Visual PinPoint and Estimate of Faintest Stars in Image

[Nicolas Schuck]

Looking at the ZeroPoint value (Visual PinPoint v7) in the plate solve, can this be used as an estimator of the faintest stars visible in the image? My values are around 22.5 (1 sec) for a 300 sec exposure.


My goal is to try to determine what magnitudes I am reaching. ±0.1 or ±0.5 of a mag would be sufficient. What is the best way to determine this star magnitude value? There is the manually compare the image with a star chart method, but I am curious if there is a better way to do this?

Thanks Bob!

[Bob Denny]

PinPoint's photometry is what I would characterize as "rough", nothing like 0.01 mag, and 0.05 mag would be sketchy. Mostly because of the rough transforms uused to go from the catalog colorbands to J-C. I would use a photometry tool. Maybe someone here would know of a good one for this. PinPoint is an astrometric tool primarily.

[Nicolas Schuck]

Thank you for the helpful info Bob,

I will look into some good photometry tools out there. Just to clarify, I mentioned ±.1 or .5 mag in my earlier post, what would you say is a good ± estimate for PinPoint's photometry? I am going to be using Visual PinPoint primarily for asteroid detection and astrometry.

Best,
Nick

[Bob Denny]

For asteroid detection and astrometry, the magnitude isn't very important. Was there something else you wanted to do?

[Nicolas Schuck]

Thanks Bob,
In this instance I was curious of a ballpark magnitude that I was reaching to give me an indication of what mag asteroids my imaging system could detect for that exposure time. If my images are only hitting say 21.0 magnitude and the asteroid is 22.5, then I know I need to make adjustments (sky conditions, exposure, or hardware changes) to be able to capture it, or at least know the limits of the imaging system. Looking at the PinPoint plate solve file got me curious if the ZeroPoint could be an indicator of my magnitude limit for an image. I'll also try and find a photometry software to help with this before using Visual PinPoint for asteroids.
Best,
Nick

[Bob Denny]

I'd just go for the gold and later figure out what you can and can't do :-)

[Eric Dose]

Ah, you're doing asteroids. Here are a couple of tips for asteroid photometric range-finding:
(1) Most asteroids vary in magnitude as they rotate, typically in the range of 0.1-0.6 magnitudes brightest to faintest. They also change over the weeks they are nearest earth due to the phase angle's interaction with surface reflection properties (and worse, the latter are usually unknown). So prior estimates of asteroid magnitude within 0.5 magnitudes is considered good work and/or lucky. If your initial exposures of a given asteroid differ from your expectations, it's probably nothing you did wrong, it's probably just the asteroid. Space rocks can be weird.
(2) MPC magnitude estimates are in Johnson-Cousins V, but all the new and accurate catalogs are in Sloan. The most useful passband for asteroids is usually Sloan r'. So here are two useful rules of thumb for solar-like illumination (as for asteroids): (1) r' (Sloan) ~~ R (J-C) + 0.22, and (2) r' (Sloan) - i' (Sloan) is typically 0.1-0.35. Using these very, very approximate guidelines, you should be able to compare your raw magnitude estimates to catalog magnitudes well enough to get started. Good luck.

[Bob Denny]

Oh man, THANK YOU ERIC!!!