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)
Darren Drake
Carpal Tunnel
*****

Reged: 10/09/02

Loc: Chicagoland
Does paracorr add any SA?
      #5836197 - 05/02/13 12:34 PM

I know that the books say it does not but I have always questioned that. I sometimes believe under high scrutiny that there is a slight amount of sa added to the optical chain. Has anyone actually tested a scope with a paracorr or paracorr 2 added to the system? I plan to do some null comparisons when I have a chance but am wondering if it has been done at all or do we just take TVs word for it???

Edited by Darren Drake (05/02/13 12:35 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5837360 - 05/02/13 10:19 PM

Darren
Not owning one I have always wondered this as well, which - for me -is also related to the question of just how well corrected a catadioptric a paraboloid plus Paracorr is.

Now we already know that simpler coma correctors such as a Rosin introduce undercorrection, and so need to be paired with an appropriate Hyperboloid but where the net result is now excellent overall correction. But the Paracorr is touted as achieving this (as can a Wynne) with a paraboloid, the tradeoff with the Paracorr being a slight negative power (barlowing effect). But with respect to definitively settling your question, one problem is that I don't think the optical prescription for the Paracorr is out there. As such I don't know if any spot diagrams are available.

All that is really needed, I think, is a paraboloid of sufficient quality to star test with virtually identical intra and extra focal patterns. Such mirors while rare are most definitely out there. Star testing is so annoyingly sensitive to SA, that now introducing the paracorr into the light path of such a mirror wold clearly show SA well below any reasonable level of optical significance. I think such a simple test as this, under good seeing, would settle this question. (at least for that particular Paracorr! LOL)

Joe


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Does paracorr add any SA? new [Re: jpcannavo]
      #5837403 - 05/02/13 10:44 PM

The best way to find out would be by using an autocollimation bench test.

JimC


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

Reged: 06/17/05

Loc: Portland, Oregon
Re: Does paracorr add any SA? new [Re: Mirzam]
      #5837494 - 05/02/13 11:57 PM

I've done extensive star testing on an 8 inch f3.3 scope for the past several months with and without a Paracorr 2. I've not seen any difference in SA with and without the P2, but there is certainly a small difference among some of my eyepieces.

Post Extras: Print Post   Remind Me!   Notify Moderator  
jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Does paracorr add any SA? new [Re: hbanich]
      #5837685 - 05/03/13 06:14 AM

Quote:

I've done extensive star testing on an 8 inch f3.3 scope for the past several months with and without a Paracorr 2. I've not seen any difference in SA with and without the P2, but there is certainly a small difference among some of my eyepieces.




Howard, Jim
Given that star testing is generally cited as being sensitive to aberration at the wavefront of 1/20th wave or better, such a result - to the extent that it is generally true (Your P2 is not some fortuitous fluke, etc) - would seem to be definitive. While other testing methods (Double pass autocollimation? Yes/No?) might yield higher sensitivity, we would be well beyond the realm of any visual or imaging significance.

I would wonder if there is a difference between P2 and the original in this regard?
Joe


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

Reged: 01/02/05

Loc: Guilford, Connecticut
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5838556 - 05/03/13 03:02 PM

According to Televue it does. However, the amount it adds on axis is essentially not measurable since it is less than the airy disc:
http://www.televue.com/images/TV3_Images/Images_in_articles/Paracorr_2_chart.jpg

Compare the charts at the left to the one on the right.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Does paracorr add any SA? new [Re: Jim Romanski]
      #5842271 - 05/05/13 07:10 PM

Jim's comment is correct. The correction does not hit the zero point on axis, as coma would. Ergo, some SA is added, but it is less than the size of the Airy disc, which means invisible, and having no effect on resolution.
Note that coma is corrected to smaller than the Airy disc down to f/3.5 in a field as wide as 40mm.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Does paracorr add any SA? new [Re: jpcannavo]
      #5842377 - 05/05/13 08:19 PM

Quote:

But with respect to definitively settling your question, one problem is that I don't think the optical prescription for the Paracorr is out there. As such I don't know if any spot diagrams are available.




No it's not. But as Al Nagler told me, anybody could reverse engineer it. Nobody has.

SFAIK the amount of SA added solely by the Paracorr is tiny. Otherwise it would mean little to point out (as Al also did) the design SA of certain of the then-new Ethos line - as in around 1/100th wave.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Does paracorr add any SA? new [Re: mark cowan]
      #5842829 - 05/06/13 01:12 AM

Quote:

Otherwise it would mean little to point out (as Al also did) the design SA of certain of the then-new Ethos line - as in around 1/100th wave.

Best,
Mark




Is that under or overcorrection?

Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Does paracorr add any SA? new [Re: dave brock]
      #5842849 - 05/06/13 01:34 AM

Hmm, didn't specify.

Best,
Mark


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

Reged: 10/09/02

Loc: Chicagoland
Re: Does paracorr add any SA? new [Re: mark cowan]
      #5843044 - 05/06/13 07:48 AM

If the light is still thrown into the airy disk does that necessarily mean that the strehl ratio is unaffected? I suspect it is.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Does paracorr add any SA? new [Re: Starman1]
      #5843174 - 05/06/13 09:50 AM

I thought the SA was a mis-alignment issue, which the SIPS solved.

