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Something for nothing: Celestron C90

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#1251 azure1961p

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Posted 21 June 2016 - 09:50 PM

Sandy, 

 

I too use the Suiter book,  yesterday evening in fact.  I'm still calling mine at about a quarter.  Wonderful that you've got 1/8!!!!  

 

I tend to believe a lot of folks with rudimentary to nil star testing experience will too often **** a mak when they unknowingly blame the SA it has by design, not by fault.  As Suiter sais,  it's not a question of If A mak has SA,  but how much. 

 

Glad you've got a gem.  Mine isn't  top tier,  though my newt and set are.  Frankly diffraction limited is fine without the higher rated waves. 

 

Pete


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#1252 t.r.

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Posted 23 June 2016 - 07:28 AM

 

 

My C90 is better than 1/8 wave on the focal plane.  That's been my experience and I love it!

 

How does one determine that?

 

I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw.

 

****NOTICE****  I READ this. I did not say it, advocate it, publish it or have anything to do with the accuracy or validity of it - so if it is wrong, please chide someone else. Thank you.

 

Sorry. Anyway, what I saw with the inside and outside focus images amounted to something like this:

 

attachicon.gifstar test 90mm Mak.jpg

 

Which shows some degree of spherical aberration, over correction I believe.

 

But at focus, I definately noticed an Airy disc and diffraction pattern very similar to this:

 

attachicon.gifstar test 90mm Mak in focus.jpg

 

When I first got the scope and tried it out on a star, I was surprised by the pattern I saw. There was the nice round Airy disc, encircled by a first diffraction ring which is larger than perfect due to the central obstruction, but then a SPACE of sorts - and then a thin, sharp diffraction ring. I thought something was wrong.

 

In reading the article, that had a chart with it, the effect of over correction that I saw, and the diffraction pattern I saw, were almost exactly as pictured in the chart - in the 1/10 wave row. 

 

The IN-Focus pattern in the chart was exactly what I was seeing for a 30% CO Mak.

 

Now, I'm not saying the scope is 1/10th wave or anything else. I don't have the knowledge, expertise or equipment to make that evaluation.

 

But I can say this. The images I see of Jup, Mars and the moon are as good as those I see with my 1996 ETX-90 with superb, American made optics and my C-80ED refractor, also a very fine scope optically - Except: the C80ED shows somewhat more contrasting and saturated colors than the Maks do, which is to be expected. But as far as detail is concerned, they are very, very close.

 

Thanks,

 

I use the "Star Testing Astronomical Telescopes", 2nd edition, by Suiter.  He has pics for obstructed telescopes and I just compare them to what I see.  It's available from Willmann-Bell.

 

 

That's a tall claim. Any chance you could get a Roddier Test image of this optic Sandy and post? Not saying it can't happen, it is just extraordinary for the price of these.



#1253 curiosidad

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Posted 23 June 2016 - 09:10 AM

Hello,

 

My question is; the C90 have some kind of coated ? multicoated or fulli multi coated? or maybe some similar to Nexstar tratement Starbright ?

Thanks,

Paul



#1254 curiosidad

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Posted 23 June 2016 - 09:11 AM

This would be very advantageous not?



#1255 Sarkikos

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Posted 23 June 2016 - 10:03 AM

The front of the corrector probably says what type of coating the C90 has.  IIRC, it's multi-coated.  Easy enough to check.  But, as far as I know, no Starbright, XLT or other fancy coating.

 

Mike


Edited by Sarkikos, 23 June 2016 - 10:04 AM.

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#1256 curiosidad

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Posted 23 June 2016 - 10:52 AM

and the mirror..?



#1257 Joe1950

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Posted 23 June 2016 - 11:35 AM

It says Multi Coated Optics which is a step lower than Fully Multi Coated. Wherever there are coatings, the coatings are multi-coatings. But, there probably aren't coatings everywhere. Of course, the only thing with antireflection coatings on it is the corrector - unless they are counting the diagonal prism.

 

Or, since the mirrors are not multi-coated, they can't say fully multi coated.

 

The categories, as I understand them are:

 

Coated - single coating but not on all air to glass surfaces.

Fully Coated - single coating on all air to glass surfaces.

Multi Coated - several layer coatings, but not on all air to glass surfaces.

Fully Multi Coated - several layers of coatings on all air to glass surfaces.

 

That's my understanding, but as I always say, I could be wrong.

 

The mirror and secondary are not enhanced coated, just likely SiO over aluminum. If it were anything else, you'd hear about it.

 

The good news is that since it is classified as a Spotting Scope, the buyer is given a lifetime warranty. Not the case with astronomical telescopes. It's probably done for competitive reasons.

 

It's just like Newts. If a scope says 114mm f/8, but doesn't mention it being a paraboloid, you can bet it's a sphere. Which is likely okay in a case like that.


Edited by Joe1950, 23 June 2016 - 11:36 AM.


#1258 SandyHouTex

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Posted 23 June 2016 - 11:24 PM

 

 

 

My C90 is better than 1/8 wave on the focal plane.  That's been my experience and I love it!

 

How does one determine that?

 

I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw.

 

****NOTICE****  I READ this. I did not say it, advocate it, publish it or have anything to do with the accuracy or validity of it - so if it is wrong, please chide someone else. Thank you.

 

Sorry. Anyway, what I saw with the inside and outside focus images amounted to something like this:

 

attachicon.gifstar test 90mm Mak.jpg

 

Which shows some degree of spherical aberration, over correction I believe.

 

But at focus, I definately noticed an Airy disc and diffraction pattern very similar to this:

 

attachicon.gifstar test 90mm Mak in focus.jpg

 

When I first got the scope and tried it out on a star, I was surprised by the pattern I saw. There was the nice round Airy disc, encircled by a first diffraction ring which is larger than perfect due to the central obstruction, but then a SPACE of sorts - and then a thin, sharp diffraction ring. I thought something was wrong.

 

In reading the article, that had a chart with it, the effect of over correction that I saw, and the diffraction pattern I saw, were almost exactly as pictured in the chart - in the 1/10 wave row. 

 

The IN-Focus pattern in the chart was exactly what I was seeing for a 30% CO Mak.

 

Now, I'm not saying the scope is 1/10th wave or anything else. I don't have the knowledge, expertise or equipment to make that evaluation.

 

But I can say this. The images I see of Jup, Mars and the moon are as good as those I see with my 1996 ETX-90 with superb, American made optics and my C-80ED refractor, also a very fine scope optically - Except: the C80ED shows somewhat more contrasting and saturated colors than the Maks do, which is to be expected. But as far as detail is concerned, they are very, very close.

 

Thanks,

 

I use the "Star Testing Astronomical Telescopes", 2nd edition, by Suiter.  He has pics for obstructed telescopes and I just compare them to what I see.  It's available from Willmann-Bell.

 

 

That's a tall claim. Any chance you could get a Roddier Test image of this optic Sandy and post? Not saying it can't happen, it is just extraordinary for the price of these.

 

It's not that unusual anymore from my understanding.  Almost all manufacturers use machines that can do this well with spherical surfaces.  Even Roland Christian at Astro-Physics uses machines.  The difficult part is if you have an aspheric surface.  The  C90 is just three spherical surfaces so it should be pretty easy.



#1259 Joe1950

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Posted 24 June 2016 - 08:37 AM

Agreed. And despite a less than perfect defocused star test, the in-focus image is on target and it puts up images very similar to my USA ETX-90, which is very good optically.

 

I had to laugh - mentioning above that the C-90 being a spotting scope comes with a lifetime warranty (which most spotters seem to have). The C-14 Edge with the nice big mount comes with a 2-year warranty, being classified as an astro telescope.

 

$179 Lifetime warranty.

 

$9999 2-yr warranty.

 

I'd still take the C-14      :lol: 


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#1260 azure1961p

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Posted 07 August 2016 - 03:04 PM

With regard the the multicoatings on the C90, it should be recalled the Ed Ting article on the C90, 90mm ETX and Questar  3.5,  the C90 was clearly the brightest.  The Meade was the dimmer  and quite so.   I don't honestly care where they put it so long as it's working!  I'll guess Celestron also used water white glass so that helps. 

 

Pete


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#1261 azure1961p

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Posted 07 August 2016 - 03:11 PM

As far as the C90 optics are concerned,  Suiter goes into these budget priced mass market scopes in particular actually forgiving them somewhat.  The high order SA isn't a matter of being there or not,  it's a given,  in these scopes it WILL have HO SA.  The question is to what degree.  The thing to remember is these smaller maks can have some of that and still turn out sharp planetary images,  DESPITE either side of focus not matching the other as you'd want.  To this end the small aperture flys under the radar to a degree.   I'm guessing my scope is 1/4 to 1/5 wave.  I wrestled with the tests for a while till a reality check brought it all into perspective: a 4mm ortho and a sharp Mars at 303x. Through a 90mm scope of any design this is very nice.  Through my C90?

 

Nuff said. 

 

Pete


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#1262 azure1961p

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Posted 12 August 2016 - 11:31 PM

 

 

 

My C90 is better than 1/8 wave on the focal plane.  That's been my experience and I love it!

 

How does one determine that?

 

I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw.

 

****NOTICE****  I READ this. I did not say it, advocate it, publish it or have anything to do with the accuracy or validity of it - so if it is wrong, please chide someone else. Thank you.

 

Sorry. Anyway, what I saw with the inside and outside focus images amounted to something like this:

 

attachicon.gifstar test 90mm Mak.jpg

 

Which shows some degree of spherical aberration, over correction I believe.

 

But at focus, I definately noticed an Airy disc and diffraction pattern very similar to this:

 

attachicon.gifstar test 90mm Mak in focus.jpg

 

When I first got the scope and tried it out on a star, I was surprised by the pattern I saw. There was the nice round Airy disc, encircled by a first diffraction ring which is larger than perfect due to the central obstruction, but then a SPACE of sorts - and then a thin, sharp diffraction ring. I thought something was wrong.

 

In reading the article, that had a chart with it, the effect of over correction that I saw, and the diffraction pattern I saw, were almost exactly as pictured in the chart - in the 1/10 wave row. 

 

The IN-Focus pattern in the chart was exactly what I was seeing for a 30% CO Mak.

 

Now, I'm not saying the scope is 1/10th wave or anything else. I don't have the knowledge, expertise or equipment to make that evaluation.

 

But I can say this. The images I see of Jup, Mars and the moon are as good as those I see with my 1996 ETX-90 with superb, American made optics and my C-80ED refractor, also a very fine scope optically - Except: the C80ED shows somewhat more contrasting and saturated colors than the Maks do, which is to be expected. But as far as detail is concerned, they are very, very close.

 

Thanks,

 

I use the "Star Testing Astronomical Telescopes", 2nd edition, by Suiter.  He has pics for obstructed telescopes and I just compare them to what I see.  It's available from Willmann-Bell.

 

 

That's a tall claim. Any chance you could get a Roddier Test image of this optic Sandy and post? Not saying it can't happen, it is just extraordinary for the price of these.

 

 

I'm sorry,  but I don't think it's possible to get 1/8 wave with a C90 without the involved touch of a custom optician or a RUMAK mak or both.  It's just not in the cards. 

 

Pete



#1263 SandyHouTex

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Posted 13 August 2016 - 01:38 PM

 

 

 

 

My C90 is better than 1/8 wave on the focal plane.  That's been my experience and I love it!

 

How does one determine that?

 

I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw.

 

****NOTICE****  I READ this. I did not say it, advocate it, publish it or have anything to do with the accuracy or validity of it - so if it is wrong, please chide someone else. Thank you.

 

Sorry. Anyway, what I saw with the inside and outside focus images amounted to something like this:

 

attachicon.gifstar test 90mm Mak.jpg

 

Which shows some degree of spherical aberration, over correction I believe.

 

But at focus, I definately noticed an Airy disc and diffraction pattern very similar to this:

 

attachicon.gifstar test 90mm Mak in focus.jpg

 

When I first got the scope and tried it out on a star, I was surprised by the pattern I saw. There was the nice round Airy disc, encircled by a first diffraction ring which is larger than perfect due to the central obstruction, but then a SPACE of sorts - and then a thin, sharp diffraction ring. I thought something was wrong.

