Number-crunching my Pluto pic

[Demonperformer]

I have been doing some number-crunching on my pluto photo (http://stargazerslounge.com/topic/248887-far-too-close-to-an-old-horizon/). This is pretty much my first attempt to add something resembling reality to my previous (theoretical) post: http://stargazerslounge.com/topic/235843-help-with-worked-math-example/. The photo was taken on the evening 23/24 July 2015, at an altitude of about 18 ½ degrees.

SQM readings:
The zenith reading was 18.78, but I also took a reading along the line-of-sight of the scope [a.k.a. "streetlight alley"], which gave me a figure of 16.36. Despite the instructions that the zenith figure is the "correct" one, I have used this latter figure in the calculations because it seems to stand to reason that I can expect to see less-faint stars against the street-lit horizon than against the darker zenith.

Photos attached and magnitudes:
(1) The original pic, with a circle drawn around the faintest star I have been able to track down on the image.
(2) The top left hand part of the photo, enlarged, with some magnitudes marked.

The bright star in the top left of each photo is ucac4-347-164615 (mag 9.41).

My faintest star is UNA 0675-30870554 (Vmag 16.40), which is clearly just a very faint dot on the image, but it becomes clearer on the enlarged version, and I maintain that I can just about make it out on the original. The 15.57 star (ucac4-347-164500) is a much more distinct object and I am going to classify this as my limiting "useful" magnitude for the photo.

FWHM:
I'm still not sure what it is for my set-up, so I have continued to use the 4" quoted in the article. In some respects, I guess this is not particularly a problem, because my analysis is based on a constant setup (and, therefore, presumably constant FWHM).

Equipment:
200mm sct, with SXV-H9 camera, 0.5x reducer, binned 2*2, 20 second exposures.

Looking at the 16.4 mag star, this gives a PSNR figure of 6.57, more than twice the "limiting" value of 3, quoted in the article. The 15.57 star has a value of over twice that at 13.74. Clearly, this theoretical figure of 3 is a bit hopeful for my setup. This could in part be due to stars not being central on the pixels (as discussed in previous thread) and could also be due to a vastly inaccurate FWHM. Whatever the cause, I have used these two "actual" PSNR figures for what follows.

Going deeper:
Based on the above figures, below are the limiting "useful" and "faintest" magnitudes I can expect to get using the above equipment for various exposure lengths. The first figure compares to 13.74 (mag 15.57) and the second compares to 6.57 (mag 16.4) above.
60s - useful mag 16.2 (PSNR=13.63), faintest mag 17.0 (PSNR=6.63)
120s - useful mag 16.6 (PSNR=13.46), faintest mag 17.4 (PSNR=6.52)
180s - useful mag 16.8 (PSNR=13.77), faintest mag 17.6 (PSNR=6.67)
240s - useful mag 17.0 (PSNR=13.27), faintest mag 17.75 (PSNR=6.72)
300s - useful mag 17.1 (PSNR=13.55), faintest mag 17.9 (PSNR=6.55)
600s - useful mag 17.5 (PSNR=13.33), faintest mag 18.25 (PSNR=6.73)

Alternatively, if I am shooting more towards the zenith (SQM=18.78), then:
useful mag 17.5 = 90s (PSNR=14.69) [faintest = mag 18.4 (PSNR=6.69)]
useful mag 18.0 = 180s (PSNR=13.48) [faintest = mag 18.8 (PSNR=6.62)]
useful mag 18.25 = 300s (PSNR=13.96) [faintest = mag 19.0 (PSNR=7.15)]

Conclusion:
Once I become proficient with the EQ mount and start guiding photos, Eris (mag 18.7), if reasonably positioned, should be within reach of my equipment from the streetlit front garden.

To totally misquote Leann Rimes, maybe I "can fight the streetlights"!

[beka]

Hi Demonperformer,

I was thinking of trying to capture Pluto with my CPC1100 and a DSLR and wanted to know the faintest magnitude I could expect to image. I came across your post from two years back when searching the forum. Nice work!

All the best!

[Demonperformer]

Hi

You should have no problem getting Pluto with your setup. My first (successful) attempts at getting Pluto were with a 102SLT/DSLR using 30-second exposures (on the SLT alt-az mount). Admittedly, I could only confirm that it was Pluto because I took a second image the following night and it had moved roughly in accordance with the CdC predictions.

And from your location it will be much higher in the sky - and you might even have some clear ones! Hope you are successful.

Thanks.