Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | (show all)
Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Secondary Size vs Reflectivity
      #5661450 - 02/04/13 10:26 AM

I should be receiving my new Protostar spider and secondary holder this month which means I need to make a decision on which secondary to install.

The scope is a 12" f/4.9 and the primary is coated with standard aluminum/SiO2 overcoat giving ~90% reflectivity (%R).

The choice of secondary mirrors that I have are:

1 - The stock 2.75" that is probably BK-7 glass, an unknown wave rating and coated with a %R of ~92-93%.

2 - Antares Optics 2.6" made of Pyrex, better than 1/20 wave and coated with a %R of ~96-98%.

When considering the diameter of the 100% illumination area, the two largest field stop sizes that I have are 1.52" (35mm Panoptic), and 1.08" (20mm Nagler).

The 2.75 will give a larger 100% illuminated area but a lower overall reflectivity while the 2.6 would have the opposite results (smaller 100% area, but higher reflectivity).

I know this is close and most might say that the difference is so small that I will not be able to tell the difference. However, I am thinking the higher reflectivity of the 2.6 might show up in other ways that the larger 100% illumination area of the 2.75 would not.

Thoughts or opinions?

Thanks,

Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: Mark Peterman]
      #5661486 - 02/04/13 10:42 AM

My suggestion:

Don't worry about reflectivity or field illumination, worry about the quality of the mirror. The quality of the secondary is nearly as important as the quality of the primary so a high quality secondary can make a significant difference in the scope's performance.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Secondary Size vs Reflectivity new [Re: Mark Peterman]
      #5661491 - 02/04/13 10:44 AM

You will not see the difference in reflectivity.

Your primary consideration now should be the balance between secondary obstruction size and the field illumination.

For visual purposes, even for 2"' eyepieces, a circle of full illumination 0.5-0 75"' is quite adequate. An illumination drop-off to 50% at the field edge is hard for even experienced observers to perceive.


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Secondary Size vs Reflectivity new [Re: Jon Isaacs]
      #5661906 - 02/04/13 02:30 PM

Quote:

... The quality of the secondary is nearly (emphasis added) as important as the quality of the primary...




Since the imaged is "processed" by both mirrors why would the secondary not be equally as important?


Post Extras: Print Post   Remind Me!   Notify Moderator  
killdabuddha
Pooh-Bah
*****

Reged: 08/26/11

Re: Secondary Size vs Reflectivity new [Re: careysub]
      #5661930 - 02/04/13 02:44 PM

Secondary quality is a point of contention and confusion. You'll even find opticians flustered

http://www.rfroyce.com/diagsize.htm

There have been threads here, and the consensus resolved to a 1/20th wave secondary as bein more than adequate, and hence the safe bet. As for, "Since the imaged is "processed" by both mirrors why would the secondary not be equally as important?" I thought the same thing. Others said no way. When I asked our optician, he said he wouldn't worry too much about secondary quality, yet you'll also find Royce (above link) sayin the same thing that you've suggested (which I quoted). So I think that we all tend to err on the side of caution.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Secondary Size vs Reflectivity new [Re: GlennLeDrew]
      #5662015 - 02/04/13 03:41 PM

Quote:

Your primary consideration consideration now should be the balance between secondary obstruction size and the field illumination.




It sounds like the known quality of the secondary should be a big part of the reason for going with the 2.6".

The difference in obstruction is only 1%, so really a non-factor in that regard. Here is how the field illumination looks with the 35mm Panoptic.



Here's the rest of the specs using the raw numbers:



* Before someone points it out, I'm not taking the beveled edge or the lip on the holder into consideration. Just using the raw MA size on each mirror to keep it apples to apples.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: careysub]
      #5663870 - 02/05/13 04:17 PM

Quote:

Quote:

... The quality of the secondary is nearly (emphasis added) as important as the quality of the primary...




Since the imaged is "processed" by both mirrors why would the secondary not be equally as important?




The reason I used "nearly" as important is that the primary mirror reflects the light back on itself so the surface error is doubled whereas the secondary reflects it at a 45 degree angle so the effect of a surface error is reduced, there's a square root of two in there somewhere.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Secondary Size vs Reflectivity new [Re: Jon Isaacs]
      #5664372 - 02/05/13 09:44 PM

Assuming then that the wave front errors combine by RMS summation, a secondary that matched the error of the primary would make the the overall error only 22.5% worse.

