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Aperture Does NOT Always Rule: a 120ST bests a 150Mak (for large exit pupil))

Eyepieces Maksutov Refractor Observing
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#1 ABQJeff

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Posted 26 February 2021 - 05:53 PM

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).

 

Exit Pupil Quality Comparison Mak150 vs 120ST.JPG

 

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).

 

CS!


Edited by ABQJeff, 26 February 2021 - 05:56 PM.


#2 Bean614

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Posted 26 February 2021 - 06:37 PM

"...150 Synta Maks have only a 30mm rear openin."    

 

Those made after 2016 are wider.


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#3 ABQJeff

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Posted 26 February 2021 - 06:48 PM

"...150 Synta Maks have only a 30mm rear openin."    

 

Those made after 2016 are wider.

Thank you!  The latest data I had found was from https://astro.ecuado...t-and-maksutov/

 

Well that definitely helps explain why I liked the 2” EPs in the Mak better than the Plossls, even if they had some vignetting at the edge.  The FOV was quite a bit larger. 

 

I will have to research what the rear opening is now (it is definitely under 40).  I guess I could take off my visual back to measure it myself...

 

CS!

 

CS!



#4 TOMDEY

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Posted 26 February 2021 - 06:50 PM

Indeed smaller apertures have their places and strengths. My rule is to acquire and enjoy the biggest affordable aperture, but also half that... and half that... and half that... progressing all the way down to naked eye... which is around 4.6mm pupil for me at age 73. Of course, virtually all of us progress from the bottom up: Born with eyes of ~7mm dark-adapted aperture. And then along the line acquiring various binoculars and telescopes from childhood through geriatric. I've maxed out at the 36-inch Dob in a Dome. But have retained incrementally smaller operational telescopes and binoscopes, to bridge the continuum all the way down to naked eye.

 

The lesson learned is to retain your smaller ~half-sized~ scopes when expanding up in aperture. They all have their unique sweet spots.     Tom


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#5 Mitrovarr

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Posted 26 February 2021 - 07:00 PM

SCTs and maks actually lose a surprising amount of light to the two mirrored surfaces (about 10% each) and the central obstruction.

 

I tried to do the math in this case (don't know if I did it right) and assuming your mirrors are at 90% reflectivity now (it depends on scope age), the theoretical light gathering of the Mak is only equivalent to ~130mm of refractor. So the ST120 didn't have as much distance to make up as the theoretical aperture difference makes it sound.


Edited by Mitrovarr, 26 February 2021 - 07:03 PM.

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#6 Jon Isaacs

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Posted 26 February 2021 - 07:07 PM

The 150mm aperture of the Mak150 did NOT rule over the 120mm for the lower mag options (3 mm and 2.4mm Exit Pupils)

 

 

Jeff:

 

With a reflector, you need to consider the reflectivity losses as well as the light lost due to the central obstruction.  The math looks like this:

 

Orion is deceptive in their specs on the central obstruction, they call it the "secondary mirror obstruction" which ignores the secondary baffle. They say the obstruction is 31%, its actually larger.

 

If you assume 90% reflectivity for each mirror, ignore transmission and coating issues with the corrector and guess at a 37% CO, the throughput is 

 

TP = 0.9 x 0.9 x (1- .372) = 0.70

 

These numbers are close to what Celestron claims for their Starbright XLT coatings. 

 

To figure an effective light gathering aperture:

 

A = 150 mm x 0.7.5 = 125 mm

 

It's a rough calculation but it basically says that the 150 Mak is very similar in terms of light gathering to a 120 mm refractor.

 

Make your comparisons at the same magnifications and the brightnesses should be similar.

 

Jon


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#7 Redbetter

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Posted 26 February 2021 - 07:11 PM

This isn't really a larger aperture comparison as there is only a rather small difference in system transmittance of a 120 refractor and a 150 Mak.  In a general sense, this is only effectively an incremental difference in aperture, ~0.2 magnitude in favor of the Mak.   The Mak will have higher resolution at small exit pupil.  

 

The problem for a Mak is achieving any sort of actual large exit pupil and wide true field of view.  That is where short ratio refractors excel.  

 

I wouldn't use a daylight test for a scope with large central obstruction like a Mak as a way of comparison, particularly to an unobstructed one.  I am guessing that your daytime pupil is what was producing the "kidney bean" you mention, and that is wasn't actual kidney bean.  


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#8 Jon Isaacs

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Posted 26 February 2021 - 07:25 PM

Red caught something I missed:

 

Exit pupil comparisons must be done at night to ensure your pupil is large enough to accept the full exit pupil.  This is particularly true with a reflector since the shadow of the secondary remains constant.

 

A daytime pupil might be 2 mm or less.

 

Jon


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#9 ABQJeff

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Posted 26 February 2021 - 08:56 PM

Ah yes, daytime pupil, I missed that as well.  Next dark night (of course it’s a full moon now) I will just have to compare head to head on actual galaxies.  I have two StarSeeker IV GoTos so can align them both easily enough at the same target.

 

(Oh and Jon, I know off topic, but continuing a conversation we had had on my StarSeeker IV accuracy, I got it figured out.  Have to use the PAE function in a sub menu to adjust for the portion of the sky I am observing in.  Now I am able to maintain tenths of a degree GoTo accuracy).

 

I like the daytime check to assess quality of image, field of view (can count antennas and trees) and vignetting across the field, that is what this test focused on.  But you are all correct, brightness comparison is meaningless in daytime (and is my main goal, to figure out what is my best OTA-EP combo to view faint small galaxies).

 

In next two weeks I should have an update, as I test the 120ST with the 15mm Stellarvue and EF 12mm against the 150mm Mak with the ES 40-68 and 30-82 under actual night conditions on galaxies, to confirm this thread’s hypothesis.

 

CS!


Edited by ABQJeff, 26 February 2021 - 08:58 PM.


#10 Jon Isaacs

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Posted 27 February 2021 - 08:13 AM

I like the daytime check to assess quality of image, field of view (can count antennas and trees) and vignetting across the field, that is what this test focused on.

 

 

These can also be affected by the observers pupil diameter.  A fast achromat at low powers during the day can look quite stunning.  The 120mm F/5 at 20x provides a 4mm exit pupil.  If your pupil is 2mm in diameter, then the effective aperture is only 60mm and you are now looking through a 60mm F/10 which has very little false color.

 

The opposite effect happens with a reflector, (Newtonian or CAT)  The size of the central obstruction stays constant, the effective aperture is reduced.  With a 150mm F/12 Mak and a 36 mm eyepiece, the exit pupil is 3 mm.  If the secondary obstruction is 33%, the shadow of the secondary is 0.33 x 3mm = 1mm.  

 

If your daylight pupil is 2mm, then the CO is now 50%, it is 1.5 mm, the CO is now 67%.. 

 

Jon


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#11 junomike

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Posted 27 February 2021 - 09:06 AM

IME the larger aperture framing the object almost always wins, especially on Galaxies and Globulars.  There can be  exceptions to the rule, however IME these are few and far between.


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#12 Jon Isaacs

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Posted 27 February 2021 - 09:18 AM

IME the larger aperture framing the object almost always wins, especially on Galaxies and Globulars.  There can be  exceptions to the rule, however IME these are few and far between.

In the case, the two scopes are very close in aperture and one is a reflector with a relatively large central obstruction and one is a refractor.  

 

I would expect them to be very similar but probably the refractor has less scatter and better baffling.  

 

Jon


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