I've reached my limit - U Leo

[Dave Smith]

Earlier this week I attempted to measure the variable star U Leo which is a current BAA project but found it at magnitude around 17 right on the limit of my observation. I am using my Celestron 11" edgeHD scope at F7 and an Atik 460EX CCD camera unfiltered and binned 4x4. The sky background was showing as around 32 000 ADU with a 2 minute exposure and the star hardly showing. I was advised that they were worth submitting to the database (BAA and AAVSO) and they are in a comparable region. No detail is revealed so not worth showing the plot here.  

I have seen somewhere a chart showing aperture of scope versus range of magnitudes that can be measured. Can anyone point me to this? 

Dave

[vlaiv]

I guess that what can be measured will depend on how fast it is changing? Right?

I mean if curve takes days to go thru cycle - you can do one hour measurements each night for one data point?

Have you thought of using LP filters, or any other kind of filter. 32000 background ADU is seriously high in 2 minute exposure. Since you binned 4x4 this means that per pixel ADU level is about 2000ADU in two minutes.

According to some calculation, you have mag18 - mag18.5 skies where you shoot? I think that using LP filter would lower background signal quite a bit and maybe improve SNR?

[Dave Smith]

On 08/02/2020 at 12:25, vlaiv said:

I guess that what can be measured will depend on how fast it is changing? Right?

Yes and in this case I believe there is some relatively rapid changes.

On 08/02/2020 at 12:25, vlaiv said:

I think that using LP filter would lower background signal quite a bit and maybe improve SNR?

That is a very interesting idea, thank you. When I did this sort of work with my 90mm refractor I did as standard have a LP filter in the optical train. I need to go and check my present setup, but I don't think I do this at the moment. When this storm has passed I will go and check. If not I do have  space on my filter wheel to add one. Normally I use a V  filter  but when the star is so dim that filter is not used to increase the possible signal.

Dave

[coatesg]

I think U Leo is a very ambitious project to have a go at for most observers!

I might have a go myself at some point with the 14" once the moon is out of the way, but will need to adjust the OAG for it - oh, for a parfocal V and L filter set... Even for me, it'll still need 5min exposures to get anywhere near I should think...

Edited February 12, 2020 by coatesg

[Dave Smith]

On 12/02/2020 at 13:41, coatesg said:

I think U Leo is a very ambitious project to have a go at for most observers!

I might have a go myself at some point with the 14" once the moon is out of the way, but will need to adjust the OAG for it - oh, for a parfocal V and L filter set... Even for me, it'll still need 5min exposures to get anywhere near I should think...

I'm inclined to agree with that. Even if a light pollution filter made a difference I don't consider it worth spending on one just for that reason. There are more than enough stars within my range to keep me busy.

Dave

[robin_astro]

On 06/02/2020 at 17:39, Dave Smith said:

 

I have seen somewhere a chart showing aperture of scope versus range of magnitudes that can be measured. Can anyone point me to this? 

 

A bit late to the party but this calculator can be used to estimate the SNR for a particular setup, conditions and exposure 

http://spiff.rit.edu/richmond/signal.shtml

Cheers

Robin