This suggests that having equal accuracy for the secondary as the primary should serve pretty well.

Of course the "accuracy cost" in a secondary is much lower to get a 1/15 or 1/20 wave mirror so there isn't much cost pressure in a scope budget to <i>not</i> go better than your primary.

If it is twice as good then the error increase is only 6.1%.

Antares offers secondaries down to 1/30 wave. You would need to have a really good primary to benefit significantly from that kind of accuracy.


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Secondary Size vs Reflectivity new [Re: careysub]
      #5664558 - 02/06/13 01:29 AM

why do the graphs of field illimination show a central flat drop of 0.06 mag?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: derangedhermit]
      #5664661 - 02/06/13 05:24 AM

Quote:

why do the graphs of field illimination show a central flat drop of 0.06 mag?




Area lost because of the secondary obstruction.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: careysub]
      #5664681 - 02/06/13 06:10 AM

Quote:

Assuming then that the wave front errors combine by RMS summation, a secondary that matched the error of the primary would make the the overall error only 22.5% worse.





This what Robert Royce says:

"To calculate the impact of errors on the final wavefront one method is to take the error of the primary mirror plus the error of the secondary mirror (if its surface error its times 2) times the cosine of the angle of incidence of the diagonal, in this case 45 degrees or .7071. So, assuming a 1/10 wave primary mirror and a 1/10 wave secondary mirror (wave front accuracy, not surface) we have the following: .1 + .1 * .7071) = .171 or Lambda/5.8. But some people think this is much too severe an application of arithmetic. I like the product of the complement method (which I developed independently) but it may only be foolishness. It goes like this: assume you have a .1 wave mirror and .1 wave diagonal. .9 is the complement of .1 or 1 - .1 = .9 so the entire formula is 1- (.9 * .9) * .7071 = .134 or Lambda/7.44 It looks nicer. Then there is the root sum squared method. This is valid for systems having many elements. This method is be as follows: ((.1^2 + .1^2)^.5) * .7071 = .141 or Lambda/7.07. Confusing?"

Sizing your diagonal Robert Royce

In any event, he includes the 45 degree cosine factor and concludes that the wavefront error of two 1/10 wave front mirrors will be somewhere around 1/7 wave. He does not seem to use the RMS summing.

My thinking is pretty simple:

You go to the trouble of making a large 1/10 mirror. That is a major investment. A high quality secondary is far less expensive and yet has a similar effect on the image so it's cost effective to invest in the best possible secondary.

As similar situation is recoating a mirror set. The reflectivity of the primary and the secondary are of equal importance in the brightness of the image but they do not cost the same. If you recoat the secondary with high reflectivity and the primary with standard coatings it will be much less expensive than recoating both with high reflectively coats and you gain about 50% of the reflectivity advantage...

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: Secondary Size vs Reflectivity new [Re: careysub]
      #5664755 - 02/06/13 07:52 AM

Quote:

Antares offers secondaries down to 1/30 wave. You would need to have a really good primary to benefit significantly from that kind of accuracy.




In fact, it has been suggested to me (by someone who has reason to know the inside story) that you would also need to have a good imagination to believe the 1/30 wave number to begin with. I think that Protostar guarantees 1/10 wave not because the numbers coming out of the interferometer aren't ever better, but rather that these better numbers don't contain any information.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: dpwoos]
      #5666601 - 02/07/13 08:00 AM

Quote:

Quote:

Antares offers secondaries down to 1/30 wave. You would need to have a really good primary to benefit significantly from that kind of accuracy.




In fact, it has been suggested to me (by someone who has reason to know the inside story) that you would also need to have a good imagination to believe the 1/30 wave number to begin with. I think that Protostar guarantees 1/10 wave not because the numbers coming out of the interferometer aren't ever better, but rather that these better numbers don't contain any information.




Optical flats for Fabry–Pérot interferometers need to be much flatter than lambda/30... it is possible.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: Secondary Size vs Reflectivity new [Re: Jon Isaacs]
      #5666736 - 02/07/13 09:55 AM

Quote:

Quote:

Quote:

Antares offers secondaries down to 1/30 wave. You would need to have a really good primary to benefit significantly from that kind of accuracy.




