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5" APO verses BEST 7" MAK CASS

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#151 precaud

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Posted 10 May 2019 - 07:57 AM

If I was limited to only one telescope - a 130mm premium refractor or a 180mm premium Mak - I really don't know which one I'd chose.

 

Or... none of the above... add an 8" or 10" dob to the choices and it becomes even harder...



#152 Astrojedi

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Posted 10 May 2019 - 08:45 AM

I’ve seen plenty of 8” SCT that look ok at 200X but there is no way they can usefully go to 400-450X. Yet I know of two 7” maks that regularly handle 400X on moon/planets and the image is fine, side by side with the SCTs concerned. One fellow has bought and sold no less than 8 SCTs trying find a good one to rival those maks.

 

 

This is completely the opposite of my experience. I think the divergence is due to the fact that folks will compare 15-20 year old SCTs with new maks and then say how bad the SCT was.

 

When folks make such blanket statements they are confusing optical design with optical quality. A good quality SCT will more than keep up with any Mak. It is a falsehood that is constantly repeated on these forums. 

 

Over the past 20+ years I have owned and /or looked through 30+ commercial SCTs of many different vintages. The quality has become a lot more consistent in the last 10 years at least from Celestron. I usually don’t even touch older SCTs due to the large variation in quality. The newer EdgeHDs are even more consistently high quality rivaling premium optics.

 

All my SCTs have no issues with 400-500x. My C9.25 is just getting started at 400x. In good seeing it quite easily goes to 700-800x on the moon.


Edited by Astrojedi, 10 May 2019 - 08:50 AM.


#153 Bomber Bob

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Posted 10 May 2019 - 09:21 AM

add an 8" or 10" dob to the choices and it becomes even harder

 

One of my points to the OP was Go Big with the Mak.  The advantage of CATs is getting that large aperture in a relatively compact & lightweight OTA.  He may have been thinking of the Meade 7" MCT, which with its ballast weight at the focuser sorta negates that advantage -- unless the owner removes it.

 

My VMC200L is crazy light.  It's also open tube (the field corrector lens is in front of the secondary mirror), and less likely to fog up than standard CATs.  Yes, an open tube Newt is another option, but he asked about Maks.  I briefly owned a 12.5" Dob, and I don't care for the eyepiece placement.  But the views... Popped.  An MCT, SCT, Cassegrain, or variant like my VMC has that refractor-like focuser position.  Regardless of the scope type, if the high-power views are good, I'm going to sit there for a while observing (and maybe sketching), and I want to be comfortable doing it.


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#154 Asbytec

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Posted 10 May 2019 - 09:33 AM

Sketching is enjoyable visual observing. We can actually share our views. I used to think of sketching as a poor man's imaging. Oh, it's so much more than that. To me, sketching IS observing. You absolutely have to observe to sketch. So, yes, get comfortable, you're gonna be there for a while. :)
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#155 elwaine

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Posted 10 May 2019 - 09:58 AM

Apples to oranges .

A 7" anything will out perform anything that's only 5".

Will go deeper photographically and visually.
Will show more detail.
Everything will be brighter.

Hmm. I disagree with your conclusion.

 

”Will go deeper photographically and visually.” Agree.

 

“Everything will be brighter.” Caveat: true at equal magnifications, but not at equal exit pupils.

 

”Will show more detail.” Not always true. FOR VISUAL, if the 5” is unobstructed and the 7” is obstructed (which is the case under discussion in this thread) it depends on the size (%) of the central obstruction in the 7” and on the target under observation (high or low surface contrast). For photography, the 7” - even obstructed - will show more detail in the final processed image (if done correctly), because image contrast can be enhanced using software.


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#156 Mitrovarr

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Posted 10 May 2019 - 11:23 AM

I'm not entirely sure all 7" designs beat all 5" designs at light gathering, either. Between the light loss due to obstruction and the reflectance losses due to two more mirrors, that might be enough to lose the aperture advantage.

