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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.


#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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