Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Interferometrically Testing Two Celestron C14 Edge Telescopes

  • Please log in to reply
92 replies to this topic

#51 Peter Ceravolo

Peter Ceravolo

    Vendor (Ceravolo Optical Systems)

  • *****
  • Posts: 24
  • Joined: 15 Apr 2014

Posted 09 March 2017 - 12:30 PM

John great article! I haven't had a chance to do more than skim the article at this point, but one testing issue is the SCT's spherochromatism - or variation of spherical aberration with wavelength, caused by the non achromatic corrector plate. The results at red HeNe 632nm will be different than what you'll see in the peak visible. At least that's what I remember with the old C14's. The HD's may have compensated/minimized this with the addition of field corrector lenses.


  • Jon Isaacs, rolo and Richard Whalen like this

#52 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 71645
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 March 2017 - 01:49 PM

John great article! I haven't had a chance to do more than skim the article at this point, but one testing issue is the SCT's spherochromatism - or variation of spherical aberration with wavelength, caused by the non achromatic corrector plate. The results at red HeNe 632nm will be different than what you'll see in the peak visible. At least that's what I remember with the old C14's. The HD's may have compensated/minimized this with the addition of field corrector lenses.

 

Peter et. al. :

 

Interestingly,  this is the exact same point made by another optical professional, Rik Ter Horst, in the Cats and Classes forum discussion of John's tests.   

 

http://www.cloudynig...-2#entry7734213

 

Jon



#53 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 09 March 2017 - 03:07 PM

John great article! I haven't had a chance to do more than skim the article at this point, but one testing issue is the SCT's spherochromatism - or variation of spherical aberration with wavelength, caused by the non achromatic corrector plate. The results at red HeNe 632nm will be different than what you'll see in the peak visible. At least that's what I remember with the old C14's. The HD's may have compensated/minimized this with the addition of field corrector lenses.

 

Hi Peter,

Thanks for your comments.  You've made a good point and you are correct.  Spherochromatism is certainly a significant factor in the Edge systems.  As you know, interferometric measurements are done with monochromatic light and power in the wavefront is typically considered to be a test alignment induced aberration so that it can be subtracted from the results.  Even if we used different wavelengths to measure the system, we wouldn't measure the full effect of spherochromatism on the white light imaging performance.  Without going to a lot of trouble and expense, interferometry is simply not a very good way to measure chromatic performance.  In the Edge systems, the problem is much worse in the blue than it is in the red so a measurement at 633 nm will give the best looking result; however, keep in mind that the design itself is almost certainly not diffraction limited at shorter wavelengths to start with.  Unfortunately, I didn't have the capability to make the measurements at any other wavelength besides HeNe, but I don't think that it matters.  The results provides valuable information about system alignment, component accuracy, and wavefront quality relative to the design at the output--even if it is at a single wavelength.

 

John


  • Jon Isaacs likes this

#54 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 71645
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 March 2017 - 06:05 PM

The results provides valuable information about system alignment, component accuracy, and wavefront quality relative to the design at the output--even if it is at a single wavelength.

 

 

waytogo.gif

 

John:

 

It's refreshing to see serious testing of an entire scope of this size by someone with your knowledge and skills.  It's a taste of reality and whatever the numbers are, I'll bet the views are pretty darn good.

 

Jon



#55 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 09 March 2017 - 06:15 PM

Thanks Jon.  If the clouds ever part, I'll have to put an eyepiece on it again one of these days.  The problem is that if the clouds part, I won't want to throw away any good imaging conditions!  lol.gif

 

John


  • Jon Isaacs likes this

#56 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 71645
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 March 2017 - 06:27 PM

Thanks Jon.  If the clouds ever part, I'll have to put an eyepiece on it again one of these days.  The problem is that if the clouds part, I won't want to throw away any good imaging conditions!  lol.gif

 

John

John:

 

There are advantages to living in Tucson.. 

 

Jon



#57 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 09 March 2017 - 06:59 PM

Yeah...don't remind me!  tongue2.gif


  • Jon Isaacs likes this

#58 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 71645
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 March 2017 - 09:24 PM

Yeah...don't remind me!  tongue2.gif

 

 

I sold my 25 inch F/5 Obsession to a young man who lives in Tucson.. I just got an Email from him.. It's clear, it's been clear.. 

