Lessons learned...First light TS Imaging Star71 - 71mm f/4.9

[nucdoc]

I have learned a great deal on this site in only one year of serious imaging so I thought I would share some lessons that I learned, and recent mistakes

I just acquired a  TS Imaging Star71 - 71mm f/4.9 for widefield imaging, and have been learning how to use my starlight OAG filter wheel on my celestron AVX with PHD.  Given that there is a hurricane about to hit the east coast of NA and I had only one night before 2 weeks of clouds  so I headed out to image M31

Below is my a single sub, 10 minutes with my QHY8Pro OSC using a Astronomik CLS filter.  There is cleary vignetting and a large area of void in the bottom left, the next picture is the bias subtracted Flat (I use a Spike-a flat fielder light panel), the third picture is the stacked, aligned and corrected master before processing.  Aside from the vignetting and large signal void there are mangled stars in the bottom left corner.

As this was my second time using PHD and OAG it looks pretty decent:)

Now that it is cloudy I set up my system in the garage and used the flat panel light source to sort things out.  The signal void and mangled stars are from the mirror on the OAG it was way to low, as I moved it up, above the plane of the filter, the void vanished. I assume the mangled stars would also be corrected, I just hope I will still see stars to use for guiding

In my controlled garage setup I still have the vignetting, it appears I am about 5mm to far back with the plane of my CCD, i can correct the problem if i use my Non-OAG filter wheel, as it is thinner and there is no vignetting, but then I would need to mount a separate guide scope

​I could also use my Monochrome SX-9 with the OAG, but then I would need to do a Mosaic

The final picture is the cropped and processed image, my first M31 (12 ten minute subs), now I want to go back and get an ever better image

I am always learning in this discipline... sometimes I wish I did not have to learn so much.....

Mark

Raw Sub

Flat

Integrated Image

Phd

Final Processed and Cropped Image

[MattJenko]

I was under the impression that the imaging circle for these scopes was huge, so bit surprised to see the vignetting. Is it caused by the filter or some connectors rather than the scope itself?

Nice image by the way

[laudropb]

Very nice image thanks for posting.

[ChrisLX200]

You need to turn the camera around ~90deg so the prism intrudes on the narrow axis of the sensor - gives you a little more room. Some drop-off in the corners might be expected with the large sensor of the QHY8 but flats will correct for it. With a little refractor having a firm focuser there's little advantage to using an OAG, I would have used a separate guidescope myself.

ChrisH

[Zakalwe]

Is that vignetting caused by the OAG? It looks very severe and not at all what I would expect, to be honest. With that scope you almost certainly won't need an OAG...perhaps a finderguider might be an alternative option?

<edit> beaten to the punch by Chris!

[r3i]

I use the same scope with an OAG and a QHY8L, mainly out of convenience rather than necessity. I've not encountered any issues with vignetting but like Chris suggests I have the pickup prism on the long axis of the QHY8L chip.

I've also used an SXV-H9 plus OAG with the scope but of course the FOV is considerably less.

[ollypenrice]

That void on the left cannot possibly be caused by the scope itself. You'd be able to see an obstruction like that when you looked down it. It pretty well has to be the OAG prism. You can test for the best prism depth by shooting flats and raising or lowering it to see when the shadow appears/disappears. Like the others I think an OAG sounds like hard work on a 71mm scope.

Olly

[nucdoc]

Thanks guys for the helpful responses..

(1) Never thought to turn it 90 degrees, will try that if I ever need to use the OAG again on this system

(2) Olly, the void on the left is defiantly the OAG prism, I proved this in my garage shooting flats with a light panel, I could make that void appear and disappear by moving the it up and down, will definitely set the depth off line using flats the next time I use it.

(3) used the OAG for guiding as at the time that was all I had, the circular vignetting, seen best on the right side resulted from the sensor being too far back (5-10 mm beyond the recommend back focus), this is also the fault of the OAG as the filter wheel is much thicker in this system resulting in the vignetting artifact.

(4) Last nite I set up with a dedicated guidescope, and a thinner filter wheel giving me optimum back focus. Guiding was good, but as Murphy would have it the QHY would not produce an image. After several hours I gave up, and by the light of day found that 2 of the pins on my 9 pin DIN cable (proprietary power supply cable for the QHY) were broken #X##@## !

Now I am off line till I get a replacement Din cable.

Mark

[gooselk]

Hi Mark,

a nice image, I wish I was able to get such results. One thing is on my mind though... Did you take darks? Because in your final image are quite eye-catching hot pixels visible. Darks would eliminate them.

Clear skies ... Lars

[nucdoc]

Hi Mark,

a nice image, I wish I was able to get such results. One thing is on my mind though... Did you take darks? Because in your final image are quite eye-catching hot pixels visible. Darks would eliminate them.

Clear skies ... Lars

Lars, thanks for your comments, yes I have darks, I shot about 25.  I did this at the same exposure and temperature as the lights, but on a different night.  I have found that with this camera hot pixels are not easily removed with darks, on a previous image 

http://astrob.in/211740/0/ 

I used cosmetic correction to clean up the hot pixels not removed using dark frames.  Using my Sx-9 I do not have such a problem, the darks clean up all the hot and cold pixels.

As I mentioned in my post above I recently found that two pins on the Din cable that acts as power supply cable were broken, these two pins feed the TEC unit on the camera.  Two sessions ago I found that this camera was heating up, despite software showing the TEC cooling function to be engaged.  A the time I assumed it was a software bug, but it may well be that my TEC cooling was intermittently failing and some of my light frames were shot at a much higher temperature than I assumed, will have to wait for a new cable and shoot an new set of darks and hope these clean up hot pixels on my lights.