#157 Magnetic Field

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Posted 10 May 2019 - 12:08 PM

I'm not entirely sure all 7" designs beat all 5" designs at light gathering, either. Between the light loss due to obstruction and the reflectance losses due to two more mirrors, that might be enough to lose the aperture advantage.

The following is pure theory (in the end of the day it this that counts what the observer thinks looks best in his focus).

 

1. You would probably have to assume very bad mirror coatings. For a mirror coating of 0.9 and lets assume 0.9 light loss of the corrector and another 0.9 for the secondary.

 

For a 7" (175mm and say 33% obstruction)

 

sqrt((175**2-(175*0.33)**2)*0.9*0.9*0.9) = 140 mm

 

2. Then you also have to assume light loss of a triplet or doublet 5".

 

I don't know maybe the light loss of each objective is say 0.95:

 

sqrt(125*125*0.95*0.95) = 120 mm

 

3. Even for coating losses of 0.8 you still get 120 mm aperture for your 7" Maksutov:

 

sqrt((175**2-(175*0.33)**2)*0.8*0.8*0.8) = 120

 

And you always end up with much better resolution capabilities of a 7" Maksutov.

 

 

4. And don't forget the MTF is not a static thing. People would really help the MTF discussions by realising that the MTF has different scales on the x-axis:

 

  For example post #212 in the following thread: https://www.cloudyni...et-scope/page-9



#158 Mitrovarr

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Posted 10 May 2019 - 01:15 PM

I hear that 0.88 reflectance for mirrors is a good estimate for most (since they drop over time) and transmission losses on lenses with modern coatings is very low, so 0.98 transmission is a good value (this is true for both the refractor lens and the corrector plate).

Still puts the maksutov ahead. I wasn't really expecting that.

#159 Magnetic Field

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Posted 10 May 2019 - 02:03 PM

I hear that 0.88 reflectance for mirrors is a good estimate for most (since they drop over time) and transmission losses on lenses with modern coatings is very low, so 0.98 transmission is a good value (this is true for both the refractor lens and the corrector plate).

Still puts the maksutov ahead. I wasn't really expecting that.

People always say the light loss effect of the obstruction is negligible. This may be true in the grand scheme of things because a 33% obstruction on a 175mm Maksutov will still give an effective diameter of 165mm: sqrt(175**2 - (175*0.33)**2) = 165mm.

 

The culprit is the loss of reflection due to mirror coatings.

 

A reduction of the light bucket to 140mm from 175mm is still quite a lot due to  mirror coatings.



#160 drprovi57

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Posted 10 May 2019 - 03:09 PM

I have owned many telescopes, to include refractors (e.g., 130mm and 178mm TMB scopes, really beauties) - but my Questar 7" Astro provides the best views of the planets, moon, and sun - given my skies.  The Q7 Astro has a 190mm primary, with a 178mm corrector.  The secondary obstruction is about 26% - with a focal length of about f/14.  Its compact size makes it a great addition to my large AG Optical 17" iDK - both mounted together on a Paramount ME 2.  It has been my dream setup for sometime.

 

JPEG image.jpg

 

IMG_0030.jpg


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#161 fred1871

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Posted 10 May 2019 - 09:35 PM

Er no... that’s a gross oversimplification that ignores the MTF of the optics.

I’ve seen plenty of 8” SCT that look ok at 200X but there is no way they can usefully go to 400-450X. Yet I know of two 7” maks that regularly handle 400X on moon/planets and the image is fine, side by side with the SCTs concerned. One fellow has bought and sold no less than 8 SCTs trying find a good one to rival those maks.

Likewise I’ve tried a Mewlon 250 last Saturday night ... owner thought it was doing well at 300X on Jupiter. I just didn’t have the heart to suggest he look through my MK91 just a few yards away, which was at 650X.

I’m beginning to think I need a bodyguard when I take the MK91 for a club meet.

