Use of experimental photometry tool to estimate magnitude of SN 2023 ixf

[DrBobAZ]

Greetings.

Wanted to share my attempt at estimating the magnitude of SN 2023 ixf using the sharpcap tool.

My workflow is as follows:

Gather data (ADU and background ADU) for the SN and a comparison star. (See the two graphics for SN and HD122865 (mag 8.13) )

The ADU (5 sec sub) are as follows:

SN 114865, background 1927

HD122865 1128159 background 2441

So first I subtract the background from each, then take the ratio of the resulting subtraction.

In this case, it turns out to be nearly 10 (serendipitously).

Then I use the formula log10(10)= n log10(2.5) where the 2.5 is the brightness multiplier for every magnitude unit, i.e. mag 4 is 2.5 times brighter than mag 5.

So 1 = 0.398 n, n=2.512, so compared to the magnitude 8.13 HD122865, it is 8.13+2.512 = 10.64

So as of 5/23, about 03:00 UT, the magnitude of SN 2023 ixf has brightened to 10.64. I think yesterday, the accepted magnitude was about 11.1.

Does this calculation make any sense? The number seems to be about what one would expect, but am I using the output from the tool correctly?

Any feedback/critique would be appreciated.

Bob

[admin]

Hi Bob,

What a nice use of the photometry feature!

I have just checked the code and I don't think you need to subtract the background value. The 'Total ADU' is the total pixel value of the star above the background level. Mind you, it doesn't change things very much.

The other thing I can see is that the maximum pixel value for the referenc star is pretty close to saturated (61809 for the star plus 2441 background - 64250). It might be better to adjust the settings to bring that value away from being close to saturation (lower exposure or throw the focus out a little to expand the star's disc). If you are getting slightly lower than expected readings on the reference star due to being close to saturated then that will tend to make the supernova work out brighter than it really is.

I think I agree with your mathematical handling of the magnitude difference.

cheers,

Robin

[DrBobAZ]

Thanks Robin for your comments and kudos. Much appreciated. Good point on the saturation, I hadn't thought of that. Yes, I will retry with a bit less gain and/or exposure.

[DrBobAZ]

The other thing I can see is that the maximum pixel value for the referenc star is pretty close to saturated (61809 for the star plus 2441 background - 64250). It might be better to adjust the settings to bring that value away from being close to saturation (lower exposure or throw the focus out a little to expand the star's disc). If you are getting slightly lower than expected readings on the reference star due to being close to saturated then that will tend to make the supernova work out brighter than it really is.

I have an update, and for reasons unknown, it seems to agree well with the magnitude of the SN at the time I did the observation. I was told that since my setup could not extract the visual magnitude (v-mag) through filters and the like, it should be too bright since it includes whatever wavelengths the asi294 can record.

Based on your suggestion, and what I got over on cloudy nights, I picked a closer, dimmer star, mag 11.88.

Reference images are below.

Both are near each other, and neither region is close to pixel saturation, and dimmer comparative star.

Star ADU 36189

SN ADU 106720

log(106720/36189) = n log(2.5)

n=1.18

So the i-magnitude of the SN at this time was 11.88-1.18 = 10.7

Similar to the brightness yesterday. However, I had some issues with that calculation not the least of which was the v-mag v.s. i-mag distinction. Likely I was brighter than the correct number because of that

I corrected a couple of those shortcomings by picking a star closer to the SN, and dimmer to avoid pixel saturation.

So is the 10.7 for 5/25/23 5:00 UT reasonable?

Well for measurements 5/24, the latest magnitude is 10.8 per this Purdue reference:

https://www.rochesterastronomy.org/snimages/

So this agreement I believe is likely coincidental, unless performing the comparative magnitude with a star of a given v-mag (the 11.88 is the v-mag AFAIK) yields the v-mag of the SN, even though the ADUs measured are uncorrected instrument values.

[admin]

Hi,

I'm not really an expert on this, but if the spectrum of the comparison star is relatively similar to that of the supernova then the errors introduced by measuring a wider band of wavelengths would be roughly the same (as a percentage) for both, so would tend to cancel out in the final result.

Whether or not it's coincidence or cancelling errors, it's a great fun to be able to see amatuer science like this being done with SharpCap - thanks for sharing!

cheers,

Robin

[DrBobAZ]

"would be roughly the same (as a percentage) for both, so would tend to cancel out in the final result"

Nor am I an expert. (fun, isn't it?!)

Anyway, I think you might be right. We are taking the ratio of 2 ADUs and putting it in a log function, so the dependence is weak of any spectrum correction in the ADUs. We then correct from the reference star which is listed as a v-mag, so there is no error there.

I am just reluctant to make any bold claims, because many serious amateur photometrists (is that a word?) spend a ton on gear and processing software, and are proud of the kit and process.

I did it here with nothing special, net cost of ZERO from what I normally do with EAA.

Thanks to you for creating such a cool tool.

Bob