Eta Carina with QHY8

[ozstockman]

Hi guys,

Here is my color Eta Carina with proper flat and bias frames.
http://picasaweb.google.com/ozstockman/QHY8/

Total exposure length: 1h 15 min(10x450sec)
Imaging scope: ED80 f/6.8 with fpl-51 glass
Imaging camera: QHY8 (offset:158, gain:91%)
Guidescope: 8" SCT reduced to f/6.3
Guide camera: QHY5

Autoguiding was done with PHD and ASCOM EQMOD. It seems it really works as good as "on camera option"

Frames including bias(x5) and flats(x5) were all taken in MaximDL/CCD.
I used DSS for staking with special settings for QHY8 FITS files and "Create super-pixels from the raw Bayer matrix" as Bayer Matrix Transformation method. Final processing was done in PS.

Disregard bad stacking in the bottom left corner of the picture. It seems that something went wrong during stacking and this is why stars in that corner look bad.

Many thanks to Narayan and Terry for their help in this thread
http://www.iceinspace.com.au/forum/showthread.php?t=32344

cheers,

Mike

[leon]

Nice Mike, very subtle colours and plenty of detail , well done indeed, I have been trying to get my images to appear like yours, but am having a hell of a time with Picasa, however looking at yours i'm getting close

Leon

[Hagar]

Hi Mike, The image has a lot of detail but I think you may wellhave clipped all three colours in an attempt to darken the background. There is a lot more detail in the shot which can be brought out but will requie the image to be restacked using the standard stacking options.
I hope you dont mind I had a fiddle with it and this is my rendition. A bit hard seeing the clipping has removed a lot of data.

[Alchemy]

Shot looks good but the offset will be too high



There are 2 basic settings GAIN and OFFSET, gain is pretty much just amplifying the signal like ISO had on the digital camera, whilst offset is a different issue It also works with the strength of the signal but adjusts the bottom end where signal starts to be received (a bit like the idle of an engine) It needs to be adjusted so that at the quickest exposure possible (bias exposure) a signal would be received from all the pixels, It could be roughly assessed using the preview function setting the GAIN and sliding the OFFSET until the signal line on the histogram was seen. In my case when I set the gain to 50% adjusting the offset saw the signal kick in at about 114, NOTE : when shooting in preview mode the image is Binned4x4 ….. now switch off the preview button (push once), change Binning to 1x1 (under RESOLUTION) Set EXPO for 1 millisecond and push MAKE EXPOSURE button, the image will be a coloured image, and I found I had to increase offset to 117 to get a signal. What you are looking for when you expand the histogram is a bell shaped curve that is not clipped at the lower end. The stretched image should show a random distribution of signal

[Gama]

This would have been so good, if it wasnt for the tracking and stacking problem in the corners.

But still a excellent shot.

Theo.

[ozstockman]

thanks guys for your comments.

Doug, I could see all these data but when I was trying to bring it the core was completely burnt. So I thought that it would be better to have a cool core even though I'd loose all dim details at the the corners. But what you did with my image is much better than I could get. Thanks.

Alchemy, 158 for offset was the only setting I sould use in order to prevent a histogram sitting to close to the left side. I was trying much lower offset but data was partially clipped. I'm not sure why it happened but I guess it may be because my scope is slow(f/6.8) or I am using Celestron LPS filter.

Theo, it's certanly because of something wrong happened when it was stacked in DSS. I stacked frames for Lagoon with the same settings in DSS and there is nothing similar at the corners. However with stacked Eta Carina I can even visually see this problem in DSS. The bottom line of the image is not straight at the left corner. The image looks like it's folded at the bottom left.

[Gama]

If the data is to the left, then you need to Increase the Ofsett, from memory. You decrease the offset when the image is going to saturate early (Heaps of Data).

Theo.

[Bassnut]

Mike,mmm, looking at all 4 corners, there seems to be vigneting there. I cant see how stacking is the problem.It seems your camera mounting is severly not flat square on. ie focus and alignment is off/drifting bottom left (is it bolted tight?), weird.

[ozstockman]

Hi Theo,

your memory is ok and it's exactly what I was doing. I had to increase offset to move farer from the left. For example at 130 the histogram was almost cut in half.

[Hagar]

G'Day again Mike I just had another look at your text... Gain of 91% WOW that is very high I usually run with a gain of between 1% and about 20%. It allows an offset of between 117 and 130 max. The thing to remember about gain is it is like the ISO setting of DSLR. The higher you set it the higher the noise level and the easier it is to oversaturate an image. You are better with longer exposures a lower gain as this reduces the noise and increases the signal to noise ratio. It has taken me a while to wake up to this fact even though the likes of Theo, Jase and Peter Ward told me I still thought diferent.