Appears to be a way of saying "rah, rah, wonderful, best of all", about the Santel.

 

I've seen a couple of examples of C8 that took 400x without difficulty. I owned one of them. My current 2004 C9.25 XLT gives very nice images at 470x, and on the closest double stars is still good at 670x. As 670x is an exit pupil of 0.35mm, I'm not surprised that I find Jupiter does somewhat better with less power due to contrast loss with such a small exit pupil. 

 

I also have a more recent Mewlon 210, with sharper optics than the C9.25. Very nice on Luna at 480x, exit pupil 0.44mm; but I prefer Jupiter at a bit lower power, again for contrast benefit. You could do worse than read Texereau on the matter of exit pupil size for planetary observing. For double stars it's a different story. 

 

Perhaps the Mewlon 250 you're dismissive of was not properly collimated? or not yet thermally settled? or looking over a car park or bodies emitting heat? A lot of factors go into final image quality.

 

I've no doubt the Santel is excellent, but strongly suspect it's not peerless.



#162 Asbytec

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Posted 10 May 2019 - 11:16 PM

I agree with Fred above. When folks say their scopes can do ludicrous magnification, well they certainly can. Even in adverse seeing if we want to put up with it. But, it helps to know which objects we're observing up that high and what one means by "image breakdown" or lack thereof.

 

At some point we'll begin to resolve obstruction and aberration effects in the afocal image (focused on our retina) with increased magnification and the image "breaks down". Also true with seeing and thermal effects, which is why some folks say seeing limited them to some arbitrary low magnification. Above that we begin to lose scotopic resolution on the eye at smaller exit pupils on select targets and they "break down" because of it. Double stars are not one of the latter and can take much higher magnification, but aberration and obstruction effects can affect what we see. The image "breaks down", or not, compared to a quality unobstructed optic. Galaxies are another story, but not all deep sky suffer with magnification. Planetaries can take magnification but planets, per se, are a mixed bag. Some of them break down because of poor or obstructed optics or seeing, while others break down because of the dim quality image presented on our retina at very small relative apertures like f/60, for example. Jupiter's afocal image is just not that bright (sic) on the eye at 0.3mm f/60 when magnified by a given aperture say, 800x through a 250mm aperture.

 

To my mind, magnification is all about balance between image scale and surface brightness (for extended objects like galaxies and planets alike), or in some cases framing an object in a star field. Find the right magnification and the image becomes productive in terms of detail visible up to the limit of the telescope's resolution and our acuity for such detail. That varies with the object type, I would never(?) observe a galaxy or Jupiter at 100x/per inch even through we observe a star's diffraction artifact without any problem. Any scope can do 100x/inch, but what is to be seen up that high depends on the object's properties as imaged by the telescope. 

 

As was mentioned above, we absolutely have to consider the observer in the equation. After all, we are the "sensor". We are not inanimate objects like a telescope with it's imaging properties set by the laws of physics and the wobbly stack. We are active observers with the ability to interpret what we see despite it all. We have the ability to learn to see and some inherent physiology allowing us to see right down to the limits of our imagination and visual noise. If we dare, of course. So, there is some variability with observers, too.


Edited by Asbytec, 10 May 2019 - 11:19 PM.

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#163 luxo II

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Posted 11 May 2019 - 05:31 AM

Fred, of course you're right.

 

All I am saying is that for given aperture the effectiveness of a design optimised for high power is underestimated (ie long focal ratio f/13 or more, high quality optics (1/8 wave  or better) and a small central obstruction), vs a-jack-of-all-trades- but best-at-nothing solution which Celestron/Meade adopted, ie f/10, a large CO and mediocre optics.

 

Of course this MK91 can be bettered, but it will take something non-trivial to do that, for example an AP 10" mak, the APM 11" f/14 recently on AM (one of just 6 made), or a premium 8" APO (haven't seen one outside an observatory). But these scopes are all unicorns - like the MK91 they're urban legends - few have ever seen one, fewer have looked through one, and even fewer own one.

 

I live in a big city and I take the MK91 to club nights (it's my only serious scope), where all the scopes are inherently constrained by having to be transportable, and there are typically 10-15 scopes on the field - a fairly predictable mix of small refractors doing imaging, SCT's and dobs, plus a handful of exotica.

 

A scope of significantly greater aperture with a larger CO and average optics might do better, but I can say after comparing several C9.25's,  10" Meades and C11's - these scopes cannot. A Meade 16" or C14 might, but they're not exactly transportable and no-one in the clubs I belong to has one. Of two Mewlon 250's, one wasn't close, and another was possibly close but I wasn't able to compare that side-by-side. I also have an opportunity to compare it to a VMC 260 soon.

 

There is one other scope in my country - if it has survived - that I easily acknowledge as superior - a 32 cm tri-schiefspiegler built by Barry Adcock in the late 1980's. I have seen images of mars from that scope and they were simply awesome by any standards. But... that was not transportable.


Edited by luxo II, 11 May 2019 - 08:57 AM.


#164 coz

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Posted 11 May 2019 - 09:23 AM

Great looking setup!  I didn't realize the Q7 obstruction was 26%. I thought it was more like the Q3.5 north of 30.

Best

 

 

I have owned many telescopes, to include refractors (e.g., 130mm and 178mm TMB scopes, really beauties) - but my Questar 7" Astro provides the best views of the planets, moon, and sun - given my skies.  The Q7 Astro has a 190mm primary, with a 178mm corrector.  The secondary obstruction is about 26% - with a focal length of about f/14.  Its compact size makes it a great addition to my large AG Optical 17" iDK - both mounted together on a Paramount ME 2.  It has been my dream setup for sometime.

 

attachicon.gif JPEG image.jpg

 

attachicon.gif IMG_0030.jpg



#165 drprovi57

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Posted 11 May 2019 - 01:11 PM

Great looking setup!  I didn't realize the Q7 obstruction was 26%. I thought it was more like the Q3.5 north of 30.

Best

You are correct that the Q3.5 has an obstruction of about 30%.  However, the Q7 has an obstruction closer to 26% based on Questar specifications.  I have a Questar Q3.5 50th edition that provides exceptional moon and planet views - however, also pricy.  There are less costly Maks that provide exceptional planetary views.


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#166 Erik Bakker

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Posted 11 May 2019 - 04:11 PM

I think the Questar 7 has around 32% CO. Mine had. AND stunning quality optics, so the end result was fantastic images, though seeing and thermal equilibrium both needed to cooperate.

 

Just had another look at the Q7 specifications per their website, where Questar shows an MTF curve of the Q7 related to a 0.34 obscuration ratio.

 

Very early Q 3 1/2's had a smaller obscuration ratio, but the CO was later enlarged leading to a similar 32% CO. Questar nowadays species per their website:

 

TYPE: Maksutov Cassegrain Catadioptric. No coma, astigmatism or spherical aberrations.
CLEAR APERTURE: 3.5 inches, 89mm (Center Obscuration, 27.9mm)

 

That would mean 31.3% CO.

 

The Q7 is a scaled design with similar CO.

 

I think no current Questar has a CO under 30%.



#167 Magnetic Field

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Posted 11 May 2019 - 04:26 PM

I think the Questar 7 has around 32% CO. Mine had. AND stunning quality optics, so the end result was fantastic images, though seeing and thermal equilibrium both needed to cooperate.

 

Just had another look at the Q7 specifications per their website, where Questar shows an MTF curve of the Q7 related to a 0.34 obscuration ratio.

 

Very early Q 3 1/2's had a smaller obscuration ratio, but the CO was later enlarged leading to a similar 32% CO. Questar nowadays species per their website:

 

TYPE: Maksutov Cassegrain Catadioptric. No coma, astigmatism or spherical aberrations.
CLEAR APERTURE: 3.5 inches, 89mm (Center Obscuration, 27.9mm)

 

That would mean 31.3% CO.

