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Darren Drake
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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)


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jpcannavo
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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


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Mirzam
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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


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hbanich
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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.

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jpcannavo
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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


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Jim Romanski
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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.


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Starman1
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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.


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mark cowan
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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


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dave brock
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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


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mark cowan
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Re: Does paracorr add any SA? new [Re: dave brock]
      #5842849 - 05/06/13 01:34 AM

Hmm, didn't specify.

Best,
Mark


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Darren Drake
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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.

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Jeff Morgan
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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.

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Darren Drake
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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.

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Jon Isaacs
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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


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Cames
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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


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Starman1
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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.


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Darren Drake
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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.

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DAVIDG
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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


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Darren Drake
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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.


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Starman1
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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.


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Darren Drake
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Re: Does paracorr add any SA? new [Re: Starman1]
      #5843568 - 05/06/13 01:17 PM

I would like to see bench testing done with and without a paracorr in place for a relatively fast mirror. 1/100 wave difference is the same as no effect and I don't believe this is the case with faster mirrors. The Telescopes Eyepieces Astrographs book doesn't get real specific on this. It doesn't even mention the paracorr by name and says it has a mag of 1.11X instead of 1.15. There is also no detailed spot diagram I suspect since the specs aren't avaialble.

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DAVIDG
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Re: Does paracorr add any SA? new [Re: Starman1]
      #5843647 - 05/06/13 01:47 PM

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.com/engine/TV3b_page.asp?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

- Dave


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mark cowan
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Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5843768 - 05/06/13 02:51 PM

Quote:

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.




You might just discuss the amount of undercorrection the original Paracorr introduced with Al himself. That idea was on the table but I passed. However anybody else who wanted to probably could. More interesting however is to build hyperboloidal mirrors to a conic designed into a Paracorr III that would also get flatter field...

Dave is correct here regarding type I vs type II, and indeed it adds more for faster mirrors, basically the price you pay for the coma reduction.

NB: 1/100th wave was re. the Ethos on-axis SA design amount, not the Paracorr I or II and not the combination of the two. In practice (of course) the optics aren't manufactured at design levels, SFAIK, but the newer designs are intended to function better with the coma correction at fast f/ratios than older ones could. A lot of it has to do with how the converging beam intersects the elements of the EP as it approaches the focal plane, and it ain't simple.

Best,
Mark


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Starman1
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Re: Does paracorr add any SA? new [Re: DAVIDG]
      #5843846 - 05/06/13 03:42 PM

Quote:

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.com/engine/TV3b_page.asp?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

- Dave



The yellow shaded area is the size of the Airy disk, not 1/4 wave of aberration.


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DAVIDG
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Re: Does paracorr add any SA? new [Re: Starman1]
      #5843979 - 05/06/13 04:46 PM

When the image is larger then the Airy disk it is no longer diffraction limited by definition. So when one has exactly 1/4 wave optics they produce an image in which all the light just fits inside the Airy disk. So this is the upper bounds of the shaded yellow area. So one can relate the theoretical diameter of the spot formed when using a Paracorr which in this case is labeled Airy disk radius to the wave front and estimate the amount of spherical aberration the Paracorr is generating.

- Dave


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Starman1
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Re: Does paracorr add any SA? new [Re: DAVIDG]
      #5844048 - 05/06/13 05:26 PM

Quote:

When the image is larger then the Airy disk it is no longer diffraction limited by definition. So when one has exactly 1/4 wave optics they produce an image in which all the light just fits inside the Airy disk. So this is the upper bounds of the shaded yellow area. So one can relate the theoretical diameter of the spot formed when using a Paracorr which in this case is labeled Airy disk radius to the wave front and estimate the amount of spherical aberration the Paracorr is generating.

- Dave



I disagree with that analysis.
1/4 wave of SA in the wavefront modifies the MTF and changes the image in a detectable way in the star test and on the test bench.

The Airy disk is determined by the aperture and f/ratio of the scope, and would be no different if the scope had ZERO spherical aberration.

The idea that 1/4 wave is somehow "diffraction-limited" or essentially produces a perfect image no one can tell apart from perfection is a historical, experiential, analysis by 18th and 19th century observers. 1/4 wave SA is very noticeably different on an MTF graph from perfect optics.

So your assumption the Airy disc size represents 1/4 wave of SA is incorrect. It represents theoretical perfection. The wavefront error that produces visible errors in the star image can be much smaller than 1/4 wave, per Suiter, and Aberrator, though the aberrations that throw light out of the Airy disk into the diffraction rings will reduce the size of the disk rather than increase it.

Though, I agree, if the SA added by the coma corrector is additive to that already present in the optics, it's not ideal. But it is obvious from the graphs the addition will be very small. I can vouch for that, since I DO know how to perform a star test (and I usually see all kinds of aberrations in scopes, the two most common of which are astigmatism and spherical aberration), and the With/Without tests using the Paracorr II display no visible axial changes at all in my f/5 scope.
Of course I cannot extrapolate those results to f/3, but I can speak about f/3.6 and a Paracorr I, where the improvement in star images was profound. And that aperture was always seeing limited anyway.

There are many things to be concerned with in optics, but the added SA from a Paracorr isn't one of them.
But don't take my word for it--call up TeleVue and ask.


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jpcannavo
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Re: Does paracorr add any SA? new [Re: Starman1]
      #5844496 - 05/06/13 09:47 PM

Quote:


But it is obvious from the graphs the addition will be very small. I can vouch for that, since I DO know how to perform a star test (and I usually see all kinds of aberrations in scopes, the two most common of which are astigmatism and spherical aberration), and the With/Without tests using the Paracorr II display no visible axial changes at all in my f/5 scope.





The star test is sufficiently sensitive that if ain't revealing an aberration to an experienced practitioner, the presence or absence of said aberration is of academic interest only.

http://www.opticsinfobase.org/josa/abstract.cfm?uri=josa-50-1-21

Also agree with Don in pointing out what may be some questionable interpretations of the Televue diagrams, perhaps involving a confusion of two entirely distinct phenomena.

A spot diagram is a purely geometric optic ray tracing that assumes light to be moving in simple straight ideal lines. As such, a spot diagram for a perfect optical system generates an infinitesimal point, while for an aberrated system it generates some blurred - i.e. a non-point - spot. But, in contrast to this geometric optic idealization, the physical optics of the situation calls for an Airy disk (wave nature of light, diffraction etc) from the perfect system - which is not an infinitesimal point at all. Now, recognizing this as something of an apples and oranges mix, the size of the Airy disk is typically included in spot diagrams to provide a metric of sorts to the depiction of blur size. (Note that in the TV website the more usual depiction is essentially mapped onto a graph - spot size and Airy disk on y axis etc - but the relevant distinctions are the same) Moreover, associated with this deliberate mixing of apples with oranges is a rough rule of thumb, whereby optical systems generating blur sizes no larger than the Airy disk are sometimes taken as being well corrected with respect to the aberration in question (coma, chromatic aberration, astigmatism etc.). But a quantification of wavefront optical quality (i.e. strehl, MTF, wavefront error) is by no means some simple (i.e. independent of the specific aberration/aberrations in question) function of the ratio of blur size to Airy disk size. So, for example, the deviation from perfect strehl for a blur size exactly equal in size to the size of the Airy disk is not the same when the aberration in play is astigmatism than when it is spherical aberration - a fact that is by no means evident from a mere inspection of said blur and Airy disk depictions, and instead calls for a far more complex level of analysis.
Joe


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Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5845105 - 05/07/13 08:38 AM

Tried that with my 15-inch F/4.5, which does have slight spherical abberation. I can see no change insie and outside of focus with and without the Paracorr. If it introduces spherical abberation of its own, I haven't seen any sign of it.

Taras


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