The previous imaging tests suggest that the more moonlight the more the problem with uneven background across the image. Not really vignetting but more like a light leak. Tonight the moon is down. Perhaps 3rd time’s the charm…….
Running same settings as before, only no moon and the Optolong LP-P light pollution filter is removed. Running the new flat field it’s noted that while the RGB lines are very narrow blips with filter they’re very broad without. As expected, I suppose.
36 X 8 minutes, unbinned, 200 gain, 15C cooling, 1 Hz guide rate
Of the 36 frames, 3 were eliminated for airplane streaks and a whopping 16 frames were discarded due to guiding problems, leaving just 17 good frames. So only 2 hours 16 minutes of the total 4 hour 48 minute exposure was usable.
Stacking the 17 frames and applying a 70% arc-sinH to bring out the nebula:
- 17 frame stretched 11w.jpg (225.38 KiB) Viewed 2543 times
17 X 8 minutes
But C9 is 45° above the horizon and there’s some local lighting to contend with:
- Christmas litess.jpg (49.83 KiB) Viewed 2543 times
Even though the refractor’s got a short dew shield there’s little or no light hitting the glass directly. But perhaps it’s bouncing off of the dew shield interior? It may be painted flat black but it’s not the same as flocking.
Processing with Photoshop:
- C9 8x11 ps s.jpg (540.02 KiB) Viewed 2543 times
Conclusions & lessons learned:
The light infringement area is now limited to the right side of the frame. That’d make sense if Christmas light’s coming in from the lower right and reflecting off of the left interior of the dew shield back to the right side of the lens.
I had a similar light infringement problem using the Hyperstar, and the solution was construction and use of a lower dome shutter that prevents any local lighting from shining on the scope. The camera filter drawer area is wrapped in black cloth so it’s unlikely that there’s a light leak in the optical train.
I’ve not fully tuned PHD2, and guiding is a major problem requiring attention.
Will try again tomorrow night if the forecast holds. Will chose a target to the south rather than to the NNE as tracking’s better when the scope’s not inverted and there’s no significant terrestrial lighting in that direction. Will install/use the lower shutter. And will extinguish/cover all interior dome light.
hgp 12/14/2023
Wednesday, 13 Dec 2023
The neighbor’s display lights are off tonight. The moon is down. Sky’s black and still. Transparency’s approaching Bortle 4. A typical lovely winter sky. And the seeing is excellent too. Thermometer reads 32°.
Rather than put up with last night’s guiding problems I’ll work M74 – 60° above the relatively dark southern horizon. Scope tracks more consistantly in the southern hemisphere. Haven’t had a chance to install flocking on the interior of the dew shield but the lower shutter is mounted, completely shielding the scope from any neighborhood light. Imaging was done remotely, with no lights (including computer monitor) on inside the dome.
Using the same setup as the last attempt at C9. 8 min, unbinned, -15C, 200 gain, 1 Hz guiding
Of the 24 frames taken 5 were rejected for guiding problems, leaving 2 ½ hours of exposure.
Here’s the stacked image after applying a 70% arc-sinH stretch – and nothing more
- stretched 70%.jpg (323.73 KiB) Viewed 2543 times
Studying this closer, using extreme stretching:
- stretched very far in PS.jpg (416.49 KiB) Viewed 2543 times
- M74s.jpg (513.51 KiB) Viewed 2543 times
18:26 – 21:23 hours, 13 Dec 2023
19 X 8 min, 1X, 200G, -20C, 1 Hz guiding
102mm ES triplet and unfiltered ASI 2600MC-P camera.
It cleaned up fine.
Notes, conclusions & lessons learned
The objective of tonight’s work was to determine the impact of local lighting and moonlight. I’ve tested for this before using the Hyperstar. And wow, what an improvement in background consistency.
I wonder if I’m overexposing for my sky conditions.
Lots of coma in the corners but it doesn’t show unless the image is blown up to a pretty good size.
M74 appears grainy when enlarged. Would have benefited from longer exposure.
This Messier isn’t a good object choice for a short focal length (900mm) scope but it was ideal for test purposes.
hgp