Crescent Nebula (NGC6888)

This session did not go that well, but I have continued to uncover things that can be fixed. I am confident that with more attention being paid to my work plan I can have a better result next trip to the Farm.


I had a pretty rough night last night. I got a started with setting up later than usual, and while waiting for the sky to get dark enough to polar align, I wanted to work with the Flats Wizard to see if I could reduce the time needed to capture Flats and Dark Flats for all four LRGB filters. For those reasons, I rushed things a little bit and never got my eyes on the work plan, so I made some mistakes. I also had issues related to items on my to-do list, which includes some items, like another Dec backlash adjustment, that am purposefully deferring until after this new moon cycle.

Start Up:

The NP101 was already on the G11. At 2000 I went out to add the ASI6200 and connect power and data cables. The temperature was 77F and the dew point was 68F. The forecast showed the temp and dew point starting to run parallel at around midnight, so I was expecting dew. Astrospheric forecasted below average to average seeing and average transparency until clouds would roll in at about 0400.

I completed camera installation by 2016 and began working on how to reduce the time required for Flats capture. With two sheets of card stock between the panel and telescope, and some real-time panel adjustments and wizard parameter adjustments, I was able to keep all exposures between 3s and 9s.

As expected, I had difficulty getting a good RDT connection, but finally a combination of Ethernet and the nano router seemed to do it. I had no unexpected drops through the night and periodic reconnections after closing the laptop to conserve power happened quickly.

The work with the Flats Wizard was completed and I was able to begin polar alignment at 2120, which was only five minutes later than the earliest that I could otherwise have seen Polaris. I accepted my first alignment run, which needed just the slightest adjustment in AZ. This tight alignment explains the absence of drift that I observed while imaging the Sun yesterday morning.

By 2130 I was working on initial focus and moved on to PHD calibration at 2148. Orthogonality was pretty good. Dec leg was a little wobbly due to backlash. An initial guiding check at 2155 resulted in RMS errors of Ra/.80”, Dec/.56”, and Total/.97”. The Guiding Assistant run showed a polar alignment error of 6.1”.

I slewed to NGC6888 at 2213, just a few minutes before end of astronomical twilight and would have been ready to begin, so I was back on track. The target was accessible through a gap in the trees, but I elected to wait for it to ascend to 40 degrees. While waiting, I checked the guiding RMS error and found Ra/.69”, Dec/.36”, and Tot/.77”.

NGC6888 was a rising target, so I ran the filters in reverse order. Test exposure filter/duration (seconds)/histogram separation (ADU) were: R/180s/1186, G/180s/1156, B/180s/1342 and L/45s/1704. Autofocus runs were set to occur on filter changes. At 2213 I was waiting for the target to ascend above 40 degrees elevation. Since end of twilight was 2238 and I began imaging at 2255, so the wait for NGC6888 to reach 40d was only 17 minutes.

Lights Capture

I initiated the imaging sequence at 2254 as NGC6888 ascended through 40 degrees. Auto slew, auto centering worked perfectly. I remembered to leave the L filter in front of the camera, but I should have left the R filter in front of the camera, because that was the first filter in the sequence. As a result of this oversight, the first autofocus run was with the L filter that I had left in front of the camera, and because the filter changed to R at the beginning of the sequence, another autofocus was initiated for the B filter.  Guiding RMS was Ra/Dec/Tot: .69”, .36”, .77”.

0015 Checkup: Guiding FWHM had increased from about 3 to 6+. There is no option to pause the sequence, or I would have paused and refocused. Since SNR was a very strong 200+, I elected to let it continue to run. Dew was starting to form on metal surfaces. I set an alarm for a 0215 checkup and fell asleep. I turned the alarm off and fell back asleep until 0440.

Guiding appears to have ended at 0042 for reasons that I cannot discern from the log file. Tracking stopped due to filter wheel contact with the pier. The counterweight bar was roughly horizontal, so this would have occurred around 0250 when the target would have been transiting the meridian.

Lights capture ended at 0350. The temperature was 69F and the dew point was 65F. There was no dew on metal surfaces when I went out to start Flats.

Flats Capture

Flats began at 0452 and finished at 0600. 30 Flats and 50 Dark Flats were captured. Capture durations ranged from 3s to 8s. Overall Flats capture time was more reasonable.

Wrap up

Following my work plan would have helped this session.

I found some limitations using NINA to get the scope pointed exactly where I wanted it, so I settled on NGC6888, which is a searchable object. In retrospect, I could have slewed to and centered on an object like NGC6888, synchronized the mount, and then slew to my desired location using Stellarium.

I will spend a little more time working on RDT with the current nano router. I’ll see if I can figure out how to do fixed IP addresses.

The coincidence of G to B filter change and guiding stopping at about 0042, it seems possible that the guide star FWHM might have gotten worse with the autofocus that happened at the filter change.

The large initial PHD FWHM number might be due to the difference between the Bahtinov mask focus position, and the focus position after the first autofocus run. An autofocus run after main camera initial focus but before guide camera initial focus might correct this.

I gained some additional experience with the Flats Wizard that will help me keep subs down to about 5s.

Glancing at the filenames, apparently I forgot to change the filter field from B to L after copying it in the sequencer. I had the 20 x 90s R subs, 20 x 90s G subs, 20 x 90s B subs and I had 120 x 45s B subs that should have been L subs. I have already thrown away the 120 x 45 B subs. Guiding stopped at around the last G sub, so I’ll probably need to throw away the 20 B subs. I’ll need to move some old captures onto the external drive so before I can further analyze what happened last night and decide whether to keep the R and G subs.

To-do List

  • Continue to work on RDT
  • Add step to work plan after initial main camera focus to do an autofocus run before focusing the guide camera.
  • Correct NINA file path
  • Process images
  • Add note to workplan to ensure that filter wheel will be turned away from the pier.
NGC6888 (Crescent Nebula) - 2022-06-25
Crescent Nebula

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