Good luck and keep posting. Maybe we can all help each other. There is a good few QHY8's about now. Theo must be bringing in container lots.

[Gama]

Yeah, lunch box containers !.

But seriously, there are quite a few out there.

Theo.

[ozstockman]

Thanks guys. I think at least now I know what gain is :-) I'll give it another try with much less gain.

Fred, it's weird for me too. The camera is bolted to an 2" extention tube as tight as possible. I am using the extender as I can't reach focus without it.
However the extender is still 10mm shorter than needed so to get my camera in focus I insert the extender into focuser with 10mm gap.

It should not cause this bad left bottom corner if all elements of my imaging train are tightened vey well, right?

[Bassnut]

OK, its tight, but is the cam square to the tube?. Or maybe dew?, or a finger smudge on the cam?.

[ozstockman]

Thanks for your help Fred.

What do you mean by "square to the tube"?

If dew can cause it that it's probably why it happens. I have a lot of problems with it and dew is forming on my both scopes. But if it's because of dew then why only the left bottom corner is affected? Also I'm usually getting bad stars at this corner at the beggining when there is no dew on my imaging scope.

[Bassnut]

square as in parrellel to the image plane, is the tube mount bent on the corner, so that the cam is not mounted flat. Im grasping at straws here :).

Dew can start creeping in from the corners on the cam. I just thought, do you have dew heaters on the scope OTA?. If you havent, get em youll have to eventually ayway, it could be dew, although it tends to grow evenly on the OTA front .

It appears to me there is vignetting in the corners too (elongated stars pointing towards the middle), I dont know tho, others may know if the QHY8 can vignet on the ED80, I wouldve thought it wouldnt have:shrug: , but it does look like it.

[Gama]

The camera itself wont vignet. There is more than enough clearance for the light to get thru. What needs to be looked at is the path, and most important, the image circle produced by the optics.

Anyways, vigneting doesnt elognate stars. It only reduces the contrast as apposed to the centre of the image.
Elognating stars at the ends of the field is a good old case of coma.

Theo.

[Gama]

A quick check to see how well your tracking and alignment is to just take 1 minute exposures with a 2 minute delay, then check the positions of the stars by just using an overlay function and not a combine function. This will show you which direction the drift is in and any field rotation, etc.
If the stars all line up, then everything in this department is fine, and you need to look elsewhere. But i think you may find a gremlin in here somewhere..

Theo.

[ozstockman]

Hi Theo,

I am lost :-)

does coma problem appears only when mirrors are used. I am using ED80 refractor with QHY8 and never thought it would suffer from coma.

re alligment: it's not perfect but it should be so bad either. Moreover I'm using PHD and QHY5 to autoguide which should somehow compensate poor alligment.

Actually I checked all my images taken with QHY8 and stars are elognated to the centre at each corner in all my images. If you want you can check original size light frame at http://picasaweb.google.com/ozstockman/QHY8

cheers,

Mike

[ozstockman]

Fred,

I am not using any dew heaters. And even if it happens that dew isn't a source of problem with elongated start I should certainly buy dew heaters.

It seems to me I am wasting my time without them. Every night I have to stop imaging and get everything back inside because of dew.

cheers,

Mike

[Gama]

Coma occurs in many optical combinations, mirror, lens, cat.
This is why you get field flattners. Normally the lower the f ratio the higher your coma, but some scopes have correctors built in to fix this.

Coma has absolutely nothing to do with the camera. But for the size of the sensor will show any coma in you optical train, because the light has to now reach further away from the centre to reach the edge and thus starts your problem.
I use a MPCC for my 22" and it does a great job.

To learn more about it, go here http://www.astrosurf.com/luxorion/report-aberrations2.htm

Theo

[Gama]

Mike, there is also a thread here http://www.iceinspace.com.au/forum/showthread.php?t=12799 plus another here http://www.iceinspace.com.au/forum/showthread.php?t=18226 from Narayan about the similar problem as you with his canon 300D about a year ago.

Theo

[Gama]

A closer look at your image seems that you may actually have Astigmatism. Here is a brief description of Astigmatism of the web :

"Astigmatism An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the centre of the field) to a tangential one (at right angles to the centre) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through best focus." taken from here http://www.aoe.com.au/telescope_dictionary.html .

Nice read.

Theo.

[ozstockman]

Hi Theo,

thanks for the links I'll check them all. I also found that it may be caused by astigmatism and you've just confirmed it.

I think I'll leave it as it's now because I hope to get a new Vixen VC200L OTA for imaging this year and use ED80 for guiding only.

cheers,

Mike

[monoxide]

ED80's suffer pretty badly from field curvature and you'll need a FF to save you from cropping just to salvage the middle of the frame.
a WO 0.8x reducer should fix the problem