What I describe below is probably common knowledge, but it was new to me, so may well be of use to others who are, like myself, in their early days of EAA.
During recent downtime waiting for a clear night, I’ve been looking at raw image sets that I took a few weeks ago to see whether PEC and /or guiding would be worthwhile for my EAA sessions.
Using the folder monitor camera, I’d seen that the drift graph showed an obvious periodic error – which appeared typical for my equatorial mount (HEQ5Pro) at about 27” peak to peak and I wanted to understand how this was affecting my images.
- Drift.jpg (367.13 KiB) Viewed 377 times
The first thing I did was to run all the frames with filtering and alignment switched off, which showed just how effective SharpCap’s alignment is, as the drift due to PE is obvious. The different alignments of the drift direction in this, and the previous image, are from different targets on opposite sides of the Meridian.
- No align.jpg (966.85 KiB) Viewed 377 times
The next thing was to run without alignment again but displaying individual frames rather than the stack. This gives a good impression of how the PE changes, with some frames having nice round stars, and some have elongated ones. It’s best here to zoom in and centre a star in the display and step through the frames. If you have captured raw frames at different sub-exposure lengths, this is a good way to check what is a suitable maximum sub-exposure length. Even if some stars are slightly egg shaped, they may well be good enough for EAA.
- Round.jpg (394.17 KiB) Viewed 377 times
- Elongated.jpg (385.41 KiB) Viewed 377 times
Even with elongated stars in some frames, FWHM filtering can be used to weed out those frames that have elongated stars. It’s easy to do with the folder monitor camera – just look in the status tab as the live stack is running, then set the FWHM filter to something above the average value and rerun the stack.
What this exercise confirmed for me was that, even at 60s exposures (which is probably as much as I’d ever want to do for EAA), the star trailing was minimal, but could be tightened up a bit with FWHM filtering. And that it doesn’t look like I need guiding or PEC, though I’m still tempted to try PEC just to see if it is useful – and it won’t cost me anything.
So next EAA session, I’m going to try checking the FWHM for a few frames for each target and switch the filtering on at a suitable value before starting the proper live stack (I normally use brightness filtering in any case). PEC may have to wait until the long dark nights of Winter when I’ve got more observing time.
Geoff
