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best telescope designs for Lunar observing

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#1 Nicole Sharp

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Posted 19 August 2020 - 06:58 AM

What are the best telescope designs for Lunar observing, ranked from best to worst?  Such as for minimal aberrations and maximum clear aperture?

 

I figure:

 

#1) superapochromatic (quadruplet) refractor

 

#2) apochromatic (triplet) refractor

 

#3) semiapochromatic (doublet) refractor

 

#4) Maksutov-Cassegrain

 

#5) Maksutov-Newton

 

#6) aplanatic Schmidt-Cassegrain

 

#7) Ritchey-Chretien

 

#8) Schmidt-Cassegrain

 

#9) Schmidt-Newton

 

#10) classical Cassegrain

 

#11) Newtonian

 

#12) achromatic Bird-Jones (catadioptric Newtonian)

 

#13) achromatic refractor

 

Not exactly sure about the ordering there though.  Let me know your own rankings!


Edited by Nicole Sharp, 19 August 2020 - 07:05 AM.


#2 junomike

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Posted 19 August 2020 - 07:26 AM

Issue here is everyone sees differently.

Some may feel that a Quad or even a Triplet is overkill for visual and a Doublet larger than said Triplet is better.

 

Also, your rating of "Doublet" as Semi-Apo is questionable as well as I've seen some Doublets better corrected than Triplets.

 

In general for me It's Aperture than wins.  So I anyone has a 12" - 16" Apo weighting and costing less than my GOTO Dob's, I'm in.

Otherwise It's the Dobs (at least in summer) for me.


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#3 db2005

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Posted 19 August 2020 - 07:28 AM

In the interest of full disclosure, I've only observed the moon trough maybe half of the listed telescope designs. But I would probably rank the (slow) achromatic refractor above any of the mirrored designs, i.e. rank it as number 4. The Newtonian (especially slow Newtonians) would also likely be ranked much higher by many observers, probably rivaling the position of the slow achromat.

 

The challenge here is that we're not doing a fair apple-to-apples comparison because the listed scopes will typically be of wildly different apertures and focal ratios. An 8" SCT will show much more detail than a 3" APO can ever hope to show on the moon. But contrast will typically be superior in the APO nonetheless. The thing is ... I don't know if that makes the APO or the SCT the better scope for lunar observing. It really depends on the observer's preferences.


Edited by db2005, 19 August 2020 - 07:58 AM.

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#4 Hesiod

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Posted 19 August 2020 - 07:42 AM

At apertures under 5" my top choice are good quality apochromatic refractors. More than the number of lenses and kind of glass it is how the telescope is crafted to really matter (I would take a superb mid/slow achro over a "run of the mill APO" without hesitation! Also take note that most 4/+ lenses designs are mainly astrographs and not necessarily especially good at high power. On the other hand CaF doublets are amazing at high power because have almost invariably superb objectives both in term of lenses and design/crafting).
From 6" upward feel that Newtonian reflectors are probably the best option unless want to use a "portable" equatorial mount: in such case a Cassegrain-derived unit would be more convenient.
Again, the best design is the one with the highest optical and mechanical quality, properly mounted and properly handled.
In general terms Intes made excellent Mak-Newt and Mak-Cass which are fairly easy to find if compared to other high quality MCTs (e.g. TEC, AP).
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#5 Nicole Sharp

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Posted 19 August 2020 - 07:58 AM

Issue here is everyone sees differently.

Some may feel that a Quad or even a Triplet is overkill for visual and a Doublet larger than said Triplet is better.

 

Also, your rating of "Doublet" as Semi-Apo is questionable as well as I've seen some Doublets better corrected than Triplets.

 

In general for me It's Aperture than wins.  So I anyone has a 12" - 16" Apo weighting and costing less than my GOTO Dob's, I'm in.

Otherwise It's the Dobs (at least in summer) for me.

 

Not all telescope designs are available in the same aperture, so a full comparison has to be theoretical for a given total aperture (since for a given total aperture, clear aperture would vary by central obstruction).  I'm more interested in the theoretical optical properties as compared to tube weight or length for mount performance, though of course for practical application, the "best" telescope would have to be affordable and mountable, as well as having good optics.


Edited by Nicole Sharp, 19 August 2020 - 08:06 AM.


#6 Nicole Sharp

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Posted 19 August 2020 - 08:11 AM

It's probably easier to compare individual designs instead of a full ranking.  Like comparing an MCT with an SCT (either traditional or aplanatic), the MCT should be superior since it has a smaller central obstruction, and then the same analysis for comparing an MCT and a semiapo or apo refractor of the same total aperture.  But am still curious if a full ranking is possible.  Having comatic and/or chromatic aberrations can be less desirable than simply having more clear aperture, when comparing two telescopes of the same total aperture.  Likewise, telescopes with spherical aberration would prevent high-magnification views.  A lot of it can be personal preference too regarding what types of aberrations you might find acceptable or unacceptable.


Edited by Nicole Sharp, 19 August 2020 - 08:14 AM.


#7 Nicole Sharp

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Posted 19 August 2020 - 08:28 AM

I figure that telescopes with slower focal ratios usually have less optical aberrations than telescopes with faster focal ratios, which is why I ranked the Maksutov-Cassegrain over the Maksutov-Newton, and the classical Cassegrain over the traditional Newtonian.  I don't actually know though if a classical Cassegrain has less optical aberrations (presumably comatic aberration) than a traditional Newtonian?



#8 Nicole Sharp

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Posted 19 August 2020 - 08:33 AM

Would also be curious to hear comparisons between a traditional Schmidt-Cassegrain and a Ritchey-Chretien.  The (non-aplanatic) SCT has more comatic and chromatic aberration, but the RCT has less clear aperture.


Edited by Nicole Sharp, 19 August 2020 - 08:33 AM.


#9 Hesiod

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Posted 19 August 2020 - 08:50 AM

Slow telescopes are easier to craft but mind that Cassegrain-based telescopes have a couple of fast optical elements, which are harder to make.

This is one of the reasons behind the peculiar choices forced upon the designers of most of the "family members", including the all-spherical design with correctors.

 

The basic Newtonian reflector is one of the most reliable design due to somewhat simple optical requirements and intrinsic control of aberrations (no CA, no SA, low amount of coma which may be easily removed; CO is meaningful only for very small ones, or widefield astrographs).

 

 

Telescopes with significant SA are not diffraction-limited and basically are junk (or narrow-purpose highly specialized devices): the attempts to get rid of SA from cheap telescopes gave the rise to Maksutov/Schmidt/Klevtsov/etc... variants of Cassegrain and Newtonian reflectors with all spherical mirrors (themselves being a simplification of the original design); as economical reasons are the strongest drive behind that kind of solution, no wonder why the "Bird Jones" are so awful: theoretically these could be crafted to excellent levels, but nobody really bother to do so since, at such cost, other options become viable.

 

Mind also that in the case of Moon-gazing, how the telescope controls glare and scatter is just as important as its intrinsic optical accuracy: smooth optics, proper baffling and lack of reflections mark the good Moon telescopes. This often cripples the low-end models more than their optical accuracy: my Heritage 130/650 has a quite decent, diffraction-limited main mirror, but I had to mod it to observe the Moon because of the critical loss of contrast caused by the internal reflections.


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#10 gitane71

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Posted 19 August 2020 - 06:57 PM

  I know the quality of many mirrors is better than ? 25 years ago, but still, I am amazed that you rank an achromat as last.  

  There are few Maksutov Newtonians, but their central obstruction is generally small enough and their spherical surfaces smooth enough to give Very good contrast.  

Many catadioptrics have fast primaries with large secondaries that amplify imperfections and decrease contrast.  

  With mirrors, the angle of reflection is equal to the angle of incidence.  With lenses, the angle of refraction is equal to half the angle of incidence, so if a mirror and a lens have the same surface quality, the lens gives a wavefront at least 4 times better, and there are other things to consider after that.  

   I would think long achromats, along with long Newtonians and Maksutov Newtonians would be near the top of the list.



#11 bobhen

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Posted 23 August 2020 - 07:37 AM

If we are talking about resolving the finest lunar details, the seeing will limit any telescope’s resolving potential.

 

For example…

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited* on 9 out of 10 nights.”

 

Some areas like Florida on occasion can get exceptional (10) seeing and on one of those nights a 10-12” telescope (give or take) “might” resolve to its full potential. In areas like the Northeast and many other locations no nights are exceptional (10) with at the most some (few) nights being very good (8) and on those nights a scope of 6-8” “might” resolve to its full potential. But on most nights in most locations the seeing is around average (5 -7) so larger scopes are not resolving to their full potential – the views might be brighter but the details will be limited to what the seeing allows and the same level of detail will be observed in a smaller scope that matches the seeing.

 

A large apochromatic triplet around 10” size would be ultimate lunar telescope in a location that could support such a size: less scatter, higher contrast and no obstruction compared to mirror scopes of any design.

 

A smaller apochromatic triplet around 5-7” size would be ultimate lunar telescope in a location with mostly average seeing: less scatter, higher contrast and no obstruction compared to mirror scopes of any design.

 

Of course, cost and bulk limit the availability of large apos so most amateurs turn to the more cost-effective mirror designs.

A Newtonian with a “very high quality” mirror, like from Zambuto, in a size that matches one’s seeing at F6 to F8 is more cost effective (although not cheap when combined with a high quality tube) and a potent performer.

 

For those that need something more compact or something that is easier to use on a GEM. A high quality folded design like the Takahashi Mewlons in 180, 210 or 250mm sizes, depending on your seeing, are excellent. High-end Maksutovs are also excellent but expensive and hard to get over 7”. High-end Cassegrains like those from CFF and other manufacturers would also be excellent, if you have the budget and the seeing that supports the larger size.

 

Most scopes can produce pleasing lunar views, but for the avid lunar observer wanting the best, the above would be some considerations.

 

Considerations for the lunar observer...

 

1. “Very high” optical quality
2. Scope and mount ergonomics (bulk, setup time, acclimation, etc.) that match one’s ability, location and observing style.
3. A size that matches one’s seeing on the vast majority of nights
4. All will be more important than a particular design.

 

With my seeing here in PA, 2 of my favorite, reasonably portable lunar scopes are…

1. My Takahashi TSA 120 refractor: when seeing is average (on many nights) or the moon is low to mid-altitude. I’m always amazed at what the Tak TSA 120 shows in average seeing conditions.

2. My Takahashi Mewlon 210: when seeing is above average (fewer nights) or the moon is higher than mid-altitude. With those conditions, the Mewlon 210 is one of my favorite lunar telescopes and will show the Alpine Rill, multiple craters on Plato that look like craters, etc., and all in extremely sharp, 3-D-like immersive detail.

 

Bob


Edited by bobhen, 23 August 2020 - 07:37 AM.

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#12 Spikey131

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Posted 23 August 2020 - 06:24 PM

I have viewed the moon through most of the scopes you have listed.  It looks good through all of them.

 

But give me a C8 or a 12.5” dob and binoviewers.  That’s about as good as it gets.



#13 EJN

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Posted 25 August 2020 - 11:02 PM

#13) achromatic refractor

 

 

This image I took with an 80mm f/11 achromat, so I think you need to revise your rankings.

Click for full size.

 

IMG_0010jpg-work2-1024x768.jpg


Edited by EJN, 25 August 2020 - 11:07 PM.

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#14 Nicole Sharp

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Posted 26 August 2020 - 12:08 AM

This image I took with an 80mm f/11 achromat, so I think you need to revise your rankings.

Click for full size.

 

attachicon.gifIMG_0010jpg-work2-1024x768.jpg

 

Yeah, should probably split up long-focus versus short-focus Newtonians and achromats but I figured that might complicate the rankings too much.  E.g. Barlowing a Newtonian can reduce visible comatic aberration for high magnifications, or stopping down the aperture on an achromat to reduce visible chromatic aberration.

 

An achromatic refractor from the time of Giovanni Cassini might have an aperture of 5 inches (130 mm) but a focal length of 150 feet (46000 mm), for a focal ratio of maybe f/350 to get the chromatic aberration low enough with the lenses of that time period.  Of course, trying get such a long refractor mounted and tracking an astronomical object is very difficult.  Big props to Cassini and his team of assistants for managing that.


Edited by Nicole Sharp, 26 August 2020 - 12:24 AM.


#15 Hesiod

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Posted 26 August 2020 - 05:09 AM

Fast (f4 or less) big Newtonians are excellent lunar telescopes.
If want get rid off coma, thete is the Paracorr.
As for achros, stopping them down make CA less apparent but if the telescope was already diffraction limited that will do more harm than good due to the loss of resolution: colored filters are IMHO a better solution

#16 Andrekp

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Posted 26 August 2020 - 03:01 PM

Resolution, as it’s normally discussed, is less of an issue on the moon.  Feature contrast is more important.  Yiu can see things on the moon that are tiny, if they contrast, and miss things that are huge, if they don’t.

 

personally, I prefer my 6” newt for the moon because it is convenient, easy to use in practice, and the Newtonian double flip matches the atlas I use.  Use what you are comfortable using for the moon.  It’s a very different thing that observing DSO’s.


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