Imaging variable stars , ,

[Malpi12]

, , or should the title be Photometry of variable , ,

My setup : a camera with lens on a  fixed tripod (pan & tilt, no tracking)

Should I use (A) 135mm (telephoto-) lens to get "close in"
 or (B) a shorter lens, perhaps a 28mm wide-field ?

I have been doing A, but that allows only short exp. of 1 or 2 sec  to avoid too much trailing.

Now, over a cup of coffee I got to thinking (! an unusual thing I know  )
With A  all the precious photons get spread over many pixels
whereas B they get concentrated on fewer ?
135 and 28 have f=2.8 or a 50mm f=1.8 , all are vintage (1960s) M42 with poor to horrid star shapes !  Camera is a Canon 60d (lots of small pixels ??)

[andrew s]

@JeremyS should be able to point you in the right direction.  Regards Andrew 

[JeremyS]

3 hours ago, Malpi12 said:

, , or should the title be Photometry of variable , ,

My setup : a camera with lens on a  fixed tripod (pan & tilt, no tracking)

Should I use (A) 135mm (telephoto-) lens to get "close in"
 or (B) a shorter lens, perhaps a 28mm wide-field ?

I have been doing A, but that allows only short exp. of 1 or 2 sec  to avoid too much trailing.

Now, over a cup of coffee I got to thinking (! an unusual thing I know  )
With A  all the precious photons get spread over many pixels
whereas B they get concentrated on fewer ?
135 and 28 have f=2.8 or a 50mm f=1.8 , all are vintage (1960s) M42 with poor to horrid star shapes !  Camera is a Canon 60d (lots of small pixels ??)

Probably the longer lens. You need to balance having a sufficiently wide field to contain comparison stars with have exposures short enough to avoid too much trailing (but that will limit how faint you can get. 
There are some examples here: https://britastro.org/wp-content/plugins/baa-frontend-tweaks/baa-check-file.php?filename=2018/10/DL_DSLR-London-18.pptx

BTW, having the star image spread over a few pixels is fine - in fact it is desirable. But you don’t want too much trailing 

Edited September 9, 2022 by JeremyS

[Malpi12]

Thanks @andrew s  and @JeremyS 

A very interesting PPT presentation, lots to think about.
Yes, the faint limit was beginning to be my worry, magnitude 9 or thereabouts was starting to be a problem in my (early) experiments with U Sge.
I'll post my results so far with that one in a bit !
 

[Malpi12]

4 hours ago, Malpi12 said:

I'll post my results so far with that one in a bit !
 

A bit later, and the poor S/N is evident down in the minimum

described in  https://stargazerslounge.com/topic/398978-u-sge-  3rd post down

Edited September 10, 2022 by Malpi12

[Malpi12]

17 hours ago, JeremyS said:

 examples here: https://britastro.org/wp-content/plugins/baa-frontend-tweaks/baa-check-file.php?filename=2018/10/DL_DSLR-London-18.pptx

Slide 5 is the answer !
" using   5second exposures
lOOmm lens (58mm aperture) down to 8 magnitude
200mm lens (72mm aperture) down to 10 magnitude
"
The clue is "aperture". yup! that makes sense 

From Wiki  n=f/d for things far away

or in my case  d=f/n
d=28/2.8=10mm
d=50/1.8=28
d=135/2.8=48

So yes, as you said, I should stay with the 135.

Thanks.