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New Celestron 7" Mak

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#26 jjack's

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Posted 07 January 2019 - 04:55 AM

Maybe this kind of scope >f/20 is not on the market because there is no market for that ?


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#27 Bill Barlow

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Posted 07 January 2019 - 10:54 AM

I would have liked to see an f/12 version.

 

Bill


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#28 Steve Cox

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Posted 08 January 2019 - 06:23 PM

I've been looking at the OTA of the Orion version for a number of years for use as a planetary scope, possibly riding tandem next to my C6R on a Twilight 2.  Will be curious to see if Celestron follows Orion and Skywatcher in selling it as an OTA only.



#29 Phil Cowell

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Posted 08 January 2019 - 07:56 PM

10” F12 or F15 would be sweet.



#30 Hugh Peck

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Posted 08 January 2019 - 08:26 PM

Maybe this kind of scope >f/20 is not on the market because there is no market for that ?

There is an 8" f/20 I'd like. 

 

https://www.orionopt...maksutovca.html


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#31 Phil Cowell

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Posted 08 January 2019 - 08:32 PM

There is an 8" f/20 I'd like. 

 

https://www.orionopt...maksutovca.html

After the exchange rate tanks after Brexit, you can probably get one for $100.


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#32 Tyson M

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Posted 08 January 2019 - 10:02 PM

There is an 8" f/20 I'd like. 

 

https://www.orionopt...maksutovca.html

I'd love one too but it seems like Orionoptics is a gamble based on reviews.  Since they are not cheap (yet pre brexit), it may be too much risk for most people to want to take.



#33 luxo II

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Posted 08 January 2019 - 10:23 PM

It would be nice if Celestron could offer the 7" Mak with a fork mount. Perhaps an addition to the current CPC lineup.


Best to keep the OTA separate from the mount. Well cared for the OTA may have a long life - 30 years or more - whereas mounts all seem to be doomed after 10 years or so either due to failure of the electronics/mechanical parts, or because technical advances make a replacement desirable.

A few 9.25” and 11” CPC mounts have been hacked as one-arm fork mounts to take other scopes - notably 7” maks. The conversion is not hard - once you have a CPC.

Edited by luxo II, 08 January 2019 - 10:27 PM.

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#34 Stopforths

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Posted 09 January 2019 - 03:06 PM

I agree re the redesign at f20 I have the f15sw which has 32% obstruction.  25-27sh would be great and make a real difference.

 

My old 715 IM had 27% but seriously vignetted at low power  something the sw doesn't do.

 

The 715 probably had an edge on planets but not so for dso's.        The SW is a good scope still amazing on Jupiter better than my 4 1/2 inch vixen apo and thats saying something.  I've done several side by sides and there is plainly more to see in the sw180 especially in better seeing.


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#35 jjack's

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Posted 12 January 2019 - 07:35 PM

What advantage could have a 7" mak-cass versus a 8" SCT ?

Virtually the same obstruction : 32% vs 35%

F/15 ratio vs F/2- 6.3- 10

heavy vs light.

Less aperture than the SCT.

Same price.

The gregory mak design have also field curvature and coma.

Maybe because collimation is more stable or it have a more beautifull corrector lens...


Edited by jjack's, 12 January 2019 - 07:36 PM.


#36 Kevin Barker

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Posted 12 January 2019 - 08:31 PM

I would have liked to see an f/12 version.

 

Bill

I agree. F 15 Maks have a very narrow fov.

F10 to f12 is better IMHO. F 12 for a 7 inch would give an 84 inch focal length.

 

Yes the Central Obstruction can be reduced with f 15 or longer but the gains are pretty minor.

One can Barlow to achieve longer focal lengths.

I own f 10, f 14, and f15 Maks.  For all round use I like the f 10. For example my IM 603 and 703. Any planetary gains at f 15 are pretty minor.


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#37 Kevin Barker

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Posted 12 January 2019 - 08:43 PM

What advantage could have a 7" mak-cass versus a 8" SCT ?

