Abell 72 Planetary nebula

[lukebl]

Hi all,

Having a nightmare getting to grips with my new CCD, collimation AND guiding, but here's my effort on Abell 72 imaged last night for what it's worth.

It's far from satisfactory but, hey, it's my first serious DSO image with the CCD and I haven't posted anything for while. I can only hope for an improvement, but I'm beginning to realise that, for me, DSLR imaging was a whole lot less hassle (and money) for a much better quality of image than the SXVF-H9 (but maybe there's something wrong with it). The overall blue cast is because I've used the OIII image for both G and B channels, but the nebula is very weak in HA anyway. HA used for the red channel. About 1.5 hour's worth of 5 & 10 minute subs for each.

Starlight Xpress SXVF-H9, HA & OIII filters, 250mm f/4.7 Newt, guiding with PHD (time to change to change to different guiding software I think!).

A very interesting nebula in Delphinius, anyway, and not often seen on the forum.

[DarkerSky]

Nice capture Luke. You've just inspired me to hunt this one down visually next time I'm out. On the CN forums there is talk of seeing this with a scope as small as 8" and an OIII from a dark sky site.

[RobH]

Nice capture Luke.

I'm surprised that you're finding the H9 to be lower quality than a DSLR.....that really doesn't sound right. My 16HR (same chip) totally wipes the floor with a DSLR in terms of quality, noise and sensitivity.

Rob

[lukebl]

Thanks gents. And Rob, yes, I am having real aggro with the SXVR! Perhaps I should have stuck out for an Atik, but this one came up and I went for it. Maybe it's just a question of getting used to it. I hope so, otherwise it's been a very expensive mistake. Now that the nights are getting longer, I hope to be able to sort things out. To its credit, it is certainly sensitive. I can pick up 19/20th magnitude stars with only a minute or so exposure.

Anyway, here's a couple of images showing the OIII and the HA separately. As you can see, it's very weak in HA:

HA filter:

OIII filter:

[RobH]

What specific problems are you having Luke?

If there's anything I can help with, just ask away.

Rob

[lukebl]

What specific problems are you having Luke?

If there's anything I can help with, just ask away.

Rob

Thanks very much, Rob.

I'm hoping that the problems aren't caused by the camera, but by other parts of the setup.

Leaving aside the folly (in some peoples eyes) of imaging with a 10" Newt (I've done it for a year with my DSLR and it's been great till now, so I'm sticking with it!), the problems are primarily fuzzy and eggy shaped stars. I know they're not due to poor tracking as they're the same shape in short exposures. I'm using a Scopeteknix Filter wheel and various adaptors to achieve the correct sensor to Coma-corrector distance. I think everything's square, and the collimation looks fine. I've collimated with a Cheshire collimator and then done a fine adjustment with a star test. The result is really nice Airy discs when viewed with an eyepiece.

Despite this, the stars aren't round when imaged. I've tried imaging without the filter wheel, and the results are the same. So it's not the position of the filter wheel.

I'm also not sure if the camera is creating too much noise. Here's a single 10-minute frame in OIII. Does the noise seem acceptable for that length of exposure?

I expected more from a specialist camera costing a fair bit more than my DSLR!

[lukebl]

As an update. here's an enlarged section from a short exposure. There are a couple of issues here.

Firstly, I would expect the diffraction spikes to be central to the star, but they're not. They are offset to the right as shown by the yellow cross. I know that the secondary is precisely central in the tube.

Secondly, you can see that the stars have some sort of coma fanning out to the lower left, as well as not being round. This is despite the presence of the coma corrector at the correct distance, and the fact that the Chesire collimator and star test both indicate that collimation is spot on. I just can't get to the bottom of this.

[RobH]

Hello Luke.

Noise levels look like what I'd expect and are easily dealt with using darks.

The star shapes look like they're caused by a couple of things, neither of which is the camera. Thye egg shapes do look like a tracking issue, but if they're the same in short subs then it's not simple drift.

How are you guiding the system?

The coma type effect is trickier....there's another thread going at the moment from Blinky where the stars have the same issue, and there are some opinions on there that it may be pinched optics, although when I had an AT65 that had pinched optics, the stars were triangular.

I would ppost the closeups in the imaging help section and see what comes up, and also have a look at Blinky's thread.

Anyway, whatever it is, it doesn't look like a camera issue to me.

Cheers

Rob

[lukebl]

Thanks Rob. I don't think there's anything wrong with the primary mirror, like pinching, as the star test shows that collimation is absolutely spot-on with perfect airy discs. Maybe the camera is sagging in the focuser so that it's not completely square. Not sure how to deal with that one.

[blinky]

I would say it is collimation, lkely due as you say to the camera causing some sagging in the focuser. I'm still trying to sort it out with mine at the moment. One other thing, you say you never had the issue with the DSLR? Does this CCD Cam have smaller pixels? That would mean it would show up problems like this as the bigger DSLR Pixels would not be accurate? enough to show the star misshapen

[blinky]

Damm fine image though!

[lukebl]

I would say it is collimation, lkely due as you say to the camera causing some sagging in the focuser. I'm still trying to sort it out with mine at the moment. One other thing, you say you never had the issue with the DSLR? Does this CCD Cam have smaller pixels? That would mean it would show up problems like this as the bigger DSLR Pixels would not be accurate? enough to show the star misshapen

Thanks Blinky! I have seen your thread about your imaging problems, but couldn't offer any advice since I have similar probs. Not sure that the pixel size is an issue as the pixels on the CCD are actually larger than on the DSLR (6.45um for the CCD and 5.2µm for the DSLR). As for the camera sagging in the focuser, that problem should have also occurred with the DSLR. Rather stumped, really, as I haven't changed anything since I got the CCD and, visually, the collimation seems spot on. It's only when attaching the camera that the problems arise.

[whirlpool5194]

Sag in the focuser would be my guess, I'm having the same issues...

Wp

[lukebl]

Sag in the focuser would be my guess, I'm having the same issues...

Wp

I'm inclined to think the same, having now eliminated everything else. Being of a longish tubular design, the SXVF sticks out quite a way from the focuser and, coupled with the filter wheel, must be the source of the problem. How to fix it, though, short of getting a new super-rigid focuser which I can't afford?

[whirlpool5194]

I'm in the same boat....can't you add teflon pads to the focuser?

Wp

[lukebl]

I'm in the same boat....can't you add teflon pads to the focuser?

Wp

What are these teflon pads, and what do they look like? Do you have any pictures?