 

The Q7 is a scaled design with similar CO.

 

I think no current Questar has a CO under 30%.

Often baffling tubes of the secondary render the "true" obstruction slightly larger.


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#168 Erik Bakker

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Posted 11 May 2019 - 04:35 PM

My guess is that effectively, Questars have around 33-34% CO. Their optical quality helps them minimize the negative effects of this. Wonderful scopes IME.



#169 coz

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Posted 11 May 2019 - 09:04 PM

Hi, I’m fortunate to have a Q50 edition as well and love it. I was always tempted by the Q7 though.... 

 

best 

You are correct that the Q3.5 has an obstruction of about 30%.  However, the Q7 has an obstruction closer to 26% based on Questar specifications.  I have a Questar Q3.5 50th edition that provides exceptional moon and planet views - however, also pricy.  There are less costly Maks that provide exceptional planetary views.



#170 coz

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Posted 11 May 2019 - 09:09 PM

Hi Erik, I always assume the Q7 was the scaled design but looking at the spec sheet I see 47mm as the secondary measurement which gets us to 26% area as drprovi57 mentioned? Or am I missing something? Close to the AP mak Cass of 23% if so. 

I think the Questar 7 has around 32% CO. Mine had. AND stunning quality optics, so the end result was fantastic images, though seeing and thermal equilibrium both needed to cooperate.

 

Just had another look at the Q7 specifications per their website, where Questar shows an MTF curve of the Q7 related to a 0.34 obscuration ratio.

 

Very early Q 3 1/2's had a smaller obscuration ratio, but the CO was later enlarged leading to a similar 32% CO. Questar nowadays species per their website:

 

TYPE: Maksutov Cassegrain Catadioptric. No coma, astigmatism or spherical aberrations.
CLEAR APERTURE: 3.5 inches, 89mm (Center Obscuration, 27.9mm)

 

That would mean 31.3% CO.

 

The Q7 is a scaled design with similar CO.

 

I think no current Questar has a CO under 30%.



#171 fred1871

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Posted 11 May 2019 - 10:27 PM

Hi Erik, I always assume the Q7 was the scaled design but looking at the spec sheet I see 47mm as the secondary measurement which gets us to 26% area as drprovi57 mentioned? Or am I missing something? Close to the AP mak Cass of 23% if so. 

CO is normally quoted, as in the various comments above, by DIAMETER, not AREA.

 

A 26% CO by area would be very, very large - more than 60% of the optical diameter (aperture) of the telescope.



#172 Peter Besenbruch

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Posted 12 May 2019 - 03:14 AM

The 180mm Synta Maksutovs advertise a central obstruction in the mid 20s, but by the time you add in the secondary baffle, you are up to just over 30%.


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#173 drprovi57

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Posted 12 May 2019 - 12:31 PM

I have attached a photo of the specifications sheet for the Q7 - that shows an obstruction of 26% for the Q7 - 46mm/178mm=0.26 or 26%.

 

IMG_0106.jpg



#174 elwaine

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Posted 12 May 2019 - 03:13 PM

Unlike the Meade ETX line, which uses a baffle immediately in back of the secondary mirror (which results in an actual CO that is larger than that produced by the diameter of the secondary mirror by itself), Questar does not appear to use a similar baffle behind the secondary; so the actual CO in Questar equals the diameter of the secondary spot ÷ the diameter of meniscus. See here for cutaway views on a Q3.5.


Edited by elwaine, 12 May 2019 - 03:16 PM.


#175 Erik Bakker

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Posted 12 May 2019 - 03:40 PM

The easiest way to solve this is to measure the secondary obstruction. 26% is on the low side IME.

Focal length in my Q7 was 2840mm by the way.




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