Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Using Exposure Calculator with Comet 46P
(2018-12-23, 07:11 PM)PMSchu Wrote: Hi Greg,

I'm using the Exposure Calculator to estimate exposure conditions for 46P/Wirtanen and having some issues. Perhaps some of the problems are caused by the comet's brightness - don't know. My imaging system is a camera-only with a photo lens & DSLR camera. The lens is a 100mm f/2.8 and the camera is a Canon 7D Mk2. My Observing Location is at 40° North. Here are some of the issues:

1) The Target List Night Bar(TL) for 12/23/2018 and the Night Bar in the EC don't show the same curve for the Exposure Quality(EQ) (blue line). The EC shows the blue line near the top of it's Night Bar for almost the whole night. The calculated SNR over 9 hr 56 min is 20000. The Target List Night Bar shows a higher EQ before moonrise, then a sharp drop in EQ afterwards. Is the EQ calculated by the EC the accurate one and the EQ shown on the TL only an approximation? Which one should I believe, if any?

2) If I change the EC's 'Expose for' from 'Main coma extent, tail' to 'Faint outer coma/tail', I have to make the new selection twice before the 'Exposure Opportunities' (EO) list is updated properly. The Night Bar does flash the green 'Calculating' message both times that I change the 'Expose For' buttons, but the update only occurs after the second change. The same thing happens when switching back to 'Main coma extent, tail'.

3) The 'Calculate SNR for exposure' part of the calculation is very slow and frequently reports: 'Aborted' with no further explanation. The Win10 Pro Task Manager shows that ST4i is utilizing a processor to max then switching to a different processor during this calculation. The calculation can run over a minute switching from processor to processor every few seconds and may return with the 'Aborted' message. The 'Done' button is inactive during this time, but changes can be made to any of the input parameters. Not sure if the original calculation continues or restarts with updated input values. A 'Stop Calculation' button might be a useful addition to regain user control. Due to the brightness of 46P, I reduce the 'Sub-exposure' to <=30 sec and the exposure time to 5 min or less. SNR values range from hundreds to thousands. 'Auto' exposure may be causing the long calc time. Once a calc completes, I can change 'Exposure Time' and 'Sub-exposure' and get a result instantly. The camera is set to allow exposure times from 0.1 sec to 300 sec. I have considered changing this to discrete values over the range to see if the calculation runs faster.

Have a Merry Christmas & Happy New Year!

Phil S.


Sorry for the delay in responding. With the holidays I hadn't had enough time to look into your reports in detail.

1) (REVISED) Your first question regarding the difference in IQ line on the Target Selection Tool and Exposure Calculator really had me going for a while. I thought maybe it was a bug that I  had worked on for the last update, but the explanation turns out to be a simple one. The Target Selection tool has a minimum moon separation setting. When the target object is closer than this angular distance from the moon, the IQ line goes to zero. The Exposure Calculator does not seem to be applying this setting properly in this case. I am looking into it.

2) I was able to reproduce this. Nice catch! I will have a fix in the next update.

3) I am looking into this. A comet this bright is really pushing the algorithms to beyond the tested limits. Normally, it should not take very long to do the calculation, so something is weird. Changing the shortest exposure time can sometimes speed things up a bit, but should not make all that much difference.

Clear skies,

SkyTools Developer

Messages In This Thread
RE: Using Exposure Calculator with Comet 46P - by theskyhound - 2019-01-03, 09:13 PM

Forum Jump:

Users browsing this thread: 1 Guest(s)