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

Reged: 10/09/02

Loc: Chicagoland
Re: Does paracorr add any SA? new [Re: Jeff Morgan]
      #5843198 - 05/06/13 10:10 AM

Jeff the only thing that SIPS solved was the ability to focus any ep without having to concern yourself with setting the paracorr to the right setting with each ep used.

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

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5843228 - 05/06/13 10:36 AM

Quote:

Jeff the only thing that SIPS solved was the ability to focus any ep without having to concern yourself with setting the paracorr to the right setting with each ep used.




It also solved the issue of what to with those extra hundred dollar bills in your wallet.

It makes sense for the guy that has one fast Newtonian, not so much for the guy that has several.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
Cames
sage


Reged: 08/04/08

Re: Does paracorr add any SA? new [Re: Jeff Morgan]
      #5843249 - 05/06/13 10:50 AM

What do you think of this test for the degree of spherical aberration introduced to an optical train by the Paracorr?

Test apparatus required:
Paracorr, a 250 line/inch Ronchi eyepiece, an accurately-collimated newtonian reflector which is known to be free of significant spherical aberration.

Method:
1. wait for a near-ideal night and observing conditions with respect to great seeing and minimal scintillation.

2. allow the system to achieve, as closely as possible, thermal equilibrium

3. critically examine (or even photograph) the images in the Ronchi eyepiece with and without the Paracorr inserted and look for signs of SA.

Would this test be sensitive enough to answer the intent of OP's query? Has anyone tried something like this before?
------
C


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Does paracorr add any SA? new [Re: Cames]
      #5843321 - 05/06/13 11:25 AM

Quote:

What do you think of this test for the degree of spherical aberration introduced to an optical train by the Paracorr?

Test apparatus required:
Paracorr, a 250 line/inch Ronchi eyepiece, an accurately-collimated newtonian reflector which is known to be free of significant spherical aberration.

Method:
1. wait for a near-ideal night and observing conditions with respect to great seeing and minimal scintillation.

2. allow the system to achieve, as closely as possible, thermal equilibrium

3. critically examine (or even photograph) the images in the Ronchi eyepiece with and without the Paracorr inserted and look for signs of SA.

Would this test be sensitive enough to answer the intent of OP's query? Has anyone tried something like this before?
------
C



I've done the star test both with and without Paracorr and found no visible difference at all with an on-axis star of magnitude 2 (not as much seeing-related flare as a brighter star). No SA at all or any aberration I didn't already know was in the scope.

However, take that star off-axis, and the WITH Paracorr star is definitely better focused and has less flare.
Near the field edge, the star is still well-focused and without flare.
Without the Paracorr, only the dead center (or a tiny portion thereof) is sharp.


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

Reged: 10/09/02

Loc: Chicagoland
Re: Does paracorr add any SA? new [Re: Starman1]
      #5843333 - 05/06/13 11:33 AM

I suspect the amount of SA added or strehl reduced is dependent on f ratio. Maybe a star test doesn't show any noticeable differences at f5 but f4 could be another story. I think you would need more than a simple ronchi grating to reveal anything but rather a true null test. That would be the equivalent of taking one edge of a line on the ronchi test and expanding it to cover the whole surface. That gives you a much more sensitive null test and requires using either Polaris or having tracking to make work. I plan to try to a null test with and without a paracorr at f/4.4 here at some point soon.

Post Extras: Print Post   Remind Me!   Notify Moderator  
DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Does paracorr add any SA? new [Re: Cames]
      #5843339 - 05/06/13 11:39 AM

Even thou the light is inside the Airy disk it is increased in size from the Paracorr so the Strehl ratio will be worse when using it with a perfect parabola.
The other problem is that your assuming that your optics are dead perfect, if not then the SA from the Paracorr will add to the spherical aberration of the primary and can result in an image that is less then diffraction limited. The Paracorr adds undercorrected spherical to the image. This is why with the Type-I units, they were best teamed up with the primary that was over corrected to a mild hyberbola.
If you use a single pass test using another telescope to produce a collimated beam, you need to remember that any error in that telescope directly adds to the test results. So if the optics have an 1/8 wave of under correction from the collimating telescope, that will adds to what ever the optics are in the telescope that has the Paracorr attached. So you can have a situtation were both telescopes are 1/8 wave under corrected and in addition to what the Paracorr is adding and when you test the system you see a large amount of undercorrection of over 1/4 wave. This would lead to false negative result that the Paracorr adds a fair amount of spherical. So just be careful that you understand the accuracy and precision of your test methods.

- Dave


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

Reged: 10/09/02

Loc: Chicagoland
Re: Does paracorr add any SA? new [Re: DAVIDG]
      #5843387 - 05/06/13 11:59 AM

Dave,
Very interesting. So the paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5843536 - 05/06/13 01:03 PM

Quote:

Dave,
Very interesting. So the Paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.



If it adds, say, 1/100 wave of undercorrection, which mirror maker would make a mirror accurate enough to add 1/100 wave of overcorrection to compensate? None.
This is a little like debating whether a 9-point mirror support is poorer than an 18 point if the 9-point deviates from perfect by 1/250 wave and the 18-point by 1/500 wave. Neither will produce anything visible.
Look at the charts. The amount of axial SA is smaller than the Airy disc all the way to below f/3. And look at the miniscule amount added. It won't be visible at all. If you see SA when star testing with the Paracorr in place, it's due to the mirror.


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


Extra information
10 registered and 25 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: 1249

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics