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7" Mak versus an 8" SCT

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#76 Asbytec

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Posted 10 August 2014 - 09:52 PM

I'd say we have the answer in the real life data, showing that MCTs have on the average better made optics. 

 

Vla

 

 

 

Beyond any design differences, I am under no illusion observing conditions play a huge role in how much we appreciate our instrument (any instrument). No doubt observing conditions greatly influences how we feel about the design we have. The MCT has stood up amazingly well, as surely any descent scope would.

 

We really get to see what it can do in real world lab-like conditions with variables controlled or otherwise favorable, cooled to ambient, collimated, clean, and with calm and clear air above. If you want to push the scope to the limits of it's theoretical and real world performance (which actually appear to be very similar), this is the time and place to do it. In the tropics, we experience almost nightly those excellent nights others crow about when their own scopes preformed at their best - those nights when their jaw dropped. My current MCT drops my jaw almost nightly.

 

Those are the nights when you can take the time to observe any differences in the end product design and production. The MCT may be better than another design in one way or another, or it may not. One might imagine it would be more difficult to observe differences in 2" arc seeing except during the fleeting best moments. We need to get both mass produced samples into a lab or in lab-like conditions to really appreciate soft contrast or high resolution capabilities of each.

 

However, if ray tracing and theory can be approximated in real world observing, then maybe speculation on production, star testing, and understanding each design can tell us what to expect. All that remains is to observe it. I dunno, I have become a believer in theory (having kind of seen it in action) observing planets, lunar, and double stars under lab like conditions. When the atmosphere allows a clear split on an otherwise more difficult Dawes double or high contrast extended object resolution down to about Lambda/D, something good is happening.

 

Maybe that something good is an appreciation or inspired love of the design that accomplished such a feat. Nothing changes the fact the design put up that jaw dropping image when observing conditions allowed it to.



#77 freestar8n

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Posted 11 August 2014 - 02:54 AM

Hi Norme-

 

If your 150mm MCT is the f/12 version, then it most certainly has an aspheric primary - presumably ellipsoid like my Meade.  Like most Mak providers, Orion doesn't specify what surfaces it has - just that it is a maksutov.  Meade does, however, indicate the primary is ellipsoid in the manual, and I have confirmed this.

 

Maksutov himself only had crude ability to do ray traces, and even with this limited ability he describes in his 1944 paper that amateurs who want faster and more compact instruments will need to retouch and autocollimate - and he provides Figure 18 depicting a spot Maksutov in an autocollimator arrangement set up for knife edge testing to produce a null - which is your exact system described 70 years ago.

 

So if your current 'scope works well that is great, but if it shows any residual spherical aberration - that just means it wasn't fully figured in the final stages.  The impact of the residual error may be negligible - but there is no reason it needs to be there, and your system most likely is not a spherical spot Mak in the first place.

 

Intes also doesn't describe in detail the actual curves on the mirrors - but they do describe final figuring by a master optician to remove any zonal irregularities.  So if a star test does reveal such irregularities in one of their instruments - again that isn't something inherent in the design and it goes directly against their claim of fully figuring the optics in an autocollimator.

 

So there is more to telescope performance than just the design - and tolerances are critical, especially when mass produced at low cost.  Maksutov himself didn't have a good handle on the tolerances, and even Gregory only had simple calculators for his work.  But people making the 'scopes learned quickly that the performance was not as predicted - even when making good spheres to the precise radii.  

 

Just as ray tracing and "theory" don't capture all that is involved in making a telescope that performs well - they also won't capture all the intangibles involved in how pleasing the view appears - in terms of contrast and so forth.  So if you find that maks provide a more pleasing and contrasty view somehow compared to sct's - that may well be true - but it also may have nothing to do with what shows in a ray trace or an MTF curve.  I prefer to go by things that can be empirically demonstrated in an actual telescope under the skies - and from what I have seen my 7" Meade is basically diffraction limited - and would be beaten by a diffraction limited 8" sct.  But that still doesn't mean the view isn't somehow more pleasing in a mak - at least for some people.

 

Frank



#78 Asbytec

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Posted 11 August 2014 - 05:54 AM

Hi Frank,

 

With careful star drifting, the focal length is actually closer to 1950mm or f/13. Still...

 

And that's the thing, it does show under correction. Star testing reveals nothing remotely similar to 1/4th LSA, and is amazingly consistent with 1/8th. Not saying it is that good, maybe on the order of 1/6th or so would be a guess along with being very smooth and no observable coma or astig on axis. It might have some of each, but I could not see it. That's actually consistent with Intes standard models and with the few IF tests available on the internet. I imagine my own is not a one off, but fairly common. Based on anecdotal evidence. Most seem to be built that way including the Intes standard model (and some mass produced built in strange ways with limited effective aperture). Intes deluxe model is better, of course.

 

If they did use an asphere or an autocollimator, one has to wonder why they didn't go all the way with it. Maybe they just made it good enough, packed it, and shipped it. Apparently they decided not to simply stay at 1/4th LSA and call that good enough. It might have been cheaper.

 

If it's not all spherical, I cannot explain the under correction that is clearly present.

 

Does your elliptical primary cancel under correction, or does it have some residual? You say it's diffraction limited, based on what observable aberration? Spherical? Because of the tolerances in the meniscus are not met? Maybe an elliptical surface is simply suitable for the design and fairly easy to make, but it's not the perfect conic. Its just good enough.

 

So if you find that maks provide a more pleasing and contrasty view somehow compared to sct's - that may well be true...

 

 

You know, I have no idea whether it's true or not. I just know the Mak is something else and that may well just boil down to excellent observing conditions currently over head...and nothing else. But it's an interesting exercise to explore the MCT to see if there is magic in the way they are designed and built.

 

I prefer to go by things that can be empirically demonstrated in an actual telescope under the skies...

 

 

Yea, I agree. Its hard to be empirical when your the only one looking through it. So, I leave testimony to my sketches and descriptions, if they are believable. I guess to be strictly empirical, they have to be believable. LOL

 

Yea, no doubt a diffraction limited 8" should just best an similar 7" which in turn would best my 6", surely, with all other variables constant. 

 

But that still doesn't mean the view isn't somehow more pleasing in a mak - at least for some people.

 

 

True. Again, that may simply boil down to the conditions it operates in. Or it may be something in the design or production process. Either or both. Dunno.


Edited by Asbytec, 11 August 2014 - 06:00 AM.







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