the stars have a trail so would that more likely be collimation than coma??

Coma and collimation of newtonian with parabolic primary are linked together.

Scope will always have coma, but if scope is collimated - that coma will have smallest impact on the image - center of the field will be coma free (coma will be too small to be noticed - it will be smaller than resolution of telescope) and as you move away from the optical axis - it will grow. It will be worst at the edges of the frame.

Here is diagram of the stars in the image with perfect collimation. Faster the scope - more coma it will show. 114mm aperture with 500mm FL is very fast newtonian and it will show a lot of coma.

Above is with perfect collimation - but if you have scope that is out of collimation - things will get even worse. First thing that you will notice is that those pin point stars are no longer at the center of the field - they will be to one side - very much like in your image above. One side of image will have much worse stars because of that. Stars will also be a bit more deformed than they would be from coma alone - there will be no sharp / pinpoint stars anywhere in the image.

If you think that you have poor polar alignment - that is easy to diagnose - stars in your image need to be streaking in same direction - all will suffer same type of distortion like this:

regardless where they are in the frame (corners or center of the frame - little streaks will be oriented in same direction).

Your image does not show that.

If you think you have field rotation - again, that is easy to diagnose, your stars should look like this:
 

There should be "center" of the frame - where stars are good, and then stars should be progressively longer streaks as you move away from that center. Also, direction of streaks should be perpendicular to line towards the center where stars are ok.

Again - that is not happening in your image.

Your image will suffer from coma (and it does suffer from coma) and you just need to make sure coma is "centered" - by properly collimating your scope if it is not collimated, and after using coma corrector to deal with remaining coma. That is probably not an option for you as there are no 1.25" coma correctors to be purchased.

 

[Lordspace]

Ive just noticed if I put the Lazer in the 1.25" slot and tighten the locking nuts then the Lazer shifts to one side so that could be giving me false reading when collimating.