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Out with ED spotting scope, in with 5" Maksutov telescope

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Old Thursday 21st March 2019, 03:44   #1
DunninLA
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Out with ED spotting scope, in with 5" Maksutov telescope

I just finished solving an issue I was having with my Brunton Eterna ED 20-60x80 spotting scope --

Basically, I became dissatisfied with the dim view when zooming over 50x up to 60x. I don't use my scope for birding, but the solution I found would definitely benefit anyone birding from their home out to a great distance.

The Problem: brightness of view in scopes, and binoculars, is determined mostly by the exit pupil size. It is generally thought that exit pupil below 2, even in bright daylight, begins to degrade both resolution and color saturation. The brand of spotting scope doesn't change this... from Swarovski to Barka, ED glass or normal glass. Tiny exit pupils kill the quality of image.

Spotting scopes of 80mm objective lens fall below 2.0 exit pupil at 41x, assuming in best case it really is a full 80mm aperture.

Now, I use my ED spotting scope for viewing out to sea, and up the coastline, anywhere from 0.8 miles up to 30 miles, so I am always at 50-60x magnification. At those magnifications, the image, to me, even in bright daylight, is just too dim, washed out color, and slightly unsharp. This isn't because my scope is mid-level ED glass, but because of the tiny exit pupil at those magnifications/

Solution: so I could either go for an alpha glass spotting scope, which would improve the view somewhat, but it would be like chasing diminishing returns down the small exit pupil hole.

Instead, I realized I don't need light weight, water proofing, or extreme ruggedness for viewing long distance from my deck. So the inherently poor optics from high magnification through an 80mm aperture scope and tiny exit pupil are not necessary for me. I assume a lot of birders would like to view from their deck, or even long distance in the field set up in the same spot for hours.

After a lot of research on astronomy forums, I decided to get a 5" (127mm) Maksutov telescope. It weights 7 lbs, and is actually shorter than my Brunton scope at 12". By its design it is more rugged than any other telescope design, so it would rarely need to be collimated like most telescopes need. Some posters at a couple of astronomy forums say they have never had to collimate their Maksutov even after travelling and bumping it around. It arrived Monday. I have it mounted, as I did with the Brunton, on a Manfrotto tripod with a Manfrotto video head.

I have compared the views in low light, and bright light, through my new MAK and my existing Brunton. There is no comparison, even with the current entry level correct image 45* prism (which I will upgrade for $80) and my mid tier eyepieces. My premium eyepiece arrives Friday. At 50x up to 60x, the colors in the Brunton are very washed out, and with the MAK they are bright and pop. The resolution is superior with the MAK as well.

Here are the advantages with this particular scope:

- The optics are superior due to a large exit pupil. I have a 32mm eyepiece installed which gives me 52x magnification with 2.5 exit pupil. I also have an Explore Scientific 26mm eyepiece coming Friday, which gives me a 2.0 exit pupil and 64x magnification.

- The cost is far less than half that of mid-level ED scopes, and 10% the cost of an alpha class 80mm spotting scope.

The 127mm Maksutov OTA, whether that be Orion or Celestron, is about $385, and the upgraded eyepieces were $35 for the 32mm and $110 for the 26mm. Even the $15 included 25mm plossl provided superior images.

Disadvantage: I lose lightweight (8 lb. vs. 4.5lb.), waterproofing, and greater ruggedness. Those I didn't mind. The real loss is zooming, and usable views below 40x magnification. Telescope Zooms don't work because their magnification range yields exit pupil sizes mostly below 2 and down to under 1. So I can no longer zoom. But again, I don't view under 0.8 miles, so that doesn't bother me nearly as much as the poor high magnification image quality in the spotting scope.

So, for all you birders who don't need a lot of portability with your scope, you should consider a Maksutov telescope instead of, or to complement, a spotting scope. It gives superior optics vs. ANY 80mm spotting scope, and costs under $600.

Just thought you guys would want to know about this alternative, since I really didn't see much written about it while I was researching.

Last edited by DunninLA : Thursday 21st March 2019 at 03:51.
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Old Thursday 21st March 2019, 12:11   #2
Binastro
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Hi DunninLA.
I have used Maksutovs terrestrially for well over 40 years.
Particularly the Custom 150mm f/10 at 95x.
From one to five miles typically.

I have written about these scopes for years also.

Points to watch.
My 127mm Skywatcher optics are not great and have some temperature problems.
The thick corrector shells can crack.
I wouldn't want to drop a Maksutov.
My 12 inch Maksutov corrector split in two on the makers bench after a lot of hard work. So I never got it.
He gave up making optics.
Large Maksutovs above 8 inch are difficult to make.
Questar made a 7 inch and 12 inch.
Quantam Maksutovs have a high reputation.
Russian ones are often excellent but heavy.
Some military ones are amazingly good, the 5.5 inch De Oude Delft Dutch one I tested was fabulous. So is my 4 inch c.1950 Russian one.

SCT scopes correctors are very thin and can easily shatter. Replacing one. Cheaper to buy a new scope.

Maksutovs. Some have collimation screws, some don't.
My 150mm has a separate secondary. The longer focal ratios usually have the silvered spot on the rear of the corrector.

Small mirror focus scopes change focus as one focuses and when adding prisms etc.
The 90mm drastically, the 127mm less and so on.
So quoting magnification without measuring the focal length at that setting with erector prisms etc. will be wrong.
My Skywatcher 90mm as tested with Vixen 8-24mm eyepiece is about 65x to 190x.
Some measures of focal length with erectors etc. can be found online. Probably accurate in some cases if the user knows what he is doing.
These scopes are optimised for one primary and secondary separation, but in practice I have not seen much difference except in the complex Sigma 500mm f/4 mirror lens. The image is only acceptable at one focus position away from that position awful. Answer, find best position and just change position of eyepiece.

Typically the amplification factor of the scope is about 5 or 6 times. An f/2.5 primary, f/12 to f/16 total focal ratio.

SCTs have f/2 primaries except the 9.25 inch Celestron, perhaps f/2.5. Which probably explains the better quality.

Maksutovs, particularly 6 inch and above aperture, can take an hour or longer to cool. So leave at ambient temperature.

I have used or at least tested about a hundred similar scopes or mirror lenses as scopes.

Welcome to the Maksutov world.

Regards,
B.

Last edited by Binastro : Thursday 21st March 2019 at 12:28.
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Old Thursday 21st March 2019, 17:22   #3
DunninLA
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Quote:
Originally Posted by Binastro View Post
Hi DunninLA.
I have used Maksutovs terrestrially for well over 40 years.

I have used or at least tested about a hundred similar scopes or mirror lenses as scopes.

Welcome to the Maksutov world.

Regards,
B.
Thank you. I am amazed at the image quality for a pittance. If anybody uses alpha glass 80mm scopes from a stationary position at the home, they would be better served (assuming they're not married to zooming) to sell, pocket $2,500, and replace it with a 127mm MAK plus two quality eyepieces and a quality 45* diagonal for $600.

P.S. I see the Wiliams ($80), Baader ($92), and Stellarvue ($69) 45* correct image prism diagonals. Do you have a recommendation about which is best for my purposes? The cost difference doesn't matter to me.
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Old Thursday 21st March 2019, 18:15   #4
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I have the Televue Chester NY 45 degree Image Erecting Prism AM1-0011 Televue Visionary here in my hand.
It is uncoated, but it is the one I use.
1.25 inch barrel.
I trust Televue.
But if I use a scope I typically go to 100x or more.

I probably have a 2 inch diagonal and prism also. Not Televue. Maybe the larger Maksutovs can take this.

At 50x or 60x I think that any of the three you list are O.K.
There is sample variation, so it is impossible to know the actual accuracy.
I would go for Williams first and Baader second. I don't know Stellarvue, I think a U.S. brand. They likely come from the same factory.
I have a 66mm Williams refractor that is good, but some report sample variation, with astigmatism? but I am not sure, so don't want to denigrate Williams, which are generally good.

For real quality with my 150mm f/15 Maksutov I don't use a prism, and only a star diagonal if I have to.
With the 150mm f/10 Maksutov I used an old straight through prism and a very heavy duty ex gov aluminium box leg tripod that easily supports a 75kg person. Two section legs and a heavy chain link around the bottom of the feet.
Incidentally bracing the tripod legs at the bottom is I think more stable than half way up.
When I set up the Maksutov by the sea shore to let people look 1 to 5 miles away the chain made a terrible racket.
I had a custom one arm mount made to attach the scope as an altazimuth.

On a deck like yours I would choose a 6 inch or 180mm Maksutov, depending on how well the 5 inch works.
If buying any Maksutov it would be great to get a really good one, as they vary. It would need an expert and preferably looking at Jupiter or Mars at high elevation and high magnification to test it properly.
If I was younger my 12.5 inch Dall Kirkham would have probably have been the thing to use on a deck like yours.
150x, 200x when possible.
People asking about long distance scoping need to be concerned more with the atmosphere than which telescope to use.

I got four professional tripods at the charity shop for about 40. One is missing the quick release plate, but some go up to 7ft 6inches. I had to make 4 journeys to carry them back. Also a tripod dolly that I gave to a friend.

If you eventually want to go for a bigger Maksutov use a heavy mount.

B.
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Old Thursday 21st March 2019, 19:39   #5
DunninLA
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Originally Posted by Binastro View Post
I have the Televue Chester NY 45 degree Image Erecting Prism AM1-0011 Televue Visionary here in my hand.
It is uncoated, but it is the one I use.
1.25 inch barrel.
I trust Televue.
But if I use a scope I typically go to 100x or more.

For real quality with my 150mm f/15 Maksutov I don't use a prism, and only a star diagonal if I have to.

With the 150mm f/10 Maksutov I used an old straight through prism
On a deck like yours I would choose a 6 inch or 180mm Maksutov, depending on how well the 5 inch works.

B.
Thanks, Williams it will be then. I had considered 6" mak, and depending upon how well the 5" works, I may still. But as you say there is an atmospheric limit to image quality, and i had though perhaps the 6" Mak would be overkill... perhaps I didn't think that through completely.
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Old Saturday 23rd March 2019, 18:52   #6
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At the level of the best mirror scopes with large obstructions vs the best APO refractors I'm not much of a mirror fan either, but if you want high magnification of fair quality for low cost I can see the attraction vs spotting scopes at the same price.

However, I'm really not a fan of the 45 Schmidt erecting prisms (usually mislabeled as Amici prisms) sold as astronomical telescope accessories. As far as I can tell none of them is phase corrected and all I have seen (including the Televue) were so poor that they would ruin the image quality of any really good scope. IMO, virtually any star diagonal prism or mirror will be much better optically for less money at the small cost of reversed images, something I easily adapted to long ago. Televue makes a pretty expensive, but excellent 60 mirror diagonal if you can't live with 90.

Henry
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Old Saturday 23rd March 2019, 19:05   #7
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Originally Posted by henry link View Post
At the level of the best mirror scopes with large obstructions vs the best APO refractors I'm not much of a mirror fan either, but if you want high magnification of fair quality for low cost I can see the attraction vs spotting scopes at the same price.

However, I'm really not a fan of the 45 Schmidt erecting prisms (usually mislabeled as Amici prisms) sold as astronomical telescope accessories. As far as I can tell none of them is phase corrected and all I have seen (including the Televue) were so poor that they would ruin the image quality of any really good scope. IMO, virtually any star diagonal prism or mirror will be much better optically for less money at the small cost of reversed images, something I easily adapted to long ago. Televue makes a pretty expensive, but excellent 60 mirror diagonal if you can't live with 90.

Henry
Sorry Henry, I think your first sentence referred to my post which I deleted because I didn't want to get caught up in a debate about Maks v Refractors.

I'll say, for half a dozen reasons I prefer a refractor to a mirror scope any day, especially for terrestrial use.
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Old Saturday 23rd March 2019, 20:08   #8
Binastro
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I also suggested a 120mm or at a minimum a 100mm good quality refractor for this application of long distance viewing.
Indeed, a 6 inch long focus observatory refractor would do well.
The 150mm Skywatcher ED refractor may also be good.

But the choice was a 5 inch Maksutov, and it has proved to be a good choice for the money.

But refractors versus mirror scopes misses the whole point.
It is the atmosphere and temperature properties of the scope that outweigh telescope quality.
Refractors are better than mirror scopes for this, especially long focus refractors, but it depends on the size.
I would say that a top quality 6 inch mirror scope, with a 25% central obstruction, equals a top quality 4.5 inch reractor.
And the cost.

Terrestrially, viewing at long distance how many have really seen high quality views with even the best APO refractor.

Spotting scopes, even a Swarovski 95mm are unfortunately only toy scopes compared with Horace Dall's 8 inch Maksutov or his 15.5 inch Dall Kirkham.
And my 12.5 inch Dall Kirkham would make short work of a 95mm Swarovski or even a 100mm Astrophysics refractor.

I have used all sorts of scopes. High quality refractors, Maksutovs, Newtonians, Dall Kirkhams, SCTs etc. and many spotting scopes also. All have a place, even medium quality scopes.

My friend's Celestron C14 is clearly superior to his fine 7 inch Astrophysics refractor.

A very skilled planetary observer gets amazing results with his 8 inch Newtonian. He also uses a 20 inch Planewave Dall Kirkham, and last year the 24 inch Lowell refractor on Mars.
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Old Saturday 23rd March 2019, 20:16   #9
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The view of a crow on the chimney pot 124 metres away looking through the Skywatcher 90mm Maksutov and using the Televue 45 degree prism at 125 times was bloody marvellous.

But I had to wait for a 20 degree elevation bright sun almost behind me to get this view and fine detail.

This is looking through double glazing at a 45 degree angle.

Until the conditions were ideal the view through this Maksutov was disappointing.

A good refractor spotter would do better in most conditions.
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Old Saturday 23rd March 2019, 23:30   #10
DunninLA
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if you want high magnification of fair quality for low cost I can see the attraction vs spotting scopes at the same price.
If only they were the same price. My mid level spotting scope retailed for $900 four years ago. To this day the only ED refracting spottting scope I know of under that price is the Celestron and it is $750. Alpha scopes start at $2,500 in the 80mm objective lens size. My MAK, Celestron Omni 32mm eyepiece, and Celestron correct image 45* diagonal cost $400 all in, and at 50-60x magnification, produce an image far superior in every way to the ED $900 spotting scope. At 20x magnification the spotting scope renders a similar, or possibly superior image to the MAK because at that magnification the spotting scope has a 3.0 exit pupil.

For 50-60x magnification, scopes of that $1000 price point just simply don't work. With 1.4 down to 1.25 exit pupil, the anatomy of the human eye and the laws of physics simply can't cooperate to produce a quality image. That image is worse than from $20 binoculars... or as some people with the scopes are wont to say, above 40x it's worthless.

Since I live almost a mile form the closest thing I observe, 50-60x magnification is my world, and the scope was frustrating. Fortuantely the MAK suited my needs.. It might suit some birders as well.

Another almost cost effective solution would have been something like the Explore Scientific Achromat in 102mm size. The image would have been superior in contrast, and about equivalent in brightness relative to the 127mm MAK, but then I'd have to deal with a 26-30" OTA that has issues with Chromatic Aberration. And the cost would $200 more than the MAK all in.

A less cost effective, but better solution would be the ED version of the above ES scope. At around $1000, the CA is mostly corrected but the 26-30" form factor remains.

So, clearly there are non-MAK solutions, but cost and length, for me at least, were unattractive.

Last edited by DunninLA : Saturday 23rd March 2019 at 23:37.
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Old Sunday 24th March 2019, 10:26   #11
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DunninLA.

Enjoy.

Regards,
B.
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Old Tuesday 16th April 2019, 18:11   #12
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I've had four weeks now with my Mak spotting scope. I can still highly recommend this upgrade from an ED spotting scope of the $1,000 price range.

Certainly the greatest impediment to high magnification viewing has been atmospheric interference in the day. I have had perhaps four days, mornings usually actually, where I could clearly see a person's face and the details of their hat and clothes at 2.5 miles on the beach -- on a fully sunny morning. To give another frame of reference, I was able to read a street sign of four inch white letters on a green background at 3 miles. I was able to achieve both of these long distance, detailed views with a cheap 9mm 50* bundled $15 plossl eyepiece giving me approx. 160x magnification and a 0.7 exit pupil. Using a 19mm BST 65* $70 eyepiece of 87x magnification I could almost make out those details on the beach, but also could read that street sign at 3 miles. I was shocked that there were mornings where 160x magnification was even possible. Of course most days my usable magnification is somewhere between 52x and 64x.

