NGC 3372 in Ha

[lazjen]

The first half of last night was clear in Brisbane, but of course it's quite near a full moon. Time for doing some narrowband imaging.

This was my second target for the night - my other one is still a work in progress and needs a lot more data. For this one, I wanted to try high gain, short exposures and lots of them. Here's the details:

Sharpstar 107PH with 0.67 FR (f4.4)
ZWO ASI1600MM-C at -15C
91 x 30 sec Ha at Gain 200, offset 50
60 Darks

211258

Much larger version is here: https://flic.kr/p/SQPfix

Processed in PixInsight. Open to CC/suggestions to improve it as I didn't really do much to it (DBE, DynamicCrop, Rotation, HistogramTransformation). The cropping is slight to remove vignetting in the corners (I'm awaiting a connector to hopefully reduce this further).

There's some weirdness to the star shapes that look as if it's some form of trailing error (or bad registration?). I was guiding and PHD2 seemed quite happy. I can't pick the problem in individual frames (using Blink), but maybe I'm missing something. I'll go back over those frames again to see if I can uncover anything else, but I'm open to any ideas?

Finally, I think that I do not have the spacing 100% correct and there's some curvature near the edges - especially near where it vignettes. After I get the connector to reduce the vignetting, I'll try to eliminate this problem too.

[Camelopardalis]

Looks nice Chris :thumbsup:

Peeping at the larger image, the star deformities look like wings, and seem to form a circle - look in the corners, they appear to be perpendicular to a line from corner to centre.

[RickS]

Looks great, Chris, apart from the minor issues you noted.

[lazjen]

Thanks Rick and Dunk.

Well, I am embarrassed to say I discovered something about the Blink tool that I wasn't aware I could do previously - I could zoom in on the view to watch sections of the image. For some reason I had it in my mind that it was fixed at the full image view. :screwy:

Anyway, I found 6 frames that were noticeably bad compared to the others, so I'll remove them from my next process, but I suspect they were being handled by PI anyway. However, more shockingly, the little "bars" are apparent on the stars on all images - all around the edges - in the middle of all the frames, they're nice round(ish) stars. I'm not sure why it's appearing like that but I have my suspicions it's something to do with not having the field 100% flat.

I've put a jpg up of a frame if you want to see an example - only thing done is an auto stretch: https://www.dropbox.com/s/1mozcnq0l9tm4p4/L_Ha_2017-03-10_22-54-52_2017-03-10_22-54-52_Bin1x1_30s__-15C_c_r.jpg?dl=0

[Camelopardalis]

Looks like astigmatism to me. If it was field curvature, the stars would just bloat as they become out of focus.

Doesn't detract from being a nice image :thumbsup:

[lazjen]

Ok, I just did some reading on that. I'll have to do some tests to try to work out what is going on and what is causing the problem. Then hopefully I can work out a solution.

[cometcatcher]

Looks very nice to me Chris. What bandwidth is your Ha filter?

[lazjen]

I believe it's 12nm.

[lazjen]

And of course, horizon to horizon cloud, no holes even to try stuff out...

[JA]

It's only visible in the extreme corners, and then only when you really zoom in. It looks like coma, but .....

It's a great image :thumbsup:

Best
JA

[JA]

You could try a bright(or not) small LED or very small light in the corner of the frame set up at a distance and framed in to the extreme corner of the shot where you noticed the issue (Top LH, bottom LH, and RH??) at night or even day possibly with LED in deep shade for contrast and then add and subtract filters, try without focal reducer, adjust etc... to find the issue, but still it is very minor and doesn't detract from a great image :thumbsup:

Best
JA

[lazjen]

Thanks JA. The issue seems to be everywhere. :) Looking more objectively, it seems to be Left and Right sides, not Top Middle or Bottom Middle. The "bars" seem to form a circle pattern around the image, with the corners sloping, and the left/right side vertical.

As for testing, I'm going to change 1 thing at a time (if possible) to eliminate the issue. I can see the effect on all the filters I've used so far (I went back and looked at L and SII images I took that night), but I have an empty slot in the filter wheel, so my first test will be to do an image through it. I'm going to wait for night skies as I know exactly what I'll be looking for in the stars and I need to look at the whole image.

And after the filter wheel test, I'll remove the FR (leaving everything else the same). From there I've got a few extra tests to do until the final one which is camera and scope only (no FW, FR, OAG - just the minimum connector to join.

[Camelopardalis]

I only looked for it because the OP pointed it out ;)

[lazjen]

Haha, should have kept my mouth shut, eh? :)

I've got to solve it as I think it's too much and I think it's going to make mosaics a lot more painful to do.

[Camelopardalis]

Sounds like a plan :thumbsup:

It's not apparent in top middle and bottom middle because the vertical dimension isn't as long as the horizontal. The corners are usually worst because they are the farthest from the field centre.

Still doesn't detract from the image at normal viewing size :thumbsup:

[lazjen]

Yeah, that makes sense Dunk.

So, even though it was clouds everywhere tonight I managed to get some testing done. I had some suspicions and they were confirmed - it seems to be the Focal Reducer causing the problem. It's the one I'm using on my RC10 and I wasn't really planning on using it on the Sharpstar anyway.

I've got a different reducer to try sometime when the weather permits and assuming I have the correct spacers for it.

[Camelopardalis]

Nice work Chris. Have you tried without the reducer? The sensor isn't enormous.

[lazjen]

I wasn't able to give a good run tonight - those clouds finally defeated me - however I've got it setup at the moment without a reducer. To confirm things 100% I'll do some more testing before changing anything.

However I am hoping to get a reducer working as I'd like a wider field and "faster" scope.

[Camelopardalis]

Yeah you just want one suitable for a refractor :D

[lazjen]

Yes, that indeed would be quite helpful. :)

I just didn't even think about it when I was setting things up. I was just too eager to try to get any damn thing imaged and start processing images again! :)

[Atmos]

It is really nice Chris, lovely FOV. Feeling like a mosaic to match Andy? :P :lol:

Are you using the Vixen 0.67x FR? What I am thinking is that even if you have the spacing correct it'll probably not look crash hot around the edges as it is designed for a F/7.7 system. As it isn't made for telescope it may be over or under correcting at the edges, changing the focal spacing may help but it could simply be an artefact of using that FR on that scope.

The seagulls in lens are a combination of uncorrected field curvature and astigmatism (my Nikon 50mm F/1.8D has it BAAAAAAAAAD).

https://dl.dropboxusercontent.com/u/27436364/Lots%20of%20stars.jpg

[lazjen]

Yes, that looks familiar. :)

I was using this FR: http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4955_Astro-Physics-CCDT67-0-67x-Reducer-2----e-g--for-GSO-RC.html

It works really well for my GSO RC10 - I think it does some field flattening as well? Anyway, it's totally unsuitable for the Sharpstar and goes into the category of "what the heck was I thinking?". :)

As for doing a mosaic of that area - baby steps. I'd really like to actually get more than 1 channel of data for a target! :)

[Atmos]

Well that explains the problem then, it flattens the field by correcting for astigmatism (RCs don't have field curvature as such) so that'll be the culprit :P