So, I have spent some time tweaking the SCT (Having worked out a trick to collimation in focus that is far more accurate than using a defocused star) and last night was too good to resist.
So It is time to move on from the really easy ones. This is as far as I am concerned my final go at Centaurus A and the final imaging night on M83, the processing on this one however is WIP. I think it is piling in time for no real benefit to go much further with data capture here, at least with this scope and camera combo. Putting these in here as I still consider myself in the beginner stage. Apart from some timelapses and the obligatory shot of M42 by strapping a DSLR to the C925 I have really only been doing this since January this year.
Taken with the c925 SCT and ASI294MC Pro camera, 2 hours with of 5 minute sibs on Cen A and about 4.5 hours worth on M83. Stacked and mostly processed in APP and polished/cropped in Photoshop.
For those curious, the best way I have found to collimate this thing relies on the coma inherent in the SCT design, I am imaging without a reducer at this point as I was using the off the shelf Celestron one but I was never really happy with the sharpness and it produced some big reflections.
Collimation wise using short exposures (5 to 10 second subs) you adjust in focus so that the coma induced tails around the edges of the FOV all point away from the center, easiest was the stars right out in the corners as the coma is most visible there. This is not quite perfect but CCD Inspector (I downloaded the trial to have a closer look) called it within 4 pixels of bang on. It moves around a little as the focuser is racked in and out but it stayed fairly well put so long as you ran the focuser down with gravity then back to to focus before starting.
Given that even cropping the size down there is still some coma visible in the corners and there is spherocromatism visible that is related to the corrector plate of the SCT, I reckon these are getting close to the best it has got to give, not bad for a visual OTA re-purposed.
For those that are not familiar with spherocromatism, the SCT main mirror is a spherical form not a paraboloid and the corrector plate is there to effectively pre refract the light to make the system behave as though the mirror is a parabola not a sphere in form. Basically it is a thin lens of a single glass type so it suffers from chromatic aberration just like a single element refractor would. But in this case it is spreading the light on a radiant from the centre of the image circle, not as a fringe around bright objects, generally blueish to the middle and reddish to the outside.
Last edited by The_bluester; 30-04-2019 at 02:52 PM.
Absolutely fantastic image there Paul.
I think we can get a bit too focused☺ on those stars at the edges but just look at the detail and colour of the object you were after. I was getting upset by oval stars but forcthe moment star tools sorts them out...I got distance and tilt to sort out☺.
Great tips also.
Well done.
Alex
I really do like round stars, but I have to accept that unless I buy another scope, I won't get them at this image scale. I am happy enough with the compromise though.
Now to do a process on M83 that I am properly happy with so I can call it done and dusted. I really did do this one over breakfast this morning.
Actually the last one I reduced the saturation significantly. That is saturation in APP. Hard to compare to my earlier stuff as I was using Deep Sky Stacker before and its colour management is much more difficualt to deal with.
Glenn, in what way do you mean size wise, the pixel size? If so by my understanding the individual pixels are very small and the 4.63um pixel size is the "Super pixel" formed by the RGGB bayer pattern, treating each RGGB block as a single pixel of 4.63um. Apparetnly that is the reason you can bin these and still end up with a colour image, the binning is at the "Super pixel" level.
Lots of great colour and detail. Is it possible that one or two of the stars which look oval are actually a little over exposed and mask another star almost behind?
I have been finding that as the oval stars are surround by round stars.
I will have to try your colimation method
I agree that collimation of an SCT is everything. I have seem some methods on CN that photograph the airy disk and analyse with amazing results but this is too complex for me. I find the simple (straight through high power eyepiece with out of focus rings ) works at my level and can be fine tuned with a Duncan mask , a very bright star and 5mm eyepiece. Try the Duncan mask , it is an interesting use of diffraction if your eyes are good and perfect for an SCT.
Possibly, though oval stars out in the corners is more likely to be just normal SCT coma showing up. There are a few in this round of data that resolved much more nicely into obvious doubles which before the collimation tweak were just blobs.
Regards the collimation, I really have to check it with the camera fitted or the benefit would be largely undone, if it did not have visible coma it would be harder to do but given it does (Just most of it is cropped out here) the easiest way I have yet found was to do it using the direction of the coma tails to sort it out. I used CCD inspector to check it afterwards but it was actually faster to do it by eye on looping frame and focus exposures.
