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Equipment Discussions >> Cats & Casses

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Cepheus Elf
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Reged: 08/01/10

Loc: Rainy, Cloudy Lancashire UK
Why 7" for Maks?? new
      #5628625 - 01/17/13 06:27 PM

I like my 5" Mak and use it a lot so I'm thinking of maybe getting a bigger Mak and letting go of my c9.25, which has got me thinking about possible options and why 7" seems to be the popular size for larger Maks. Is there a particular reason for this? Or is it maybe because Questar set the 7" standard and other manufactures followed suit? I suppose an 8" Mak would be my ideal, but the options are limited at this aperture....any thoughts????

Cheers,
Mick


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junomike
Carpal Tunnel
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Reged: 09/07/09

Loc: Ontario
Re: Why 7" for Maks?? new [Re: Cepheus Elf]
      #5628634 - 01/17/13 06:32 PM

7" is the largest for most people due to cost and weight. Once you hit the 8" Mak barrier a CG-5 class Mount is no longer an option as the weight almost doubles.

That being said, I'd think your C9.25 would be close in performance to a 7" Mak, so swapping the SCT for the Mak might be a lateral type of move.

Mike


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JohnH
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Reged: 10/04/05

Loc: Squamish BC Moved!!!!!
Re: Why 7" for Maks?? new [Re: junomike]
      #5628635 - 01/17/13 06:34 PM

I would agree, being the owner of an 8" Intes Mak/Cass.

That extra weight means I absolutely need the biggest mount I have to get useful viewing done.


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titanio
sage


Reged: 02/15/09

Loc: Alicante, Spain
Re: Why 7" for Maks?? new [Re: JohnH]
      #5628805 - 01/17/13 08:13 PM

If you make your own tube and make several changes, it will be something different, 8 inches Mack is a very nice telescope.

Toni


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Why 7" for Maks?? new [Re: titanio]
      #5628987 - 01/17/13 10:13 PM

Weight might have something to do with it, but I think the more likely reason is higher order SA. It increases exponentially with the aperture. Anything over 6" to 7" begins to require an aspheric surface or a longer focal ratio. The former just costs more and the latter becomes less desirable. I think this is the reason the Meade 7" uses an elliptical primary and the Orion uses f/15.

http://www.telescope-optics.net/maksutov_cassegrain_telescope.htm


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maknewtnut
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Reged: 10/08/06

Re: Why 7" for Maks?? new [Re: Cepheus Elf]
      #5629054 - 01/17/13 10:50 PM

Great question!...or perhaps making a good point.

I became, for lack of a better term, a fan of that aperture point over the past decade or so. When asked for an opinion on the matter, I've often made the same point other contributors have offered to this point. More punch than a 6", without the heft of an 8". To expand on that, many so called 7" Maks are actually 7.1" in aperture (180mm).

Much of that derives from a long love affair with Intes Micro Maks, where design and build tends to make a bit of leap from 150-180mm models as compared to 203mm models.

The aperture point where planetary observation might be characterized as 'getting serious' varies from person to person. IMHO, that point is 180mm. Whether a person might fill in that blank with 6-10", 8-12", or maybe even 14" and up...it's difficult to argue that those apertures on the lower end of any range will often provide the potential to come the closest to their potential performance at high mag (given a high baseline across the board optical quality)....ie, consistency.

FWIW, I 'let go' of a C8 with decent SCT optics with the hopes the consistent performance of a 5" Mak could be parlayed in a 7.1" Mak. That bet paid off in spades for me, although both were MakNewts. Depending on average local conditions, observing habits, and other factors; some Cassegrain variants can be more demanding than Newtonian variants when it comes to consistency.

What is it about the C9 you're not all that fond of?


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Why 7" for Maks?? new [Re: maknewtnut]
      #5629078 - 01/17/13 11:02 PM

I figured this: Meade bought the machinery from questar for the 7s including some glass as it was at a better price than fabricating it anew. I could be way wrong but that was my feeling. I never felt a 7" Mak nor a 90mm was coincedence. Next I recall Dennis Dicicco in saying that over 7" and thermal cooling became problematic. Not nsurmountable but problematic.

Pete


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freestar8n
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Reged: 10/12/07

Re: Why 7" for Maks?? [Re: Cepheus Elf]
      #5629309 - 01/18/13 03:25 AM

In the case of the Meade 7", which I happen to own, I think a driving force is that it let them use the same size tube and primary as the 8" sct. The meniscus is an expensive part, and the savings on a 7" vs. 8" meniscus, plus the ability to use common parts with the 8" sct, would have been driving factors. In the Meade the entrance pupil is at the front so the mirror has to be larger. This is the way it is in I think all the higher end maks.

As for why Questar went to 7" - I guess it's partly because it is twice 3.5, the size of their other one. When Meade made one later - I assume it was just coincidence that 7" worked well for them - and also reminded people of the 7" Questar.

That's my take, anyway.

Frank


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why 7" for Maks?? new [Re: freestar8n]
      #5629589 - 01/18/13 10:01 AM

I'm not sure but I LOVE my Meade 7"
I'll sell my 12" Meade LX200 with great optics before the 7!!
Could be cooling or like the refractors, price of blanks over 7-8"
The Questar tie is probable, I'd never heard of that, I sure wish the never discontinued the larger ETX Maks.. That being said I love my 127 NexStar SLT!!! Perfect Grab&Go
I sold a perfectly good 6" se instead of it!


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frebieAdministrator
Chief assistant to the assistant chief


Reged: 04/29/08

Loc: Neither here nor there.
Re: Why 7" for Maks?? new [Re: azure1961p]
      #5629624 - 01/18/13 10:22 AM

Quote:

I figured this: Meade bought the machinery from questar for the 7s including some glass as it was at a better price than fabricating it anew. Pete




Questar does not make their own optics. They are made for Questar by Cumberland Optical in Maryland.


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bierbelly
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Reged: 01/23/04

Loc: Sterling, VA
Re: Why 7" for Maks?? new [Re: frebie]
      #5629670 - 01/18/13 10:49 AM

Re Questar, my expectation is that they were making them to a military spec, and commercial sales merely followed suit.

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Why 7" for Maks?? new [Re: orion61]
      #5629686 - 01/18/13 10:56 AM

The argument for Meade using it's existing parts makes sense. But, as mentioned above, larger than 6" MCTs require an aspheric (expensive) term or longer (undesirable) focal ratio. Higher order SA is exponentially proportional to the meniscus surface radius (as I understand it, that changes with aperture and focal ratio. I don't believe you can simply scale up a 90mm MCT to a 250mm MCT.)

I think this is why we see most MCTs in the 90 to 127mm range, with a few special companies making them larger (and more expensive in apertures above 7", or so.)

"Since the aberration contribution of the corrector is (approximately) inversely proportional to the forth power of R1...While it varies somewhat with the specifics of corrector, acceptable optical quality in this respect doesn't extend significantly beyond a 6" ƒ/2.5 primary level."

http://www.telescope-optics.net/maksutov_cassegrain_telescope.htm

"Higher-order spherical is more of a problem in larger apochromatic refractors and, particularly, in instruments using strongly curved meniscus correctors. As the wavefront error of 6th order aberration increases inversely to the 5th power of surface radius, meniscus corrector can generate enormous amount of higher spherical aberration, that cannot be minimized to an acceptable level. This limits relative apertures of mirrors used in these instruments to ~ƒ/3, or slower, even in moderate to small apertures."

http://www.telescope-optics.net/higher_order_spherical_aberration.htm

One question we might ask is why there are not more MCT's on the market in the 6 to 8" range. Too expensive? Bad rep for cooling? Too much competition from SCT? They are fine scopes, I am happy to have gotten one while it was available and relatively inexpensive.


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Why 7" for Maks?? new [Re: Asbytec]
      #5629702 - 01/18/13 11:04 AM

Some time ago I saw an 8" Mak from Bosma, but never
have seen any images taken with it.
Pricewise it was very apealing to me, but nowadays they
seem to have gone.
Would this 8" Mak be an alternative?


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Eddgie
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Reged: 02/01/06

Re: Why 7" for Maks?? new [Re: Cepheus Elf]
      #5629774 - 01/18/13 11:38 AM

Intes Micro makes a variety of larger MCTs.

How many choices do you need?

I suspect that the real question is more like "but at the price I am willing to pay, the options are limited."

So, which option do you want? The very expensive f/10 809, or the very expensive f/15 815?

And it gets better. These are available in both "Standard" and "Deluxe" models. While the optics in the Deluxe models are a bit better, I promise you will not be disappointed with the Standard model. No one will be able to tell the difference at the eyepeices between the Standard and Deluxe.

So, there you go. Four different options.

But again, my guess is that you really want a less expensive option.

And if that is the case, then the answer is that there is not enough demand to make it worth Synta or someone else to produce them in the quantaties they would need. These are volume business companies and a limited production runs will hurt their bottom line. Better to build 1000 8" SCTs and make more in volume than build 100 MCTs that while can command a higher price, will cost more to make and take longer to sell.

It is the f/15 that hurts. I can get a bigger true field out of a C11 that will cost less and only be a bit heavier (24 lbs vs 27.5 lbs for the C11)
.


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ColoHank
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Reged: 06/07/07

Loc: western Colorado
Re: Why 7" for Maks?? new [Re: Eddgie]
      #5629874 - 01/18/13 12:25 PM

Questar made a limited number of 12" Maks, one of which, I understand, has been and perhaps still is used to track launches at Cape Canaveral.

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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why 7" for Maks?? new [Re: ColoHank]
      #5630126 - 01/18/13 03:36 PM

Boy would I love to have that Q 12"...If I could lift it!
I am wondering why the market has set the Price of a used Meade 7" with LX200 mount at around $1100.00-1200??
I'd never sell mine for that!
Perhaps it is like anything recently discontinued, people wake up about 5-10 years later and we see prices going back up again. The only beef I have with the 7 is mirror shift.
I am wondering if anybody has taken the Spring that assists pushing the mirror up out?
I could buy a rear cell focuser but then you start messing with the mirror Meniscus spacing, also it doesn address mirror flop from Horizon to Horizon, it actually effects finr collimation a bit.. a tiny bit, but I'm OCD about proper alignment, I guess thats why I have gone through about 30 SCT's keeping the best one untill The one I have that was owned by Leo Henzl is better than any I've had..


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EddWen
Pooh-Bah


Reged: 04/26/08

Re: Why 7" for Maks?? new [Re: ColoHank]
      #5630168 - 01/18/13 04:03 PM Attachment (47 downloads)

Sometimes big Maks are confused with hot water heaters...

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Yu Gu
member


Reged: 06/18/06

Re: Why 7" for Maks?? new [Re: EddWen]
      #5630180 - 01/18/13 04:08 PM

but they are so cool!

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Cepheus Elf
member


Reged: 08/01/10

Loc: Rainy, Cloudy Lancashire UK
Re: Why 7" for Maks?? new [Re: Yu Gu]
      #5630348 - 01/18/13 06:15 PM

Many thanks to all for some excellent answers to my question.
I suppose my question should really have been ….“as 8” is an almost “institutionally” recognised aperture standard, why don’t the telescope “mass producers” make an 8” Mak?

In reply to Mark, My C9.25 is a good grey tube USA made OTA and it star tests very well, but it’s at the upper end of what I would call portable, so something a little less bulky, but with a similar performance, would be a better option for me now. There is an excellent shoot out article between a Meade 7” Mak and a C9.25 and the C9.25 won. However, I reckon it would have been a closer contest if the Mak had been an 8” and I think a 8” Mak would be easier to manoeuvre than a C9.25 (weight is not an issue for me..it’s the bulkiness of the OTA).

I could go for an 8” sct, but as the scope will be purely visual, I like the thought of a Mak’s smaller CO.

Norme makes a very good point about optical manufacturing practicalities, while Eddgie’s point about demand also makes sense…so I guess the answer is a mixture of the two.
Some years ago, Telescope Service (Germany) sold 8” Maks, but they don’t seem to stock them any more and it looks like Orion Optics UK are only making their OMC Maks in 140mm aperture now, so the only option for me seems to be the Intes Micro scopes which Eddgie mentions. I'm gonna have a close look at those!
Thanks again to all for some very interesting answers!

Mick


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Cepheus Elf
member


Reged: 08/01/10

Loc: Rainy, Cloudy Lancashire UK
Re: Why 7" for Maks?? new [Re: EddWen]
      #5630363 - 01/18/13 06:25 PM

Quote:

Sometimes big Maks are confused with hot water heaters...




Now that is what you call a Big Mak!!!


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