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There is an old discussion on calculating SNR here. Basically I ended up calculating all of the non-photon sources directly by measuring the RMS value in the sky estimation. To calculate the photon, sqrt(N), noise you need to know the camera gain. I’ve implemented this in an automated aperture photometry tool that I use and it seems reasonable to me.
Thanks. That works very well. I get a pretty good ensemble fit against Gaia G and a magnitude for SN 2021hpr of 14.21. This compares to 14.29 using my aperture photometry tool, presumably affected by the galaxy background. I think the SNR you quote is too low though. I get 168, you have 27. SNR is a difficult thing to calculate correctly. How do you do it?
Maxim – Thanks, I’m almost there. I had forgotten the “Get comparison stars” step! Now I get a fit. The only problem remaining is when I click on the object the flux/magnitude etc are all zero. What am I doing wrong?
One other question – Am I right that the ensemble fit graph is upside down, i.e. the brightest magnitudes are in the lower left corner. Not a problem but I would just like to understand.
Michael – Yes, it seems to work fine without the astrometry.net key although my files are already platesolved. Here’s an example of the V1391 Cas field with the fit using V.
Maxim – Could you add Gaia DR2 G to your list of catalogues? Also, the image contrast stretch doesn’t seem to work very well for me.
Maxim – Thanks that works fine, see attachment. I’ll have a look through your documentation later and will do some experiments on known fields.
I’ve just converted the FITS that I was using from int16 to float32 and it now works. I have an image. I’ll play around a bit more.
Yes, I get the same on Win 10. FITS data but no image. See the attached screenshot. FITS file is attached. Note that it is a gzip compressed fits but I can only attach files with an extension .fits so you may need to change that to fit.gz to get it to work.
I get the same problem trying to install 3.9.4 direct from the Python website into a fresh Win 10 20H2 instance in a VM. Trying to run pip from Powershell gives a “command not recognised” error. Installing from the MS Store worked. The rest of the install then ran smoothly although it did moan about ‘wheel’ not being installed as per the attached screenshot and has various warnings (see screenshot). I’m assuming these are benign. I’ll try out the program now.
Yes, I get 13.8 unfiltered ref Gaia DR2 G tonight. Image here.
CBET 4945 designates this nova as V1405 Cas. My image of it and M52 which is a degree to the north is here.
The astrometry I get using Gaia DR2 is 23:24:47.70 +61:11:14.8 with a very high SNR and 1.29 arcsec/pix sampling. There are four stars within 5″ in Gaia DR3. The closest to that position is 15.36G offset by 0″.47 in RA, 0″.04 in Dec.
My latest observation was on March 9.8 when it was 15.39 (ref Gaia G). My data is in the VSS database.
The nova has been steadily brightening for a few weeks and tonight I get it at 17.42. At this rate it will be brighter than the close star just to the north in a few days. An interesting object to follow just as Cassiopeia is getting low in the north. How bright will it get?
Yes, the first comet photos, by Underwood and Bond, were of C/1858 L1 (Donati) in 1858. With the help of Mike Maunder we estimated that these would have been taken using Collodion plates that had a sensitivity of around 0.05 to 0.1 ISO . By the time of Lockyer’s attempts in 1874 he would have been using gelatine plates with a sensitivity of maybe 0.2 ISO. No wonder it was so hard to photograph a comet but even then people knew to use short, fast lenses. Did Lockyer try any photography using a portrait lens? Eight years later, in 1882, Gill got his famous photo of C/1882 R1 which led to the Cart du Ciel project.
