M27 Dumbbell nebula, RC8 + ASI1600mmc

[vlaiv]

First light of RC8" and ASI1600mmc (with OAG in the mix), and I'm somewhat unhappy with result

Data: 60x60s Lights, x256 Flat / Flat darks, x64 Darks, taken at -20C (gain 139, offset 25)

Equipment: RC8, ASI1600mmc, UHC filter, OAG + ASI185mc for guiding, HEQ5

Conditions: good transparency, changing seeing (from good to fair), red zone (SQM around mag 18.5)

Image is binned 2x2 for effective resolution of ~1"/pixel

I've probably went over board with processing, still not used to processing images with bright target in rich star field, and I wanted to see if UHC would help me capture outer H alpha and OIII regions extending around nebula itself (and it appears I did manage to capture hints of it).

What makes me unhappy about the image is star shapes in the corners, I did expect some field curvature but this seems a bit too much for 4/3 sensor size (apparently one should be able to use RC8" with APS-C sized chip and have FC under control). I did focus at about 1/4 - 1/3 of the edge, and not in center of the image. I also notice that star shapes are different in different corners, could this be due to poor collimation of scope rather than FC? (check out full size version to see star shapes in detail).

Here is the image:

All comments, suggestions and critique are welcome.

[MarsG76]

I don't see any reason to why you should be unhappy with this image, looks good to me.

 

[vlaiv]

2 hours ago, MarsG76 said:

I don't see any reason to why you should be unhappy with this image, looks good to me.

 

Yep, I guess you are right, I probably whine too much. I guess I'm surprised with the shape of the corner stars, I mean they are quite out of focus compared to center field, and a bit misshapen.

This is comparison to top right corner, top left corner exhibits astigmatism, while bottom right one looks a bit like coma. I'm just wondering if this is normal, or I did something wrong (like wrong focusing), or is collimation of scope a bit off. If properly collimated I would expect star shapes to be symmetric with respect to image center.

I'm yet to try out focal reducer (CCD47 by TS) with this setup, and I'm slightly worried since that is only FR and not field flattener and any edge of the field aberration is just going to be emphasized.

[MarsG76]

1 hour ago, vlaiv said:

Yep, I guess you are right, I probably whine too much. I guess I'm surprised with the shape of the corner stars, I mean they are quite out of focus compared to center field, and a bit misshapen.

This is comparison to top right corner, top left corner exhibits astigmatism, while bottom right one looks a bit like coma. I'm just wondering if this is normal, or I did something wrong (like wrong focusing), or is collimation of scope a bit off. If properly collimated I would expect star shapes to be symmetric with respect to image center.

I'm yet to try out focal reducer (CCD47 by TS) with this setup, and I'm slightly worried since that is only FR and not field flattener and any edge of the field aberration is just going to be emphasized.

When you try a field flattener you'll notice that the edge distortion is either completely eliminated or a lot better. Quite often the edges are slightly out of focus on bigger sensors revealing the coma. 

[VentsiBo]

The stars in the corners don't look quite ok to me... I have GSO 8" RC and QHY9 and there are also some issues in the corners but not that much. Did you try the CCD47 reducer?

 

 

[ollypenrice]

The image is very deep. You really need Ha and OIII filters to get the outer exptensions so to find as much as you have is promising.

I can't help with the corner stars, though.

Olly

[vlaiv]

2 hours ago, VentsiBo said:

The stars in the corners don't look quite ok to me... I have GSO 8" RC and QHY9 and there are also some issues in the corners but not that much. Did you try the CCD47 reducer?

 

 

Yes, sorted out some time ago - it was collimation issue - primary was out of collimation, once I fixed that, edge stars improved considerably, now there is almost no distortion up to the edge of field with ASI1600 sensor.

I have not yet tried CCD47, hope to give it a go in next couple of weeks (now expecting shipment of full set of filters, both RGB and narrow band, so I'll probably toy around with NB filters when the moon is up - this will give me chance to test out reducer). I think that I'll probably still have issues with CCD47 in the edges of the image, it might be that careful focusing (2/3 - 3/4 of the center of the image) will help somewhat, but field curvature is such that for sensor size such as ASI1600 starts showing it at the furthest corners unless there is perfect collimation. I suspect that with perfect collimation and focal reducer there will still be some noticeable FC in the corners.

[VentsiBo]

I also think that CCD47 being a reducer will not make a change. I'm asking about it because I'm considering buying it and want to know what to expect...

[vlaiv]

1 hour ago, VentsiBo said:

I also think that CCD47 being a reducer will not make a change. I'm asking about it because I'm considering buying it and want to know what to expect...

It will certainly not flatten the field, and since it is reducer it will "widen" field of view - which can only mean more curvature in the corners, but how much, I can't really tell until I try. I'll certainly post results once I use it, so you can see the results and then decide if you want to invest in buying it. There is however flattener for RC without focal reduction - you might want to take a look at it. For me, ideally there should be focal reducer and field flattener - I can live with field curvature on my sensor without focal reduction, but no one makes such thing yet.