In fact, it has been suggested to me (by someone who has reason to know the inside story) that you would also need to have a good imagination to believe the 1/30 wave number to begin with. I think that Protostar guarantees 1/10 wave not because the numbers coming out of the interferometer aren't ever better, but rather that these better numbers don't contain any information.




Optical flats for Fabry–Pérot interferometers need to be much flatter than lambda/30... it is possible.

Jon




Never said it wasn't possible, but rather that (I was told) that it isn't possible in this case.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: dpwoos]
      #5668042 - 02/07/13 10:56 PM

Quote:

Never said it wasn't possible, but rather that (I was told) that it isn't possible in this case.




I suspect that there are folks here like Glenn, Mike Jones, Ed Jones, David and others who probably know and if it is possible, some have probably made lambda/30 flats.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: Secondary Size vs Reflectivity new [Re: Jon Isaacs]
      #5668429 - 02/08/13 07:58 AM

I don't think that the problem is production - some number out of every batch might very well be 1/30 wave. Rather, the problem is creating and maintaining an interferometric testing setup that can find them with certainty. If you accept, based on provided interferometry, that a flat advertised as 1/30 wave is so, then that is up to you. I don't, and so have (and will continue to) purchase secondaries with what I think are more realistic and trustworthy numbers.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size vs Reflectivity new [Re: dpwoos]
      #5668782 - 02/08/13 11:30 AM

Quote:

I don't think that the problem is production - some number out of every batch might very well be 1/30 wave. Rather, the problem is creating and maintaining an interferometric testing setup that can find them with certainty. If you accept, based on provided interferometry, that a flat advertised as 1/30 wave is so, then that is up to you. I don't, and so have (and will continue to) purchase secondaries with what I think are more realistic and trustworthy numbers.




I am not buying any flats. But I would be interested in an explanation of why you think it is it not possible to manufacture and interferometrically test flats to 1/20 or 1/30th wave.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: Secondary Size vs Reflectivity new [Re: Jon Isaacs]
      #5668862 - 02/08/13 12:08 PM

As I have written several times, I don't think it is impossible, but rather that in practice claims of 1/20 or 1/30 wave secondaries as sold to the amateur astro community are not (to me) credible. The reason I think this is because someone I trust in the optical fabrication business with knowledge of the testing methodology and equipment told me so. I also know that it is very easy to confuse information and noise - even bona fide scientists/researchers struggle with this. I also know that businesses can easily find themselves in a spiraling race, where facts are stretched in an effort to distinguish themselves from the competition. All of this makes me very skeptical of these claims, and I am happy to stick with more modest numbers (e.g. 1/10 wave).

Edited by dpwoos (02/08/13 12:11 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Secondary Size vs Reflectivity new [Re: dpwoos]
      #5669009 - 02/08/13 01:16 PM

I don't make flats, except an 18" long octagonal flat for a 1m f/2.5 LIDAR telescope. But it had a very much looser requirement on flatness than 1/30 wave (thank goodness.)

If sizable batches of flats are made, lambda/30 (P-V, of course) specimens are certainly possible to find, being more likely from the center of a bunch. But I wouldn't go out of my way to find one. A 1/10 wave unit is more than good enough, and the difference between it and a 1/30 wave job is probably impossible to discern in use.


Post Extras: Print Post   Remind Me!   Notify Moderator  
dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: Secondary Size vs Reflectivity new [Re: GlennLeDrew]
      #5669111 - 02/08/13 01:52 PM

I certainly think that it is possible that some samples from a batch of secondaries would be 1/20 or even 1/30 wave. What I am questioning is the ability of the vendors of secondaries to the amateur community to pick them out via interferometric testing. If a 1/10 wave flat is plenty good enough (as you say, and I agree) then why the 1/20 and 1/30 wave claims? To me, this sounds like all marketing hype which I guess also fuels my suspicions that the testing is not legit. Of course, I am not claiming that the secondaries in question are not good 1/10 wave - they probably are.

Edited by dpwoos (02/08/13 03:07 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | (show all)


Extra information
15 registered and 27 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1091

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics