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Yes, stacking would counter act that, but isn’t something that I generally do with photometry. Though having said that I have done it on some DSLR photometry to improve the SNR in the past.
But for a given magnitude, to avoid saturation you need a short exposure, therefore fewer photons and a so a lower SNR.
I’ve always found that increased gain leads to a larger scatter in the photometry readings and larger errors reported by the photometric software. The error (sigma) increases in greater proportion as the signal (photons) decreases (sigma being the square root of the signal) and this is not counteracted by the reduction in read noise. At least that’s what I suspect, I’m no mathematician though.
I use the ZWO ASI1600MM for variable star photometry, you need to keep the gain low as increasing it will reduce the signal to noise ratio. I mainly use a gain of 0 and at most 80, you basically need to capture as many photons as possible without saturating the sensor. Increasing the gain does not increase the number of photons captured, it reduces them, that in turn means that the SNR is less.
Another place you can submit minimum to is http://www.as.up.krakow.pl/add-minima/
That’s where mine go.James
I’ve had similar steps in time series photometry with a newtonian at meridian flip. I think they were caused by the main mirror moving in it’s cell as the weight shifted.
About 23:00 BST last night (11/07/2020).
Comet C/2020 F3 NEOWISE, Canon 7D Tamron 200-600 at 200mm, f5, ISO800 on a fixed tripod.
I’ve used both a CCD (Atik414ex) and CMOS (ASI1600MM) with equal success. The Atik is a little more sensitive due to it’s larger pixels but the quality of the photometry was similar.
I’ve never come across Russ Laher’s Aperture Photometry Tool, I use Muniwin, although it can take a bit of experimentation to get some of the parameters correct. You could simply use larger apertures to overcome poor tracking if there are no other stars very near the variable or comparison stars.
I’ve never used anything like 10 minute exposures, the longest I’ve used is 1 min, most of the time I use 10-20sec. 10sec will get me down to around mag 11 with errors about 0.03. I would only accept an error of 0.1 for secondary variables in the same field. Some fields I observe I can get photometry of 4 stars in the same field.
As for DSLR with my setup, a modified 500D with a 200mm f2.8 lens (about 70mm) I get good photometry to mag 8.3 with ISO400 and 20sec green channel only, unguided of an alt-az mount. I use a plan in APT that re-centres targets (with Goto++) about every 1 to 1.5 hours, as it does drift off centre without guiding on a low quality mount.
Extrapolating isn’t easy as the magnitude will be a function of the camera pixel size, lens/telescope focal length and aperture.
I’ve been using an ED70 scope with a finder guider for years, with both CCD and CMOS cameras. There is certainly no need for a large scope there are a lot of stars that can be followed by small portable systems.
James
Hi Andy,
I’m using Chrome on Windows 10. I cannot get onto the main VSS page, if I enter britastro.org/vss as I’ve always done chrome is removing the slash between .org and vss and so it tries to access the wrong domain.
James
It’s still not working for me. Though I have noticed that the photometry database is much faster, instead of take minutes to upload and commit observations it now takes 1-2 seconds!
James
There are still issues with the vss web site. Refresh doesn’t help, I keep getting re-directed to //britastro.orgvss/ when I enter //britastro.org/vss the slash after .org keeps moving.
James
I have used AIP4WIN, but have found it unstable (it would crash if processing more than a few hundred images at a time), and the attitude of it’s developers is that they weren’t interested in fixing the problems. They considered the BAA VSS spreadsheet workflow used an obsolete part of AIP4WIN when I contacted them about it. If I were you I would stick with something else, AstroImageJ, CMuniPack or I understand ASTAP does photometry though I’ve heard no report of how good it is.
James
Hi David,
I use C-MUNIPACK (http://c-munipack.sourceforge.net/) for DSLR (and CCD) photometry. The help isn’t very good but it will output to the BAA VSS format. If you process a lot of files (1000+) at once, for example doing a time series of a eclipsing binary minimum, be prepared for it to take a while. On my old laptop it can take a hours, only to find that I need to tweek a setting and start again.
James
I’ve been using a CMOS camera for a few years (ASI1600MM-C) without a problem. Most (if not all) DSLR cameras use CMOS sensors, so if they work a dedicated cooled astro CMOS camera is fine.
James
Hi Andy,
This should give access to my apps.
https://www.dropbox.com/sh/skt7dd0lxwdb447/AADyI-mXUbpsilxEtOr31mGia?dl=0
James