Virtually the same obstruction : 32% vs 35%

F/15 ratio vs F/2- 6.3- 10

heavy vs light.

Less aperture than the SCT.

Same price.

The gregory mak design have also field curvature and coma.

Maybe because collimation is more stable or it have a more beautifull corrector lens...

If 8 inch sct s can be made with consistent optical quality then they should be better than a 7 inch Mak made to the same wavefront error.

Maks seem to have a reputation or are perceived to be made to a higher optical standard leading to better images at the eyepiece. This perception could well be Wrong?  

I do think there is some truth in the assertion that the high strehl Maks intes micro, Questar, TEC etc do out perform average or poor sample sct s of similar or slightly larger aperture. There are lots of other factors in play here, seeing,  cooling, collimation and  obstruction etc

A well made high quality sct can perform as well as a Mak of similar aperture.


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#38 luxo II

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Posted 12 January 2019 - 10:09 PM

Kevin,

 

Intes (as per their website) guaranteed P-V errors < 1/6 wavelength for their complete scopes and P-V < 1/8 wavelength for the deluxe models. Questar, TEC and AP similarly have defined quality criteria.

 

However, both Celestron and Meade carefully refrain from anything resembling a statement about the optical quality of their SCT's so it should come as no surprise many have P-V errors > ¼ wavelength.

 

Secondly the size of the secondary obstruction does have a significant effect. 

 

The choice of focal ratio for the Gregory-Maksutov design however is not arbitrary, it is a result of trying to find a diffraction-limited solution which can be achieved with all-spherical surfaces, and where there the radius of the secondary mirror matches the radius of the back surface of the meniscus and also its separation from the primary matches that of the corrector to primary spacing. 


Edited by luxo II, 13 January 2019 - 05:17 AM.

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#39 jjack's

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Posted 13 January 2019 - 04:06 AM

Hi Luxo II. Are you shure a 7" mak can reach diffraction limit with all spherical lenses only ?

I had a Meade 7" mak. It has an aspherised primary. Cool, heavy and beautifull look but not better than a perfectly collimated SCT.


Edited by jjack's, 13 January 2019 - 04:11 AM.


#40 luxo II

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Posted 13 January 2019 - 05:19 AM

I would expect all the 7" (or 180mm) Gregory-Maksutovs are slightly aspherized, the aspherization to correct SA perfectly is easily accomplished. 

 

The Rumaks are different, however.


Edited by luxo II, 13 January 2019 - 05:23 AM.


#41 RichA

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Posted 13 January 2019 - 01:23 PM

It's possible but unlikely as they would face the same issues Meade faced 20 some odd years ago unless It would ride stable on the Evo Mount?

Meade only faced them because they cheaped-out and used fork arms that were  too short.  They should have used Meade 10" SCT fork arms and they wouldn't have need an 8lb piece of iron in the back.  The plain OTA didn't have the weight.  But this is all moot as their original 7" was an exotic design and it would have been nice if they'd brought it back.



#42 RichA

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Posted 13 January 2019 - 01:24 PM

Well given the parent company involved "clone" wouldn't be the term I'd use.

 

Specs (size, weight, optics, etc) are essentially the same (given slight rounding often seen) and the extra weight from the larger dovetail bar used on the Celestron.

 

My guess is that this is an easy low risk product for Celestron to introduce to cover any interest in the GSO classical Cassegrains.

Are the GSO's "Classical Cassegrains" or just Dall-Kirkhams?



#43 Magnetic Field

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Posted 13 January 2019 - 01:44 PM

I would have liked to see an f/12 version.

 

Bill

f/10 would have been a sweet spot.



#44 luxo II

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Posted 13 January 2019 - 05:06 PM

f/10 would have been a sweet spot.

On the contrary - F/10 is a horrible compromise - the result is a scope that is trying to be jack-of-all trades but doesn’t excel at any. The resulting CO means the scope won’t excel at planetary, and on the other hand is not a wide-field scope either.

Far better imho to design casegrains to do what they’re good at - planetary, ie f/15 - and have a faster scope such as a 6” RFT around f/5 for wide fields.

Edited by luxo II, 13 January 2019 - 05:09 PM.

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#45 Asbytec

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Posted 13 January 2019 - 05:58 PM

I am not sure what the wavefront error really means with a Mak. It has quite a bit of HSA from the corrector that cannot be eliminated without an asphere (ellipsoid like the Meade.) An SCT has the HSA correction built into the corrector, the MCT does not. But, one can have a large wavefront error up to about 0.4 PV due to HSA and still have a diffraction limited optic with RMS at or better than 1/14 waves. To be a true 1/4 PV would be a fine Mak with a high Strehl ratio as most of that is probably residual HSA which has less impact on the image than pure 1/4 LSA or HSA alone.

 

This is also the reason we find commercial Maks in the 180mm range and smaller and usually at f/15 for larger apertures due to the required magnification of the secondary. The HSA is exponentially proportional to the radius of the steeply curved meniscus. In this sense, they are really no different than a fine APO with it's steeply curved lenses and residual HSA. 

 

https://www.telescop...n_telescope.htm

 

As said above, I am not sure Celestron "makes" anything. Synta produces a certain Mak model and it is rebranded and marketed by Celestron and others. I doubt seriously Synta would redesign any of it's production models to suite a single distributor, but they might to suite the market as a whole so long as the redesign R&D and production are cost effective. Even if the coatings and paint are different, the model is the same as the Orion US and Skywatcher versions. I do not know why, but Maks do seem to be terrific scopes. Probably not better than a well executed SCT, but maybe more consistent in quality with smooth optics and good correction (in my experience). 

 

Nice scope, though, from Celestron. It's probably as good as Intes standard models. Not sure why the Meade 6" GEM version didn't seem to catch on. I wonder if this one will at that price point with a mount. 


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#46 mclewis1

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Posted 13 January 2019 - 06:35 PM

Are the GSO's "Classical Cassegrains" or just Dall-Kirkhams?

The new GSO's are apparently classical Cassegrains.



#47 luxo II

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Posted 13 January 2019 - 08:44 PM

This is also the reason we find commercial Maks in the 180mm range and smaller...

Incorrect. There are no technical obstacles for making maksutovs far larger than this.

 

it's the cost of the glass blank for the corrector which is prohibitively costly for maksutovs > 7" aperture, and the fact that the production volume is so low it cannot sustain a business, as demonstrated by Santel, TEC, AP  and even Zen who made a few but also showed its something they did more out of passion, than as a serious business proposition.

 

Given that Synta owns Celestron, Meade and Skywatcher it's just a marketing decision - in the US the "Skywatcher" brand is virtually unknown for OTAs, but Celestron and Meade are. Hence the decision to brand it as a Celestron product for the US market.  Whereas here Skywatcher has been well established for many years so we see them under that brand.

 

In another decade Synta might rationalise its brands and drop one or two IMHO, there's really no need for three.


Edited by luxo II, 13 January 2019 - 08:54 PM.


#48 skyward_eyes

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Posted 13 January 2019 - 09:00 PM

Given that Synta owns Celestron, Meade and Skywatcher it's just a marketing decision - .


Synta’s home brand is Sky-Watcher and they acquired Celestron. They do not own Meade.

#49 Asbytec

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Posted 13 January 2019 - 09:13 PM

"Incorrect. There are no technical obstacles for making maksutovs far larger than this."

For purely spherical there is. Going much larger requires an asphere to perfom well. In that case, though, no technical obstacles, to my knowledge, when aspheres are applied. Some argue they all require an aspheric term.

#50 luxo II

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Posted 13 January 2019 - 09:16 PM

They do not own Meade.


Oops sorry you’re right - Ningbo Sunny Electronic Co Ltd does.


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