Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 02/20/2015 - 20:25
I use calibrated iTelescope files for producing images. If I stack several calibrated images with VPHOT, what additional processing does VPHOT do and/or what additional processing would I need to do to produce final images (i.e., alignment, registration, stretching etc.)? If I need to do some additional processing not done by VPHOT, does anybody have suggestions which software packages would be best for this?
Dave:
I regularly take replicate images with itelescopes. They are uploaded directly to my vphot account. I always stack them with vphot. They are aligned properly. No additional processing is necessary.
Try it with your images. There is nothing to loose doing this. I do assume that your replicates were taken with identical binning so their is no obvious scale difference? They will show WCS and Cal green dots in vphot. The stacked image will be in bold with correct mean time as time of observation.
Is that what you have tried? Are you trying to do photometry or something else?
Ken.
Hi Ken,
I thought VPHOT did it all but just wanted to be sure. I'm not doing photometry in this case but rather taking multiple images of some faint nebula that require a fairly long exposure time. I'm limiting each image to 300 seconds as recommended by iTelescope, so I need to stack. I've already played with VPHOT with these images and everything looks good but I just wanted to be sure that I wasn't missing something.
Thanks.
Dave
Dave:
Now I know some more! A critical step in aesthetic imaging is to align and register images extremely well (sub-pixel) to avoid softening the final image. You may also bin colors differently than luminance and need to address that difference.Your needs are likely more critical than with aperture photometry where it is even acceptable to slightly defocus to avoid saturation issues. The centroid algorithm in VPhot works well but I cannot attest that it is as good as centroiding in some other software.
So if VPhot is good enough for your aesthetic images, then go for it. If not there are sophisticated options including pixinsight at the high end. I suspect you have already looked at many of them?
Ken
Ken,
Yes this is what I was wondering about. I've been looking at other image processing software in addition to VPHOT (Fits Liberator, Deep Sky Stacker, Astrometrica). They all seem to have most if not all of the capability I'm looking for, but I'm familiar with VPHOT and prefer using it if it does the job (and it seems to). I've talked to another astronomer that swears by PixInsight but has indicated that it has a pretty steep learning curve.
PixInsight has a 45 day free trial that I'll probably play with but I wanted to be sure as I can that VPHOT will do the job.
Thanks again for all you help on this.
Dave
I believe VPhot uses a different method for stacking than do most other image processing programs since the requirements for photometry are not quite the same as for pretty pictures. I think VPhot aligns at the whole pixel level while most AP software aligns to a sub pixel level.
If I'm right about this images aligned with VPhot may not look quite as sharp as those aligned by software designed for esthetics rather than photometry.
Phil
Thanks Phil. I'm not looking for "pretty pictures" but rather for imaging faint nebula for another pro-am project. These nebula require a fair amount of time-on-target, hence the need for stacking. I think pixel-level alignment should be sufficient. So it does look like VPHOT will do the job. Thanks for your input.
Dave