This post is just to share a personal 'ah-ha' moment for others newish to astronomy.
I am presently working thru the DeepMap 600 DSO list on Sky Safari Pro and we are in the midst of galaxy season. I have had pretty good success with my Mak 150 using an ES68 24mm. However, after checking off the 'easier' items I am now trying to get those pesky low surface brightness (eg face on) 10.9, 11.3, 11.7, 12, etc. Mag galaxies in Bortle 3-4 skies.
So I had the brainstorm (that I am sure many others have), why not try a longer focal length EP to get a larger exit pupil. Sure I may lose mag, but maybe I will pick up more 'good light' than the contrast I lose by not cutting thru LP. Furthermore, I have a 2" visual back on my Mak. What if I use my ES 30mm-82 or 40mm-68 EPs? The 40-68 would give me 50x (for my Mak's approximate effective focal length of 2000mm) and thus a 3mm exit pupil with a decent FOV to boot.
A concern of course is the 150 Synta Maks have only a 30mm rear opening (so yes a 2" visual back, but get vignetting on larger focal lengths). But, the human eye is relatively insensitive to vignetting. So to see if the narrow opening mattered, I did a comparison between an ES 40-68 and a regular 40mm Plossl, the ES 30-82 vs a 32 mm Plossl, ES 24-82 vs ES 24-68, and ES 18-82 vs Baader 17.5-76. Since my 120ST is really my low mag scope, and I bought it specifically to get large Exit Pupil and large FOV (for nebula), I added that also to the comparison, using a Stellarvue 15-82, an Orion EF12mm and Baader Morpheus 9mm and 6.5mm EPs. It is easier to get large exit pupils in the 120ST, but at the expense of lower magnification.
These OTA-EP combos provide a 3 mm exit pupil, a ~2.25-2.4 mm exit pupil, a 1.8mm exit pupil and a ~1.3mm exit pupil. For each exit pupil size, I did a subjective/qualititative comparison in daylight looking at a remote mountain scene (Sandia Mountain top). I judged based on 'quality' (vignetting, blackouts, magnification, effective field of view), and ease of use of eyepiece, with the basis being 'what would be best to see galaxies?' The results had some suprises.
- The 40mm-68 did not appear to be vignetted (in that it didn't have a 'soft' edge, I could see sharply to the edge of the field stop) on the Mak (even with the ES 40-68 45.5mm field stop), however this EP in a Mak had large demonstrable blackouts (from CO?). I still prefered it over the 40mm Plossl on the Mak.
- The 150mm aperture of the Mak150 did NOT rule over the 120mm for the lower mag options (3 mm and 2.4mm Exit Pupils) because of optical aberrations cause by the tight Mak rear opening and because those big 2" EPs are HUGE ('ease of use' criteria). But when then Mak was using 1.25" EPs, it was in its element and then came out on top (providing higher mag for the same exit pupil, which is nice for small galaxies).
Below is a table of my subjective/qualititative assessment ranking the results 1-2-3 for each OTA-EP combo (again based on what would be 'best' to observe small faint galaxies).
Bottom Line: I wrote this because going in my thought was always when observing was, if it can fit in the FOV, use the largest aperture. As mentioned, I am newish to astronomy and the drumbeat of 'aperture always wins' was in my head. So for other newish folks, realize that is only if comparing apples to apples.
Aperture does NOT always win if there are other factors (like small rear openings causing vigenetting) or specific use critieria. One also has to look at optical performance of system or how one is using the OTA. Think of a comparison of a 120ST vs a NP-101...there are MANY ways an NP-101 bests a 120ST even though the 120ST has 20mm more aperture (think splitting bright doubles or planet performance).
Edited by ABQJeff, 26 February 2021 - 05:56 PM.