Advice needed on Celestron C8 images please.

[GrahamD]

Hi all,

Although not new to Astro imaging I’m new to imaging with a Celestron C8.  I’ve recently also got hold of a Celestron f6.3 flattener/reducer and tonight is the first time using the combination on a DSO. I’m shooting M51 with 420 second subs on a ZWO 294mc pro, Mount is a NEQ6 and Polar alignment is good. Using an ASIAIR pro to control the rig.  Guided and guiding is ok.
 

backspacing is as per specification and I focussed on  Capella with a Bhatinovmask. 
 

ive attached a couple of screenshots of the subs, one overall and a second closing in on the right hand side at around 3 O’clock - I noticed the weird shaped stars, brighter towards the centre - is this a collimation error, a backspacing error or something else ?  
 

Thanks, 

Graham

[B_ickringill]

I’m no expert but I would start by checking the collimation and at least eliminate it as a potential source. Could also be the size of the imaging chip is pushing the limits of the scope / reducer and giving you coma due to the optics.  Looks like you can get a decent crop of M51 though. 
 

Good luck! 

[Feccy]

+1 on what B_ickringill said

I had the same issue and fixed it last night - it was bad collimation - I had the same artefact but over the entire image not just the outer edges. Get some Bob's Knobs - I installed these over the weekend and despite 'bricking it' about messing with collimation the process was very easy and I had it sorted in 2 mins - glad I did

 

That said, I reckon you are just seeing the limits of your setup

[iapa]

On 12/04/2021 at 23:28, GrahamD said:

Hi all,

Although not new to Astro imaging I’m new to imaging with a Celestron C8.  I’ve recently also got hold of a Celestron f6.3 flattener/reducer and tonight is the first time using the combination on a DSO. I’m shooting M51 with 420 second subs on a ZWO 294mc pro, Mount is a NEQ6 and Polar alignment is good. Using an ASIAIR pro to control the rig.  Guided and guiding is ok.
 

backspacing is as per specification and I focussed on  Capella with a Bhatinovmask. 
 

ive attached a couple of screenshots of the subs, one overall and a second closing in on the right hand side at around 3 O’clock - I noticed the weird shaped stars, brighter towards the centre - is this a collimation error, a backspacing error or something else ?  
 

Thanks, 

Graham

Collimation.

If you find a good start to focus on in the center of the field of view and go off focus you will get a dark circle, if that is not centered in the ring of the star you need to collimate.

https://www.celestron.com/blogs/knowledgebase/sct-edgehd-collimation-guide

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwidr_O5wv3vAhUFt6QKHWf_C7wQtwIwAHoECAQQAw&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DuJ1u3eHBYVY&usg=AOvVaw2HyqcNEixZIy7y2eF--C01

The ASI294MC has a 23mm diagonal sensor, certainly good enough for the reduced C8

Edited April 14, 2021 by iapa

[iapa]

Just remembered you can do it in day time too using an artificial star.

 

https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html

 

Edited April 14, 2021 by iapa

[iapa]

5 hours ago, iapa said:

Collimation.

If you find a good start to focus on in the center of the field of view and go off focus you will get a dark circle, if that is not centered in the ring of the star you need to collimate.

https://www.celestron.com/blogs/knowledgebase/sct-edgehd-collimation-guide

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwidr_O5wv3vAhUFt6QKHWf_C7wQtwIwAHoECAQQAw&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DuJ1u3eHBYVY&usg=AOvVaw2HyqcNEixZIy7y2eF--C01

The ASI294MC has a 23mm diagonal sensor, certainly good enough for the reduced C8

Correction.

The reducer has a 41mm aperture, which when reduced gives 25.83 image circle - so, not quite as much I as thought, but still more than the sensor diagonal.

Any vignette good tho’, can be removed by flats.

[GrahamD]

Thanks all for coming back to me - strangely, when I stacked the lights with the calibration frames the stars were ok !  - I will check collimation next time I'm out (I've got a home made artificial star so will have a go with that as I only collimated without the corrector in !

Thanks,

Graham.