 

In reading the article, that had a chart with it, the effect of over correction that I saw, and the diffraction pattern I saw, were almost exactly as pictured in the chart - in the 1/10 wave row. 

 

The IN-Focus pattern in the chart was exactly what I was seeing for a 30% CO Mak.

 

Now, I'm not saying the scope is 1/10th wave or anything else. I don't have the knowledge, expertise or equipment to make that evaluation.

 

But I can say this. The images I see of Jup, Mars and the moon are as good as those I see with my 1996 ETX-90 with superb, American made optics and my C-80ED refractor, also a very fine scope optically - Except: the C80ED shows somewhat more contrasting and saturated colors than the Maks do, which is to be expected. But as far as detail is concerned, they are very, very close.

 

Thanks,

 

I use the "Star Testing Astronomical Telescopes", 2nd edition, by Suiter.  He has pics for obstructed telescopes and I just compare them to what I see.  It's available from Willmann-Bell.

 

 

That's a tall claim. Any chance you could get a Roddier Test image of this optic Sandy and post? Not saying it can't happen, it is just extraordinary for the price of these.

 

 

I'm sorry,  but I don't think it's possible to get 1/8 wave with a C90 without the involved touch of a custom optician or a RUMAK mak or both.  It's just not in the cards. 

 

Pete

 

Well you are certainly allowed to believe what you want to believe.  I do have a lot of experience making and testing telescope optics though.


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#1264 barbie

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Posted 13 August 2016 - 11:24 PM

I agree with Sandy. I also have a number of home made optics to my credit and my C90 matches Sandy's description as well as that of Suiter's book.  My new C90 is also much better than the one I had 30 years ago. It is possible for me to use magnification in excess 50 x per inch when my local conditions permit, which is not often enough for my tastes!  Also, flocking the primary baffle has enhanced the contrast to a point that the scope compares very favorably to similar Maks far more than 200.00.


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#1265 Asbytec

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Posted 14 August 2016 - 02:34 AM

"I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw."

 

 

Do you have a link? I am having a hard time with the star test image posted. Hard to make out, but it does not appear to be 1/8th wave. It looks to be between 4 and 8 waves defocus, I'd like to see the image at 8 waves. The outer soft ring is confusing. Is it a halo of some sort? I have seen very thin halos outside my defocused pattern in good seeing very close to focus (including the color image below.) 

 

Now, in my experience, the Maks I have tested certainly appear to be better that 1/4th LSA, but likely not 1/8th. Even though my own patterns resembled Suiters version of a 33% obstructed, 1/8th PV LSA, when you add some residual astigmatism of coma, which are likely present in very small amounts (like the Skywatcher 150 link below), that drops the total wavefront to about 1/6th PV and the (estimated) Strehl remains relatively high. Maks tend to be very smooth and free of any obvious zones. 

 

There is some question as to whether the smaller Maks are aspheric with an elliptical primary (which may be a form of balanced low/high order aberration by adding lower order of the opposite sign, i.e., lengthening the radius of the primary.) There's a good argument that they almost have to be to perform as well as they do. This complicates the star test, IMO, as we likely will not be seeing textbook LSA. The good news seems to be, inside focus we get a shadow breakout pretty early with energy near the shadow. This is indicative, IMO, of descent correction probably in the vicinity of about 1/6th PV total wavefront deviation, including some residual lower and higher order SA. (I am not sure how to test for HSA, specifically or in conjunction with other aberrations.) 

 

http://www.microsoft...41874#post41874

 

The bottom line seems to be, Maks test pretty well and are likely better than classic 1/4 PV LSA. Below are my star tests of a 150 MCT. It's kind of difficult to know exactly how many waves defocus due to the focal ratio not being exactly f/12 as stated. But, if the math is right, that's near 8 waves defocus as Suiter uses in some of his illustrations. Notice the lower contrast outside focus indicative of under correction (after removing the baffle mod.) But, the important thing is the emergence of the inner ring inside focus which, to me, is not indicative of an obstructed ~30% 1/4th PV LSA pattern. So, I presume the correction is better than that, at least. In focus the patterns look very nice, too. The first bright ring is uniform to within my ability to detect otherwise in the most excellent seeing. 

