PHD2 Guiding advice

[Celestron4]

I have had another go. I gave the polar drift alignment and Guide Assistant more time (roughly the worm period for my Celestron AVX mount (approx 400-500s, I think). I decided to have a go at photographing NGC 281 (Pacman Nebula), it doesn't show up as well as M51 but starting to show itself in s a stack of about 15, 5 minute exposures. The tracking in the centre of the image is much better but the issues now lien the outer edges. Is this something to do with my camera? I have attached a test image, the Guide Assistant readout and the log file, I hope they help!

 

PHD2_GuideLog_2020-07-31_000005.txt

[michael8554]

RA 1.0 arcsec and Dec 0.75 arcsecs are lower and getting closer to each other, which results in rounder stars.

RA is guiding a bit north then south, north then south. The smaller excusions are well handled, but bigger ones are overshooting.

I can only suggest reducing RA Aggression and/or upping RA Hysteresis , leave each change long enough for the mount to settle.

You guiding is now good enough to show that there is a small amount of coma in the corners of your image.

I don't remember your equipment profile, so if you have a Coma Corrector, then a chart often shown on this site suggests the corrector is slightly too close to the sensor.

As mentioned previously, you said you have an Aux Mount connection but the RA and Dec positions of the mount aren't showing in the logs.

Definitely good progress.

Michael

[Celestron4]

I have a Celestron focal reducer, not sure if it acts as a coma reducer though. I’m looking at one-shot colour CCDs at the moment. Would a different camera help with the coma?

Edited July 31, 2020 by Celestron4

[michael8554]

There is some debate as to whether the Meade/Celestron 0.63 FRs also reduce coma.

The Celestron Focal Reducer is optimised for the 8" SCTs, the 6" SCT may have different mirror shape etc, but it seems to work okay.

A lot depends on the size of your Imaging sensor.

An APS-C sensor will be picking up the edges of the image circle, so some coma is to be expected, a smaller sensor will probably be coma free.

But you added the FR to increase your FOV, so a smaller sensor would be counter-intuitive.

Also the 105mm back focus is for the 8", the best figure for the 6" may be different

As I said, the coma orientation suggests you should move the camera away a few mm.

I checked and see you don't have an Aux Mount connection after all.

It's lack won't affect your guiding in any way, except that without it you will have to Calibrate on every new target, and the GuideLogs will be missing useful info. 

Michael

[Celestron4]

The focal reducer has definitely helped improve the quality of the images I am getting. I am thinking about getting a one-shot cooled CCD camera, which as you said will make the FOV smaller. Cooling may make a difference and the increased sensitivity to the red end of the spectrum should bring the nebula out more in the images. 

The Aux connection is something I will look at. I assume it’s the INDI mount connection I should be using?