American Association of Variable Star Observers (AAVSO)
Sun, 09/27/2020 - 14:27

Hi All,

I am looking for some feedback on a situation I encountered with my system.  I have been using ACP to control my backyard observatory and I have had "auto flip images when GEM points west" in my settings.  Recently, I discovered that when performing lon time series collections for EBs and exoplanets, I am seeing a change in magnitude between eastern viewing and western in VPhot time series analyses as well as photometric processing in MaxIm.  I am wondering if anyone has encountered this, or if anyone has some insights as to whether this is likely a camera issue, systemics, or just misapplication of my calibrations?  I'm posting here as I am leaning toward an issue with my calibration frames. I welcome all inputs. Thanks, Chuck


PS- Looks like I'm unable to embed a screen shot and not able to attach it either. The screen shot shows that my SNRs for target, check and comp stars were consistent before and after the meridian flip.  I saw anywhere from 0.02 to 0.06 mag shifts in the target, comps and check stars after the flip (some stars birghter and others dimmer).  The mag is very much a straight line before and after, just a noticeable shift. (Note: I was collecting out of transit observations for a EB to get a baseline before starting to collect eclipses).


American Association of Variable Star Observers (AAVSO)
anomalous magnitude shifts

Hi, Charles:  There are two things that can create what you're seeing (maybe more...). 

   Suspect #1 is that your flat frames are imperfect by a few-percent.  When the mount flips, the comps move to differnt parts of the chip, which of course have different dust donuts and vignetting, etc. Getting "perfect" flats -- better than a couple of percent -- is hard, and convincing yourself that you've succeeded is even harder.  I think that it is pretty common to accept a few percent photometric shift when you flip, and adjust for it during analysis (e.g. for eclipse timing). 

Suspect #2 is image aberrations (coma, etc) combined with a too-small measuring aperture.  If the shape/size of the point-spread-function is different at different parts of the image, then you have to e sure that the measuring aperture is large enough to capture all of the light from the largets/mis-shapen star image.  I think that this is most common if the field of view is large, or telescope collimation is badly off.

   I'm not familiar with ACP's "flip image" routine.  It might be worthwhile to confirm (or test) whether you do (or don't) need to flip your flats to match the "flipped" sceince image.


Bob B.

American Association of Variable Star Observers (AAVSO)
Prefer late image flip, if any

Agree with Bob. When I had a German Equatorial run by ACP (300-500 images/night), I never let ACP flip the images, precisely because I wanted to be 100% certain that my downstream calibrations were uniform and absolutely correct in orientation.

For many analysis workflows (as for mine), flipping to get uniform image orientation just isn't needed. And even if one's workflow does require it, it's better to trust the plate solutions (rotation angle is your best pier-side indicator) and flip after calibration than to trust what the mount and ACP *think* the pier side was. Either way, I see no advantage to letting ACP flip the images based on mount information, perhaps aside from convenience in visual inspection.

Then if pier-side bias persists, at least you'll know it's not imperfect flats etc, and you'll favor physical causes and solutions (loose dew shield, mirror flop, loose filter, etc).


American Association of Variable Star Observers (AAVSO)
If you are using an SCT

If you are using an SCT "mirror  flop" or movement of the mirror as you execute the meridian flip is probably the culprit.  It happens to almost all of them.

Variable Stars South (VSS)
If I understand the stated

If I understand the stated problem correctly, you are seeing magnitude shifts after meridian flips with an equatorial mount. This is a well known problem.

My solution to this (suggested to me by a colleague) is to rotate the camera 180 degrees at the time of the meridian flip. When I do this the magnitude shift simply disappears on almost all occasions.