 

Cropped 150MCT f13 30pct CO.jpg

Cropped Orion 150 MCT Star Test (Final).jpg

 

Got a better look at the focused pattern one night, Arcturus I believe. 

 

Orion 150 MCT in Focus (Final).jpg

 

Testing a complex telescope is, for me anyway, not that easy. Basically, the thing I like to see is energy near the shadow inside and not too far from focus like a more perfect optic has and unlike the dark central pattern of pure 1/4th LSA. This means, whatever they did, they managed to get a more uniform, even if dissimilar, star test pattern and good images. 

 

Below is one I took at just outside (IIRC) focus. Thought is was quite beautiful during the most steady seeing. Not sure what it tells me, other than the seeing was most excellent. If you look close, the first bright ring that breaks out is actually comprised of very thin rings close together. I found that quite striking. 

 

color.jpg


Edited by Asbytec, 14 August 2016 - 02:41 AM.

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#1266 duck2k

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Posted 15 August 2016 - 02:16 AM

Holy cow! This thread is still going?  I have not used my C90 in a couple of months.  I have been spoiled by my giant binos and mounts.  I think it is time to get that little jewel out.

:sct2:


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#1267 azure1961p

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Posted 16 August 2016 - 09:57 AM

Sandy and friends, 

 

LOL,  guys I'd love to believe the C90 can get 1/8!!!  I'm a big C90 fan -  but my understanding if I read Suiter correctly is the systems with an aluminized spot on the back of the corrector  run into problems with such a design that require either a rumak  or custom optician hand corrected fix. 

 

 

Pete


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#1268 SandyHouTex

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Posted 16 August 2016 - 10:08 AM

Sandy and friends, 

 

LOL,  guys I'd love to believe the C90 can get 1/8!!!  I'm a big C90 fan -  but my understanding if I read Suiter correctly is the systems with an aluminized spot on the back of the corrector  run into problems with such a design that require either a rumak  or custom optician hand corrected fix. 

 

 

Pete

As I said before, you are free to believe whatever you want.



#1269 elwaine

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Posted 16 August 2016 - 10:34 AM

Sandy and friends, 

 

LOL,  guys I'd love to believe the C90 can get 1/8!!!  I'm a big C90 fan -  but my understanding if I read Suiter correctly is the systems with an aluminized spot on the back of the corrector  run into problems with such a design that require either a rumak  or custom optician hand corrected fix. 

 

 

Pete

 

A few comments... 

 

First, a quote from RF Royce ( http://www.rfroyce.com/accuracy.htm )

"Also, optics targeted to and sold within the amateur community have often been advertised as not only possessing high accuracy, but also as inexpensive; as if both were simultaneously achievable. The fact is, this is not, and really can not be, the case. In the world of optics, as in virtually everything else, you get what you pay for."

 

Second, a quote from you know who: "... what difference does it make?"

 

Does it really matter whether or not the C90 has an accuracy of 1/8th wave if so many C90 owners enjoy their scope and are delighted with the views that these instruments provide? If wave front accuracy, RMS, P-V, Strehl, and MTF were code names for battle implements, we amateur astronomers would comprise the fiercest fighting force on the planet.  :lol:

 

It's wonderful that so much pleasure can be derived from such a small investment.


Edited by elwaine, 16 August 2016 - 10:37 AM.

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#1270 Asbytec

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Posted 16 August 2016 - 11:04 AM

All good points. They are certainly not premium scopes with the high prices attached. Though they may cost more if they were hand crafted and tested. In fact, the premium versions are. But they are pretty good, good enough, IME with a few Maks.

I have not seen one that tests near the classic 1/4th LSA they get blamed for having being mass produced. And I doubt they are as good as 1/8th. But they are likely better than "diffraction limited" at .70 rms. You can get down toward 0.40 rms and have a nice Strehl. I suppose with care, a premium optic is better still. You do get what you pay for, it seems. 

