Dusty Eta

[Benjamin]

This was more in the nature of an experiment to see how my gear (with OAG) works with my 8" Newt. Its all a bit heavy for the HEQ5 but the guiding seemed really stable and PA seemed good (drift aligned with PHD). Sadly all the stars came out quite misshapen and these distortions were as evident in my accidental 6 second Ha subs as they were in the 18x 120 second Ha subs. Thus I'm assuming these distortions were in the imaging train (camera tilt, collimation, incorrect distance from chip to coma corrector). The distortions seemed to be radial (see screen shots from each corner) and off centre (one corner is better than the rest), making me think it was a combination of uncorrected coma and either camera tilt or collimation? Would love opinions from more experienced heads. I tried to process out some of the dodgy stars but have included both images, the last of which is the more final image.

In processing I was enjoying the contrast with the dark central dusty sections rather than worrying about the region around Eta Carinae itself.

[gb44]

Ben, looks good to start with.
I used a f4.5 reflector and would see coma similarly, not as much though. You are showing it in 2 of the pics. You should see the coma in all the corners. If the optical axis isnt the exact same as the mechanical axis then the coma will be asymmetrical. The coma will point towards the centre of the optical axis.
Then you should be able to adjust things. You want the exposure centre to be in the optical centre. I was able to move the primary and/or focusser mount a little and using repeated 4sec exposures and adjustment iterations I could centre the optical axis. See what you can do in real time and recheck collimation.
I do have a Baader multipurpose coma corrector available. What are you using?

GlennB

[doppler]

Hi Ben,
That's looking pretty good to me, so much dust there and nice and sharp.
I would be fine tuning the collimation before worrying about tilt. Sensor spacing is always the hard one to get perfect, but it looks like you are close there.
Personally I think the Heq5 is under rated in regards to weight carrying and performs better with a heavier load on top. A big tube and a bit of wind is not a good mix though.

[Benjamin]

I'm using a Skywatcher f5 coma corrector that doesn't screw particularly well into the Orion TOAG I'm using - setup is Coma Corrector, TOAG, 16.5mm spacer, 20mm filter wheel and then ASI 1600MM-Pro (6.5mm). I think the extra room between the TOAG and Coma Corrector might be causing an issue (need to measure this but assumed it was around 12mm). However the focuser has been replaced on the Newt and the primary has been moved up a little. My thinking is to make sure the focuser is square to the tube (necessary?? and if so how to do this??), and make sure the secondary is centred under the focuser tube, then collimate. Then also accurately measure the distance from imaging chip to Coma Corrector to aim around 55mm. I gather form what you're saying Glenn I could, for sake of alignment, get rid of the coma corrector and use the coma to help align the optical and mechanical axis?

Here is the Astrobin link so you can see in detail how the background stars distort - https://www.astrobin.com/345100/

[raymo]

Rick, you're right about the HEQ5 [and the 6], both will happily carry
more than their rated loads; one member here carries a 12" Newt on
his HEQ5. I used to carry around 11.5-12kg on mine for AP, and had no
weight related problems whatsoever.
raymo

[Benjamin]

Every image had equally bad stars (i.e. the same direction and size of distortion) so assume the guiding was great with the HEQ5 (!) and weight was not an issue. Even less of an issue with the shortish exposures I think? Oddly made me convinced the mount is a keeper but think refractors might be a bit easier in some respects.

[raymo]

Horses for courses Ben; I personally preferred 8" and up for the light gathering ability, 4x faster than a 4", and I like diffraction spikes, and
most importantly, with my $5 D.I.Y. tube rotating device I could always have the eyepiece/camera in a comfortable position. I also like the greater focal length; 1000mm is a great compromise.
raymo

[Atmos]

The images look quite nice and contrasty so I don't imagine that there is anything too far out.
If you're getting misshapen stars at 6s in the centre of the frame then I'd be suggesting that your collimation might be off a bit. Or at least be off in that part of the sky, it isn't unusual for collimation to shift as you more around the sky.

I'd try to get the centre of field as good as possible before worrying about what is happening as you move towards the corners.

[Benjamin]

Pulled the 8" Newt to bits and a found a few issues. Focuser was not square due to a bracket I had installed for autofocusing, which then impacted on where the secondary was placed etc. Also looked at the imaging train which was way too long. I adjusted that yesterday and headed out to experiment in some "on and off" weather and realised that the shorter imaging train threw out the focus of my OAG, so rather than try to adjust this in the wee small hours I switched to a finder guider. This impacted on guiding I think, or else my polar alignment was too rough as I got quite eggy stars across the image (all in one direction, although stronger on one side - some camera tilt perhaps), but the image I got seemed to my eyes not to be impacted by coma? Adjust the focus of the OAG this afternoon so now I should have everything adjusted and guiding will be perfect right!!!!??? Seems typical that one problem solved (hopefully) reveals another, and another. Anyway, feel like some progress is being made with it. Another Eta Carina 30x 120s Ha subs. Astrobin: https://www.astrobin.com/full/345400/0/
Also tried to image the Cats Paw but the stars were much worse which as much as guiding my have been problematic it could well have been that collimation had shifted as you mentioned Colin. Appreciate the advice and thoughts on this.

[gb44]

Yeah, you will find the optical centre then. Hopefully then you can manipulate the OTA.
The full res image in Astrobin showed good stars shapes centrally and top right. They were odd in the lower left but coma showed top left and bottom right.
That focusser tilt issue ought to have been revealed when you use the laser and rack thru the range of the focusser and see the spot move on the primary.

GlennB

[Benjamin]

Thanks for having a close look Glenn. Is it the first Astrobin image you are referring to? The second recent one I’m hoping seems a bit better but maybe I’m just being hopeful! I thought the issues in the recent one were more on the right and particularly the lower right and it seemed the stars were all angled roughly in the same direction without much coma? I’ll rack through the focuser and check out the position of the laser and see if it moves. The first optical collimation I did after reassembling seemed to get everything very much inline and the laser collimation confirmed things (a slight adjustment of the secondary and primary). I did fiddle with the the imaging train when imaging to see if I could quickly improve things and there was some small movement I could correct.

[Benjamin]

Checked the focuser with the laser in the draw tube and watched the spot on the primary move as I racked it out! Adjusted the focuser to a degree and then re-collimated and found that in the zone I focus in the spot is stable but at certain point when the focuser hits some kind of resistance, on the way in, the spot (or draw tube) moves more distinctly in one direction. I’m guessing this won’t matter so long as I collimate near the focus point that is unaffected by whatever is causing the tube to divert.