Coma or field rotation

[Magnus_e]

On friday night i got to take out my new Equinoq 80 for it's first ligth. Sadly the nights are still to bright. Ended up doing 4 min subs at iso 200 and still overexposed! Really want to give the scope a try in decent conditions.

On the brigth side i managet to get the goto to work 100% for the first time. I Did the two star align with one calibration star and polar aligned on the last star. 

I was going to start working on a mosaic of the "Cygnus Loop" with the Eastern and Western Veil. So i typed in ngc 6960 and the scope slewed rigth on target

What i noticed was that i looked to me that the Equinox had a little more coma than i was expecting? Or is it field rotation? On the center stars it looks like i have good tracing, so don't think thats the issue. The guidescope is a ST80 that is mounted on top of the Equinox.

So what is it coma or rotation? And if it's coma is there something i can do to fix it other than a field flattener. Would [better focus, shorter exposures, post prossesing tricks, ... ] be able to make it less visible?

Here is the stacked image

and a unedited but a werry bright sub

[steppenwolf]

This isn't field rotation, it is field curvature - quite normal with a refractor like this using a DSLR camera and no field flattener.

Field flattener or focal reducer/field flattener would make a big difference.

[Magnus_e]

Thanks

Did not know what it was. Had a lot of coma when i was imaging with th ST80, but then the stars looked like "comets". And if it would be field rotation the stars would trail like in startrail images, just less pronounced. The stars in this image looked focused, but pointing to the center.

I have the field flattener next on my list

Anyway looking forward to darker nights so i can start to test it out.

[jambouk]

Coma is mostly confined to telescopes with mirrors:

https://en.m.wikipedia.org/wiki/Coma_(optics)

Wait till you start looking into coma correctors spacing distances... That is fun

[Magnus_e]

Coma is mostly confined to telescopes with mirrors:

https://en.m.wikipedia.org/wiki/Coma_(optics)

Wait till you start looking into coma correctors spacing distances... That is fun

Sounds like a fun saturday night

Anyway, my ST80 had a lot of coma. This is the edge of a image with the ST80. The image is already cropped, but the coma is there

[jambouk]

I meant field flattner spacings...

If the ST80 is a refractor, the artefact was unlikely to be coma. Again, either field curvature, or a tracking artefact, or chromatic aberration, or all three.

[Magnus_e]

Then it must have been all three, The ST80 is a frac...

I know some use this for the Equinox 80, And FLO says it's a good match, so i'll tink i go for it.

Just have to wait for the next paycheck

I guess i'll hold off my mosaic plan untill i get the field curvature fixed. Don't think that would look great in a mosaic.

[lensman57]

This is not coma, you need a field flattener with your scope, the one form SW works fine and is cheap. I have also read your post regarding " Terrestrial Polar Alignment ".

With all due respect I think this idea is nuts. You can use a Terrestrial object to get the scope in a similar position as when had a good PA but you need to Polar Align it properly even after a visual alignment. You can use your handset PA routine. May I suggest that you invest some time reading up about the basics of imaging first. You could buy the book " Make every photon count " as it is very good book on the subject.

Regards,

A.G

[Magnus_e]

This is not coma, you need a field flattener with your scope, the one form SW works fine and is cheap. I have also read your post regarding " Terrestrial Polar Alignment ".

With all due respect I think this idea is nuts. You can use a Terrestrial object to get the scope in a similar position as when had a good PA but you need to Polar Align it properly even after a visual alignment. You can use your handset PA routine. May I suggest that you invest some time reading up about the basics of imaging first. You could buy the book " Make every photon count " as it is very good book on the subject.

Regards,

A.G

Thanks for the tip.

I have previously heard of the Make every photon count book, and guess it's time to get a copy of it.

The only reason i wanted to use the terrestial object PA is that my scope have been missing the first alignment star so badly that i hav been having problems getting the correct star in FOV.

But i can admit it was a strike of laziness....

[lensman57]

Thanks for the tip.

I have previously heard of the Make every photon count book, and guess it's time to get a copy of it.

The only reason i wanted to use the terrestial object PA is that my scope have been missing the first alignment star so badly that i hav been having problems getting the correct star in FOV.

But i can admit it was a strike of laziness....

You can mark the position of the feet of the mount's legs . once you have a good PA you can then return the mount to the same position as marked and do a hand set PA routine. with the FL of your scope, 480mm , you should be able to get a good enough PA after a couple of cycles of the PA routine to be within or less than one minute of arc.

A.G