This skywatcher is pretty typical with tested (if you believe his test results) at rms ~0.032 and a Strehl of 1 - (rms * 2 * pi)^2 ~ 0.96 and 1/6th pv. I guess the obstruction helps a little, too.
http://www.microsoft...41874#post41874

 

They seem to test and are advertised, sometimes, near 1/6th pv. That sounds about right. Even the new AT refractors are coming in this good according to some sample tests. I believe it. Good, but not premium. And they tend to be smooth which counts for something. They are good scopes, but not premium, hand crafted scopes. But, it appears we live in an era of some decent mass production these days. 

 

"It's wonderful that so much pleasure can be derived from such a small investment."

 

You bet. 


Edited by Asbytec, 16 August 2016 - 11:12 AM.

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#1271 barbie

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Posted 16 August 2016 - 02:41 PM

Again, I will agree with Sandy.  I've also had the premium scopes both Rumak and conventional style maks and they didn't show me anything different from what the mass produced scopes showed. PERIOD!!

 

You are all free to believe the marketing hype if you want.


Edited by barbie, 16 August 2016 - 02:42 PM.

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#1272 SandyHouTex

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Posted 17 August 2016 - 09:42 AM

 

Sandy and friends, 

 

LOL,  guys I'd love to believe the C90 can get 1/8!!!  I'm a big C90 fan -  but my understanding if I read Suiter correctly is the systems with an aluminized spot on the back of the corrector  run into problems with such a design that require either a rumak  or custom optician hand corrected fix. 

 

 

Pete

 

A few comments... 

 

First, a quote from RF Royce ( http://www.rfroyce.com/accuracy.htm )

"Also, optics targeted to and sold within the amateur community have often been advertised as not only possessing high accuracy, but also as inexpensive; as if both were simultaneously achievable. The fact is, this is not, and really can not be, the case. In the world of optics, as in virtually everything else, you get what you pay for."

 

Second, a quote from you know who: "... what difference does it make?"

 

Does it really matter whether or not the C90 has an accuracy of 1/8th wave if so many C90 owners enjoy their scope and are delighted with the views that these instruments provide? If wave front accuracy, RMS, P-V, Strehl, and MTF were code names for battle implements, we amateur astronomers would comprise the fiercest fighting force on the planet.  :lol:

 

It's wonderful that so much pleasure can be derived from such a small investment.

 

What you fail to mention is with the advent of grinding and polishing machines, spherical optics can come off the machine at 1/8 to 1/10 accuracy.  Even Roland at Astro Physics uses machines to grind and polish his optics now.  He only touches them up if required by hand, because for some reason he wants them to be good to 1/40 wave.  Which is undetectable by the way.

 

So now, a big company like Synta can make telescopes with spherical optical surfaces that have 1/8 wave accuracy inexpensively.  I would maintain that that is what's going on with the C90.

 

It's also true that Celestron's optics sets have gotten notably better since they partnered with Synta.  Probably for the same reason.  Not to mention that I can't remember the last time that Celestron raised their OTA prices.


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#1273 Joe1950

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Posted 17 August 2016 - 11:33 AM

I recall many years ago, I believe in the Ingalls books, that machine polishing always required finishing by an optician since the machines had a tendency to generate zones - their motions were just too regular. They have improved so much over the years with software control (I assume) that with a spherical curve they are very accurate.

 

Someone sells 4" f/10 Carton mirrors in the classifieds. You can get them in a spherical shape or parabolized. I've often wondered if the spherical mirrors would be a safer bet in the long run. Parabolizing a 4-1/4" f/10 would seem difficult and likely result in over correction - at least I would think with my very limited optical knowledge.

 

 

By the way, what is a rumak?


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#1274 rmollise

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Posted 17 August 2016 - 12:01 PM

A Rumak is the familiar design that uses a separate secondary.

 

Actually, the Chinese parabolic f/4 primaries can be surprisingly good. I know the one in my StarBlast was.


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#1275 Joe1950

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Posted 17 August 2016 - 12:42 PM

Ah, as opposed to the spot secondary. Thanks very much Rod!

 

The quality and consistency of Chinese optics is improving from what I hear. I think they are capable of good quality, but the quality control in the past was not up to par. You could get great, good and not so good. That seems to be getting better.

 

joe


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