 

http://arizonastartours.com

 

Jon



#59 TOMDEY

TOMDEY

    Apollo

  • -----
  • Posts: 1406
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 10 March 2017 - 09:49 PM

Hi again, John et al. I just caught up on the discussions... Good Stuff !  Just one mention here regarding Axial Coma. My favorite scopes are Newtonians. So, coma can be aligned out by tilting the PM, effectively redefining the field center. This is tantamount to treating the PM as a (very slightly) off-axis paraboloid. Yet another way to look at it is that the mechanical and optical centers of the PM are a bit different. I believe this is why recipes for field-aligning a Newtonian on a star involve a final tweak that may disagree with the alignment tools. I guess this ~could~ percolate over to more complex systems. But, because of the multiple elements, could be tricky to execute, OR the system might lack adjustability where it would be needed?  Also, when tweaking alignments, such things as field tilt and even baffling could manifest.  Ahhh... the simplicity of just ONE mirror. Thanks, Isaac!  Tom



#60 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 11 March 2017 - 01:18 AM

Tom,

I'd like to discuss this with you but it is completely off-topic.  Perhaps you could start a new thread in another forum where this could be discussed in more detail.  [CN really needs two (at least) new forums:  1) Optics and Optical Design and 2) Optical Testing.   They need a few others but I'll stop at that for now.]  The ATM forum might be the most appropriate.

 

Let's see if we can keep this thread focused on topics more closely related to the article.

John



#61 TOMDEY

TOMDEY

    Apollo

  • -----
  • Posts: 1406
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 11 March 2017 - 10:43 AM

John, yes - that's good. I tend to drift. In context, I DO like the luxury of AC testing off a (good) flat. And that also allows null-testing at any field, so is luxurious.  Tom



#62 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 11 March 2017 - 10:51 AM

Yes indeed.  I could have also tested the field performance but that wasn't my objective.  If the on-axis performance is good and I get nice round, small star images over the field (as demonstrated,) I'm a happy camper.

 

John


  • jrbarnett likes this

#63 jrbarnett

jrbarnett

    Eyepiece Hooligan

  • *****
  • Posts: 29443
  • Joined: 28 Feb 2006
  • Loc: Petaluma, CA

Posted 11 March 2017 - 07:16 PM

Yes indeed.  I could have also tested the field performance but that wasn't my objective.  If the on-axis performance is good and I get nice round, small star images over the field (as demonstrated,) I'm a happy camper.

 

John

John, how would one go about empirical field testing?  The conditions in the wild are ever changing which introduces a lot of variables that impact performance and are hard to measure or quantify.

 

Thanks!

 

- Jim



#64 xHarry

xHarry

    Vostok 1

  • -----
  • Posts: 163
  • Joined: 02 Nov 2016
  • Loc: Lafayette, Ca

Posted 11 March 2017 - 08:30 PM

Wow. I was not expecting an article of this depth or rigor. Thank you.

Perhaps we could add optical math to one the the proposed new forums.

 

Harry



#65 BillP

BillP

    Hubble

  • *****
  • Posts: 17771
  • Joined: 26 Nov 2006
  • Loc: Spotsylvania, VA

Posted 14 March 2017 - 07:50 AM

How about one new forum to cover them all: Optical Theory, Design, and Testing.


  • jhayes_tucson likes this

#66 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 71645
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 March 2017 - 09:00 AM

How about one new forum to cover them all: Optical Theory, Design, and Testing.

 

I think that is included in the ATM, Optics and DIY forum. Such topics do get discussed there and that's where folks like Ed Jones, Mike Jones, Mike Lockwood, etc hangout.

 

Jon



#67 jfrech14

jfrech14

    Viking 1

  • *****
  • Posts: 721
  • Joined: 17 Jan 2012
  • Loc: Virginia, USA

Posted 14 March 2017 - 02:54 PM

This is awesome! I love the writeup. Great work :)



#68 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 6186
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 14 March 2017 - 06:53 PM

 

Yes indeed.  I could have also tested the field performance but that wasn't my objective.  If the on-axis performance is good and I get nice round, small star images over the field (as demonstrated,) I'm a happy camper.

 

John

John, how would one go about empirical field testing?  The conditions in the wild are ever changing which introduces a lot of variables that impact performance and are hard to measure or quantify.

 

Thanks!

 

- Jim

 

 

Jim,

I was referring to the optical field--as in "off-axis."  Regardless, you are right that taking the telescope outside under the night sky introduces some additional variables.  The two main things that happen are that the temperature changes and the telescope pointing angle changes.  The lab test shows the telescope wavefront when everything is stable and that's what we try to achieve outside. It's never perfect but fans and thermal conditioning can go a long way to mitigating the problem.  Changes in alignment due to pointing angle are things that have to be controlled in the mechanical design.  In the Celestron design, the largest indicator of misalignment is field independent coma and none is visible in the star images that I supplied so the alignment in this particular scope is quite stable.  It is indeed possible to use precision metrology to measure the wavefront using a star as a light source.  The IFI Shack-Hartman wavefront sensor is just one way to do it.  Just keep in mind that the atmosphere often introduces significant wavefront errors.

