I only used the Celestron NexStar 90SLT once before returning it, so I had only a few hours to capture some photos and decided to try for Neptune, which is the only planet I've never been able to photograph before (I was able to photograph Uranus without GOTO during the Venus-Uranus conjunction). Unfortunately, I didn't know which "star" was Neptune, but the GOTO was accurate enough (after a Solar System Alignment on Jupiter) to still put it within the field of view. Overall, I was impressed with the performance of the 90SLT, and was disappointed that I had to return it due to the fact that it did not have sufficient clearance to go above 77 degrees in altitude angle without the optical tube assembly hitting the tripod legs (the 90SLT needs a pier extension or wedge to go to zenith when used with a DSLR camera).
I couldn't find any photos online of Triton taken with an SLT mount, so I figured I should share it here:
Neptune can be identified as belonging to the "asterism" of four bright "stars" in a line, the third "star" (from lowest to highest in altitude angle) being Neptune.
Triton is most clearly visible at 33% zoom. Increasing the image scale past 33% washes out the signal of Triton with the increased visibility of ISO grain and altazimuth field rotation. This was taken using a 30-second exposure at ISO 51200, so there is up to 8 pixels of trailing (proportional to the angular separation from the optical axis). The offset of Neptune from the optical axis is because I didn't know which "star" was Neptune in the DSLR liveview after GOTO Neptune.
Here is the photo scaled to 34% (1365 x 2048), in which Triton makes Neptune appear like a tight double star:
I had no idea that Triton could show in a 90-mm Maksutov-Cassegrain with a DSLR camera, and I actually thought it was a background star at first, until I looked on Stellarium.
It should be noted though that I "cheated" --- I used my Omegon 1000/90 (f/11) Maksutov-Cassegrain instead of the Celestron 1250/90 (f/14) Maksutov-Cassegrain. The faster focal ratio makes it much more suitable for long-exposure astrophotography on an altazimuth mount.
This is a single-shot JPEG photo (no stacking or processing) taken with an un-modified Canon EOS Rebel SL2 (200D) DSLR camera at prime focus, using the Farpoint low-profile T-adapter and the Explore Scientific low-profile T-ring on the Omegon MightyMak 90 with the Celestron NexStar SLT (90SLT) mount.
datetime = 2021-10-27T23:05:13-04
(phi, lambda, h) = (+39D:42M:23.84S, -78D:38M:34.82S, 323 m)
(T, P) = (+3.9 deg C, 1013.21 mbar)
focal length = 1044 mm = (1000 mm) + (44 mm)
sensor = 6000*4000 px @ 22.3*14.9 mm^2
pixel scale (at 6000*4000) = 0.734"/px
aperture diameter = 90 mm
Rayleigh resolution at 502 nm = 1.40"
Triton altitude angle = +44.1203 deg
Triton apparent magnitude = +13.65
Neptune apparent magnitude = +8.02
Neptune-Triton elongation = 15.3"
Neptune angular radius = 1.17"
Triton angular radius = 0.065"
However, I am posting this to the DSO forum instead of the Solar System forum because of a stellar curiosity. If you look on the center left edge of the photo, there is a small asterism of three stars, with the brightest of the three stars on the very edge of the photograph. Of these three stars, Stellarium says that this one should be the dimmest, with an apparent magnitude of +14.34 versus +13.74 and +13.09 for the other two stars. However, in the photograph, the star at magnitude +14.34 appears to be brighter than the star at magnitude +13.09. So I am wondering if maybe the magnitude in Stellarium is wrong, or if this is a variable star? Unfortunately, Stellarium does not give names to any of these stars. How do I get Stellarium to provide names for every star? What would be the best way to find out the name and correct magnitude of this mysterious star?
Note that there may be some stars missing from the photo since I had automatic noise cancelation enabled (this is the infamous "star-eating").
Edited by Nicole Sharp, 12 November 2021 - 06:30 AM.