So, in additional to a far superior image at 52x, the switch from ED spotting scope with 20-60x non-interchangeable zoom lens to a Mak gave me dozens of possible eyepieces varying from normal to extremely wide angles. And the beauty of the very slow f13 maksutov telescope is that almost any eyepiece starting at $20 (50* fov) up through $90 (*65-70* fov) gives a far superior image to even the best zoom on a spotting scope priced less than $4,000.

The bottom line is that people considering a spotting scope for long distance terrestrial viewing (stationary birding, stationary hunting, rifle or archery range, sea or land observing) should really ask themselves whether a Maksutov 127mm telescope might get the job done better at about 10% the cost vs. alpha scopes.

And even for those who hike some distance to reach a viewing spot where they will spend some hours, packing in the Maksutov is not burdensome... it is only about 4 lbs. more than an alpha scope of 80mm, and of equal length though 50% fatter.

Last edited by DunninLA : Tuesday 16th April 2019 at 18:56.
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Old Tuesday 16th April 2019, 20:57   #13
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Hi DunninLA,
Good to read your report.
You seem to have a good example of the 127mm Maksutov.

I am not surprised at all that you can use 160x on a sunny morning with good Seeing.
Is the magnification actually nearer 180x if you are using a 45 degree prism?
This is with the large central obstruction and maybe a non phase coated prism.
But it does need bright sunshine, although cloudy bright might also work.

At 3 a.m. even better results might be possible with a very well lit street sign, or if you can find a large back lit clock face, preferably high up.

The spotting scope adds waterproofing and lighter weight, and probably is more robust, but more expensive.

Maybe one day something bigger might come your way.

Regards,
B.
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Old Tuesday 16th April 2019, 22:09   #14
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Hi,

first of all congratulations to the o.p. to what seems to be a good example of chinese made Maksutov-Cassegrain. Also the diagnosis seems to be a mild case of aperture fever - be careful to hold it at bay - otherwise you might find yourself observing with a 24" dobsonian soon.

Some statements like any scope and eyepiece combination which results in an exit pupil below 2mm can not be sharp due to physics seem quite confused though.
As a matter of fact the resolution of perfect instruments depends on aperture (and the diameter of a possible central obstruction as in your Mak) and not on exit pupil size.
The perceived brightness of the image does indeed depend on the exit pupil size, but in bright sunlight 1mm exit pupil is perfectly usable in my experience - 0.7mm also in a pinch. At night on planets or the moon the sky is the limit - 0.5mm (which equates to the common 50x per inch of aperture rule) is often deemed the maximum useful but people have been using more.

Joachim, whose 77mm fluorite spotter is very sharp and sufficiently bright at 52x even at dusk - 90x in good light is also usable.
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Old Sunday 21st April 2019, 00:44   #15
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Hi DunninLA,

I am not surprised at all that you can use 160x on a sunny morning with good Seeing.
Is the magnification actually nearer 180x if you are using a 45 degree prism?
B.
Yes, actually I was going by memory but you are right... per a couple of posts from posters who took measurements, with at 45* diagonal (I am now using the Williams Optics version), the FL is about 1650, so you're right, the magnification with the 9mm eyepiece is 183x.

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Old Sunday 21st April 2019, 09:32   #16
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Because the focal length of the eyepiece will not be exactly 9mm and the focal length of the scope not exactly 1650mm, I would round the magnification to 180x.
This is probably the sweet spot for best terrestrial conditions.
It is possible that on a good night for high elevation astro objects the scope could take 250x well using a good star diagonal or straight through, no diagonal, but this depends on how good the Maksutov actually is.
A 6 inch Quantam Maksutov has been used at 400x on Mars, but this was a really top scope, probably with a smallish central obstruction. Probably straight through, no diagonal.

As to the success on four mornings at 180x, reading road sign at 3 miles. Also seen at 87x.
Good observer.
Good eyes.
Good telescope.
Good location.
Elevated location.
Good local Seeing.
Good terrain.
Good conditions.
Good weather assessment.
A settled morning observation.
Sunshine.
Sun position.
Spring season.
Patience.

So if one chooses the time and place then high magnification can be successful.
But if general bird watching, it may be that the birds determine the time and location and only modest magnification can be used.

Regards,
B.

Last edited by Binastro : Sunday 21st April 2019 at 09:41.
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