John



#69 BillP

BillP

    Hubble

  • *****
  • Posts: 17771
  • Joined: 26 Nov 2006
  • Loc: Spotsylvania, VA

Posted 14 March 2017 - 07:07 PM

 

How about one new forum to cover them all: Optical Theory, Design, and Testing.

 

I think that is included in the ATM, Optics and DIY forum. Such topics do get discussed there and that's where folks like Ed Jones, Mike Jones, Mike Lockwood, etc hangout.

 

Perhaps so...but I would never lump ATM & DIY topics with Optical Theory, Design, and Testing.  Sort of like having a forum on Ice Cream and Steak lol.gif



#70 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 20724
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 14 March 2017 - 07:49 PM

 

 

How about one new forum to cover them all: Optical Theory, Design, and Testing.

 

I think that is included in the ATM, Optics and DIY forum. Such topics do get discussed there and that's where folks like Ed Jones, Mike Jones, Mike Lockwood, etc hangout.

 

Perhaps so...but I would never lump ATM & DIY topics with Optical Theory, Design, and Testing.  Sort of like having a forum on Ice Cream and Steak lol.gif

 

So says someone who does a shoot out of diagonals and doesn't even take one apart to contact test the flats.

 

Unbelievable



#71 jrbarnett

jrbarnett

    Eyepiece Hooligan

  • *****
  • Posts: 29443
  • Joined: 28 Feb 2006
  • Loc: Petaluma, CA

Posted 14 March 2017 - 10:52 PM

 

 

Yes indeed.  I could have also tested the field performance but that wasn't my objective.  If the on-axis performance is good and I get nice round, small star images over the field (as demonstrated,) I'm a happy camper.

 

John

John, how would one go about empirical field testing?  The conditions in the wild are ever changing which introduces a lot of variables that impact performance and are hard to measure or quantify.

 

Thanks!

 

- Jim

 

 

Jim,

I was referring to the optical field--as in "off-axis."  Regardless, you are right that taking the telescope outside under the night sky introduces some additional variables.  The two main things that happen are that the temperature changes and the telescope pointing angle changes.  The lab test shows the telescope wavefront when everything is stable and that's what we try to achieve outside. It's never perfect but fans and thermal conditioning can go a long way to mitigating the problem.  Changes in alignment due to pointing angle are things that have to be controlled in the mechanical design.  In the Celestron design, the largest indicator of misalignment is field independent coma and none is visible in the star images that I supplied so the alignment in this particular scope is quite stable.  It is indeed possible to use precision metrology to measure the wavefront using a star as a light source.  The IFI Shack-Hartman wavefront sensor is just one way to do it.  Just keep in mind that the atmosphere often introduces significant wavefront errors.

John

 

Ah!  Got it.  Thanks John.

 

- Jim



#72 precaud

precaud

    Skylab

  • *****
  • Posts: 4100
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 15 March 2017 - 07:52 AM

Perhaps so...but I would never lump ATM & DIY topics with Optical Theory, Design, and Testing.  Sort of like having a forum on Ice Cream and Steak lol.gif

So says someone who does a shoot out of diagonals and doesn't even take one apart to contact test the flats.

 

Unbelievable

I bet it's a mistake he won't repeat...



#73 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 20724
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 15 March 2017 - 10:24 AM

ice cream and steak gives me gas...like m42.



#74 treadmarks

treadmarks

    Viking 1

  • -----
  • Posts: 683
  • Joined: 27 Jan 2016
  • Loc: Boston MA

Posted 15 March 2017 - 02:48 PM

Thank you very much for publishing these tests. There is much speculation about the quality of various telescope manufacturers and the average stargazer has no way of verifying a manufacturer's claims. As someone who recently purchased a Celestron C8, I've been pretty curious as to how Celestron's quality control compares. Based on your tests, I'd say they're pretty good about it. I'm not sure how transferrable your results are to a base 2017 C8 but at least I have one data point now.



#75 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 20724
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 17 March 2017 - 01:40 PM

 

and the average stargazer has no way of verifying a manufacturer's claims.

that's not true.

 

you can test it against an oil flat with a knife edge and a led. nothing really special.

MVK has a post about it in the ice cream...I mean ATM forum.

 

very simple setup.

 

what the average stargazer doesn't have access to is testing the quality of the diagonal

 

here's my 2in WO CF diagonal. not very flat is it.

 

med_gallery_106859_3508_24685.jpg


Edited by Pinbout, 17 March 2017 - 01:45 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics