Jump to content


Photo

Newtonian- glass secondary support?

  • Please log in to reply
34 replies to this topic

#1 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 03 August 2014 - 11:36 PM

First sorry, I'm sure this has been covered, but can't find it Thinking of making a coated float glass support for the secondary of an 8" GSO f/5. Edmund Scientific has a premade and coated "window"  http://www.edmundopt...-windows/48-859 . I enjoy making simple stuff, so I thought this would be a good way to get rid of difraction spikes and protect the innards also. Any thoughts or suggestions appreciated.--Jack



#2 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44751
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 04 August 2014 - 05:48 AM

At that price, I can't imagine it's very flat.... The trade off:  Swapping diffraction spikes for a Dew Magnet...  I'll take the diffraction spikes.

 

Jon



#3 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 8270
  • Joined: 22 Feb 2010
  • Loc: You can't see me...

Posted 04 August 2014 - 06:35 AM

you'll be cuttng off some of the light with the same size window.

 

f5 is great for a travel dob. :)

 

 



#4 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2623
  • Joined: 20 Jan 2011

Posted 04 August 2014 - 10:18 AM

Look at the surface quality -- 80-50. These numbers are part of the MIL-REF-13830B standard for scratches and digs in microns over a designated area. These imperfections add to light scatter and loss of contrast. Typically, high quality optics are 10-5. You be the judge.

 

Mladen


Edited by MKV, 04 August 2014 - 10:19 AM.


#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44751
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 04 August 2014 - 06:16 PM

Look at the surface quality -- 80-50. These numbers are part of the MIL-REF-13830B standard for scratches and digs in microns over a designated area. These imperfections add to light scatter and loss of contrast. Typically, high quality optics are 10-5. You be the judge.

 

Mladen

 

Mladen

 

What are you thoughts on the flatness,I saw no spec.

 

Jon



#6 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 04 August 2014 - 10:52 PM

@Jon I contacted Edmund and the engineer/tecnician looked up the flatness--4-6 waves. I realise that would not be usable in a lens or mirror. If the surfaces are reasonably parallel, the refraction index constant, and the clarity at 96-99% would not the effect on the image be minimal?--Jack



#7 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44751
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 05 August 2014 - 03:28 AM

@Jon I contacted Edmund and the engineer/tecnician looked up the flatness--4-6 waves. I realise that would not be usable in a lens or mirror. If the surfaces are reasonably parallel, the refraction index constant, and the clarity at 96-99% would not the effect on the image be minimal?--Jack

 

 

I am no expert but it would seem to me that 4-6 waves of flatness would cause  wave front errors of approximately N-1 x E where N is the index of refraction and E is the flatness error.. This would be 2-3 waves..   How all these add up, I couldn't say but I tend to think it would not be a good thing.

 

Jon

 

Jon



#8 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 05 August 2014 - 03:51 AM

@John, for a mirror, or a lens, true. For a window? I have strong doubts. In a mirror or lens there are surface variations in refraction, and in the length of the path of adjacent photons. I fail to see how this would be possible in a window, but what do I know?-Jack



#9 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 8270
  • Joined: 22 Feb 2010
  • Loc: You can't see me...

Posted 05 August 2014 - 06:31 AM

@John, for a mirror, or a lens, true. For a window? I have strong doubts. In a mirror or lens there are surface variations in refraction, and in the length of the path of adjacent photons. I fail to see how this would be possible in a window, but what do I know?-Jack

 

 

 

look at the roughness in this sct, it can be either the mirror or the corrector plate. one of them was produced too quickly and it produces zones, many zones in the airy disc which destroys contrast.

 

https://www.youtube....h?v=BqnP3a3X07s


Edited by Pinbout, 05 August 2014 - 06:32 AM.


#10 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4915
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 05 August 2014 - 08:39 AM

 Jean Texereau in his excellent book "How to Make a Telescope" has an excellent chapter on making an optical window for a Newtonian and what the specs need to be. The window can be a few waves from flat on each side since this forms a very long focus lens but the surface must be optically smooth.  What is critical is that the image formed by  this  "lens"  needs to be smaller then the Airy disk formed by the mirror. What is also critical is there is no or very little wedge error since then the window would forms a weak prism and will introduce lateral color in the image ie the star will turn in small spectra. Texereau also makes the point that unless the telescope is nearly optically perfect, the improvement in the image from the use of  the window will have no effect on the image quality.

 

                 - Dave


Edited by DAVIDG, 05 August 2014 - 04:22 PM.

  • kalasinman likes this

#11 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 05 August 2014 - 09:47 AM

Thanks Dave, most informative response I've had to date. When the time comes, I'm going for it. The glass costs $48. Could be a fun project.-Jack



#12 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 8270
  • Joined: 22 Feb 2010
  • Loc: You can't see me...

Posted 05 August 2014 - 10:36 AM

<<most informative response I've had to date.>>

 

well Dave has won too many awards to keep count, for optics as well as the builds.  ;)


Edited by Pinbout, 05 August 2014 - 10:37 AM.


#13 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4915
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 05 August 2014 - 04:31 PM

Thanks Dave, most informative response I've had to date. When the time comes, I'm going for it. The glass costs $48. Could be a fun project.-Jack

 Jack,

  Glad to be of some help. Texereau chapter in his books covers grinding , polishing and testing an optical window pretty well.  You might consider purchasing a large piece of standard float glass and then scanning it for areas that are optically flat and then cut out that area for your window. There was an article back in the 90's in Sky and Tel were the author made Schmidt camera and scanned a large but inexpensive piece of float glass and found large areas that were optically smooth and flat enough to make the corrector plate from.

 

             - Dave


  • kalasinman likes this

#14 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 05 August 2014 - 09:04 PM

Dave, I'd planned on using one of these pre-made windows from Edmund

http://www.edmundopt...-windows/48-859

Being in a very rural part of N.E. Thailand, off the shelf is attractive.--Jack



#15 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2623
  • Joined: 20 Jan 2011

Posted 05 August 2014 - 10:22 PM

I'd planned on using one of these pre-made windows from Edmund

http://www.edmundopt...-windows/48-859

 

Jack, the industrial surface quality rating 80-50 of that window refers to surface error (digs, scratches)  size in microns and is not the best. Precision optics typically expected of astronomical instruments are rated 10-5. The greater the numbers the greater the light scatter and loss of contrast, to mention just one parameter.

 

regards,

Mladen



#16 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 06 August 2014 - 03:44 AM

Mladin, Thanks I am aware of that.-Jack



#17 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 06 August 2014 - 07:22 AM

As I see it, a risk assessment is always useful. I like to tinker so doing something I enjoy is a win regardless of outcome. Cost of materials $48, I can handle that. Nothing would be done that cannot easily be undone. Potential loss of contrast from low polish glass could well be less than the gain in contrast resulting from elimination of spiders. If I don't try this, no fun, and I don't learn anything. Risk looking very low to non-existent, possibly a negative number.

As a proof of concept I could give it up, or like it and keep it and later upgrade to a high standard window (quoted under $300).--Jack



#18 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2623
  • Joined: 20 Jan 2011

Posted 06 August 2014 - 07:36 AM

Jack, I feel the same way. The purpose of providing information is not to persuade or dissuade anyone, but to provide them with as much information as one can find -- for a better assessment. Eliminating the spider can be a lot more beneficial then a slight loss of contrast on extended objects. And who can make such a thing for $48! :)

 

Mladen 


Edited by MKV, 06 August 2014 - 07:40 AM.


#19 jzeiders

jzeiders

    Viking 1

  • -----
  • Posts: 524
  • Joined: 07 Nov 2012
  • Loc: SF Bay Area

Posted 06 August 2014 - 12:36 PM

Something to consider, you have a marginal to poor piece of glass working as a window and secondary support. Consider the weight of the secondary assembly adding torque to the piece of plate glass. How thick is it?   If you want a window that works fairly well do the stress calc and if the glass is 1/4=3/8 of an inch hunk ok. Buy at least three of them and regrind and polish them flat with the traditional 1 on 3, 2 on1, 3 on 2 cycle. Then you can get the best one a/r coated.  You may still get ghost reflections from bright stars. ALso you have a dust and dirt magnet. You may also need to redesign the secondary holder to allow for collimation with the window mounted in the tube. Look at a mMead Schmidt-Newtonian for an example.

 

Or, you could just make a curved vane spider.

 

Jack



#20 amicus sidera

amicus sidera

    Skylab

  • *****
  • Posts: 4223
  • Joined: 14 Oct 2011
  • Loc: East of the Sun, West of the Moon...

Posted 06 August 2014 - 03:15 PM

The optical window on the Edmund Astroscan was intentionally designed to be non-plano-parallel; while the front surface is (reasonably) flat, the side facing the primary has a very slight negative curvature; in other words, deeper in the center. The Edmund engineers I've spoken with have said that this was done specifically to avoid ghost reflections of brighter objects.



#21 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 06 August 2014 - 07:37 PM

@Amicus, Thanks. Interesting aside.-Jack



#22 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 06 August 2014 - 08:06 PM

@jzeiders, The thickness of the window is listed in a link posted earlier. If you have not done the stress calc, on what basis are you judging that 1/4-3/8" would be ok? I would think that any potential deflection would depend on several other factors including the weight of the secondary, it's support, and the diameter/thickness.

As regards dust and dirt, I'm not sure I follow your reasoning, but the outside of the window will be easy to clean in contrast to the primary and secondary it would protect.

I see no reason to presume failure and start grinding. See above risk assessment.

The window has both sides mufti-coated to reflect less than .4% as shown in the specs of the window in the previously posted link.

Building a custom secondary support will obviously be required.

Curved spiders have been considered, but elimination of the diffraction related loss of contrast that all spiders induce is a major goal of this.



#23 jzeiders

jzeiders

    Viking 1

  • -----
  • Posts: 524
  • Joined: 07 Nov 2012
  • Loc: SF Bay Area

Posted 06 August 2014 - 10:50 PM

I have no idea of whether it will work well or not. It may be fine. Just wanted you to consider before going ahead. As I have a few  regular Newtonians 8", 10", & 17", a Schmidt-Newtonian 8" and a Maksutov- Newtonian 7" I am familiar with Newt. scopes that have corrector supported secondaries and spider supports. I find the hassle of correctors less enthralling the more I use them. With any optic, the more often it is cleaned the sooner it will will be damaged. The MN and SN have far more problems with dewing and dirt collection than any of my regular Newtonians. Perhaps you have less issues with dewing where you live  than I.  Last new moon weekend I had the SN out at a local star party and was shut down by dew around 11 PM  along with another fellow with a SCT while others with regular Newts. were observing happily even after I  I left at 3 AM.

 

My ATM experience tells me to suspect the flatness of most inexpensive windows, Having actually made one many years ago. a 4", I'd say it wash't worth it. I bought 5 blanks of untempered water white plate and followed the instructions found in the old ATM books. A friend used the leftovers and made a Wright-Schmidt. Eventually Igot feb up with it dewing up and replaced it with a regular 3 vane spider.

 

What ever floats your boat. :=)

 

Jack



#24 kalasinman

kalasinman

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 12 Jun 2014
  • Loc: N.E.Thailand

Posted 06 August 2014 - 11:13 PM

@jzeiders, I do appreciate the post. If you read back a ways, Dave from Germany quotes from a great book on making telescopes where the subject of windows was explored. The upshot being that parallelism was critical and flatness was not within reason, also that any lens like effects would be at a longer focal length. I have considered these factors.

You're right about the dew not being much of an issue here. Our night time temps average in the low 20's c. There may be some chance in our 3 week winter period when temps plunge to near 10c, however.

It will be some time before I attempt this, but there seems to be limited current definitive information. I'm considering methods to get a fair A-B test going. As seeing is so variable, I was thinking of using a DSLR at prime focus to image a large piece of newsprint at a long distance, then compare crops.



#25 MKV

MKV

    Mercury-Atlas

  • *****
  • Posts: 2623
  • Joined: 20 Jan 2011

Posted 07 August 2014 - 07:47 AM

Sputnik, I think you have thought this through and have very little to lose by going with that Edmund window. As I mentioned earlier, you can't make it, edge it, coat it, etc. for $48, plus work involved. If it works -- great! if it doesn't it's just $48  We waster more on Starbaucks in a month! If I were you, just for the heck of it, I would ask Edmund to supply the wedge data which is not specified, because, as you said, the wedge is much more important here than any slight curvature. Such a window only needs to be reasonably flat (by a spherometer  or by one of those Ed Jones wonderful DIY 3-nail contraptions :)

 

http://www.youtube.c...h?v=Z0jCycLpXSM

 

Mladen








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics