[Craig Towell]

Thanks for the kind words Paul and David. I must say it is intensely satisfying seeing the data points plot out for the first time. Addictive too. Not great for the wallet though as I’m already eyeing up larger scopes and a monochrome camera!

Craig

[Craig Towell]

Well I think I just about managed it… some scudding clouds led to data gaps and cut short the post egress ‘tail’. The data is not pretty but it shows what its supposed to (just).

Interestingly I measured ingress a bit later than predicted. Will need to check those timings.

Thanks for all your help!

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[Craig Towell]

Thanks Paul, that sounds like good advice. I am using a doublet refractor though, and whilst the colour correction is good, it’s not perfect. Would that make any difference to the decision to go with an IR/UV cut filter over a clear filter.

Just thinking the stars with a clear filter will probably be quite bloated. But I guess that won’t matter much for photometry?

[Craig Towell]

I tried to install and use APT but just could not get it to work. I did manage to get AstroimageJ up and running which has allowed me to measure SNR on my images, so I did a little bit of testing on a previously captured image.

With my 102 frac, colour camera and Lum filter, I took a 2 min exposure of a Starfield near Polaris.

Looking at that image in AstroimageJ I can see stars at mag 12.10 have a peak value of 35,520 and an SNR of 550. Very usable I hope.

Edit: Interestingly the SNR calculator I linked to earlier predicted an SNR of 595 for that star and capture parameters. Not a million miles off.

• This reply was modified 3 weeks ago by Craig Towell.

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[Craig Towell]

Right so I was woken at 3am by the youngest sprog and couldn’t get back to sleep, so I did a load of internet searching and reading around SNR, AstroImageJ and aperture photometry (totally normal behaviour, right?).

Stumbled across this useful looking calculator…
https://mirametrics.com/sn_calculator_mvn.php

Plugging in the pertinent data, or estimates of, and it thinks using a 102mm refractor operating with 2 minute exposures, and assuming my minimum acceptable SNR to be 500, then it looks like I should be aiming for stars of at least mag 12.5 with a 2% transit depth.

Edit: Plugging in numbers for my 300mm/1500mm Newtonian says it should be good down to mag 14 ish with a 2% depth.

• This reply was modified 4 weeks ago by Craig Towell.
• This reply was modified 4 weeks ago by Craig Towell.

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[Craig Towell]

Thanks Mark, I hadn’t considered nearby stars to the target star so that is a very useful tip for me.

I’m having a little trouble with establishing what the SNR is on any given star. Tring to do it in Pixinsight, and to be honest as there is so much conflicting information online I’ve no real idea what I’m doing!

I took a calibrated 2 min sub, debayered and extracted the green channel (OSC camera).

The I created a preview box right around a star, and another on a patch of background sky.

Then I used the e statistics function to look at the mean ADU value of the star (3420) then divided that by the stddev of the background (57) to get 60. So is that a SNR of 60? Pretty flow for a star close to saturation!

I have attached a pic of the preview boxes I used, coupled with a .Fit file of the green channel I was inspecting.

Any help greatly appreciated!

Edit: seems like the .fit is too large

• This reply was modified 4 weeks, 1 day ago by Craig Towell.

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[Craig Towell]

Thanks Paul that is very helpful. I had a short clear spell when I got home from work and managed to capture a series of images of varying durations of a star field.

I’m just looking at the images now and identifying the stars in the image and their magnitudes.

What do you use to measure the SNR of a particular star? I have pixinsight and I can see the ADU values of the centroid and the background, is it simply a case of taking the ratio of those two values?

Thanks again
Craig

[Craig Towell]

Thanks for the response Paul, that is very helpful.

I also have a 305mm/1500mm Newtonian I could use, however the FoV would be tiny (0.28 deg x 0.16 deg), not sure if that would be enough to get any comparison stars in shot.

[Craig Towell]

Thanks Ray and Bill, that’s very informative. I will have to have a look at that 3D model function. Quick map is such a great resource. 

Craig 

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