I think this is M22 in Sag? Taken with the Canon 40D at ISO400, it is a single, 600 second exposure through the Mewlon 180 F12 with x0.8 Flattener/Reducer at an effective fl of 1728mm, at F9.6.
There are a few things that are ‘wrong” with this image; amp glow in the top left hand corner, a little noisy due to it being just a single frame, some coma at the edges despite the x0.8FR and evidence of star elongation (flexure?), although this is less noticeable after the full 3888x2592 frame has been re-sampled down to 1024x680.
Usually, when using the ST7, CCDSoft and The Sky 6 Pro, the target details are written to the header of the FITs file but when using the Canon 40D, I’ll have to start writing details down about each target, so pardon my uncertainty.
Auto guided using a WO 66mm F6 Petzval and Orion Star Shooter (old model).
Not bad for a single exposure Dennis, maybe shoter multiple exposures would cure some of those woes
Cheers
Hi Trevor
Thanks, I appreciate your comments and suggestions. The single 600 second image was taken as part of my ongoing investigation into improving my side-by-side imaging capabilities at focal lengths of over 1500mm, something which I have really struggled with.
I am pleased to report that I have made inroads into the problems, mainly by mounting the guide scope nearer to the imaging scope and by supporting the focuser of the guide scope in a guide ring. Whilst these two most recent changes have not eliminated star elongation, they have certainly contained the magnitude of it so I can now concentrate on other contributing factors.
Having to set up each night and suffering long, long periods between experimentation has really hindered my progress, but it is nice to see some real improvement at last!
Sounds like I'm going to need to engage you in some consultancy efforts at Astrofest Dennis. How do you go adjusting the guide scope position if the focuser is in a guide ring?
The image looks pretty good to me despite the issues you mention. It also looks like there is some sort of gradient (darker at the top than the bottom) - is this sky glow related?
Kudos for not picking a usual target Dennis. I like what you've put together. The issues you've noted aren't major and are addressable. Its the problems that are difficult to find or reproduce cause the most frustration. You've managed the star colours well. Subtle blue cast over the image perhaps, but it wouldn't be suppressing the hues of K and M spectral type stars by much. Thanks for sharing. Well done.
That's a nice globular cluster Dennis, you have certainly bought out a lot of detail for a single exposure. Would love to see it again when you get your set up fine tuned
Thank you for all you comments and helpful suggestions, they are much appreciated. The M22 image was a means to an end, in my search for the holy grail of round stars at longer focal lengths, so at this stage, I’m not too concerned about its shortcomings.
The M180 F12 is a Dall-Kirkham Cassegrain and one of the characteristics of this design is that it produces off axis coma. I think that it is generally accepted that D-K’s are best for Lunar, Planetary and double star work. Even with the x0.8 Reducer/Flattener, it remains a painfully slow F9.6 system. With my ST7 chip at 6.9x4.5mm (765x512 pixels) off axis coma is not a problem; it only becomes noticeable with the much larger 22x15mm chip (3888x2592) of the 40D.
Mating the 40D to the Mewlon has certainly provided me with a real insight into the astonishing difference in Quantum Efficiency between my SBIG ST7 and the non-modified Canon 40D, especially on faint objects. The ST7 produces a very recognisable image of the M16 pillars in less than 10 seconds whereas the 40D struggles even at exposures of 180 seconds, a huge difference!
My experiments have also given me a much greater appreciation and insight into the quality of the recent work posted by Barb and Dave using their C9.25 at F6.3 and standard Canon 40D. I have a long way to go!
I am happy to be making progress and each challenge that I overcome, leads to another one to grapple with. I can see that gradients and stellar profiles are fast working their way up the list! One day, I might even post some images I am happy with!
Hey Dennis, I reckon you have a gradient rather than amp glow. On screen there is a dark band nearly all the way across the top 1/8 of the image. There is no amp glow in the image and I think the gradient is giving you the impression of amp glow.
Keep going though, I am sure you will conquer the issues.
How do you go adjusting the guide scope position if the focuser is in a guide ring?
Hi Peter
Here is a photo showing how I slid the WO 66mm Petzval further up the assembly, so that the rotating focuser tube was now clamped by one of the guide scope rings.
Previously, that rear ring was located on the main OTA body. I think that this change in position helped reduce flexure, although I find that the long time between tests makes it difficult to quantify these changes with any certainty
Another improvement has been to replace the approx. 3inches of extension tube with a star diagonal so the guide camera overhangs by less of a cantilever.
The cable clips ensure that cables do not transfer any strain to the guide camera when the mount is operating.
What I don’t yet know, is, will all my improvements be repeatable during my next session, when I have to assemble and set up everything once more? I hope that I will have succeeded in making these tiny changes become permanent.
Having to set up each night and suffering long, long periods between experimentation has really hindered my progress, but it is nice to see some real improvement at last!
I know how you feel Dennis, the setup procedures every night can become tedious and somewhat counter productive in the end. (Not to mention 2 and a bit something hrs out of every session.) Quite palatable at the beginning, however your learning seems to plateau off after a period ... your equipment setup is only a fraction of what it could be etc .. you know your mounts accuracy and stuff like that, because every session you have to rehash stuff again just to get back to the same point you were at last time .. The weather is making sure its a long time between sessions up here this year too .. Im waiting now until I can get my observatory together, hopefully have some worthy stuff to display here. Anyway ... Good Luck
Nice image Dennis. Yes I can see some of the problems you state. Experimentation and thinking outside the square seem to be the way to go.
Flexure seems to be the PC phrase of late but I think you may need to look a little further and look at primary items such as focuser sag etc. to come to a resolution. The aberation at the top could well be repaired with Darks and flats and maybe even checking sag and camera alignment. Some of the field curvature may be fixed by adjusting the distance between the FR/Flattener and camera.
Nice single image but to achieve less noise and better clarity (S/N ratio) you need more exposures. Nothing you do will make such a massive improvement.
I know how you feel Dennis, the setup procedures every night can become tedious and somewhat counter productive in the end. (Not to mention 2 and a bit something hrs out of every session.). Anyway ... Good Luck
Thanks Jeff!
The biggest restriction of setting up and tearing down each night, at least for me, is that I no longer pull a one night stand. If there is a clear spell over 2 or 3 nights, I then set up and leave the observing hutch and tripod out overnight.
Good luck with the observatory!
Quote:
Originally Posted by Hagar
..but I think you may need to look a little further and look at primary items such as focuser sag etc. to come to a resolution. The aberation at the top could well be repaired with Darks and flats and maybe even checking sag and camera alignment.
Nice image and good luck.
Thanks Doug!
Some very useful advice there - thanks. I have a Moonlite focuser on the Mewlon so I have never even considered looking there, so I might just give that one a once over…just in case!
