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Equipment Discussions >> Eyepieces

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bgavin
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4650264 - 06/21/11 12:01 AM

OPT has a nice deal going on the Televue TRG-1072 t-ring adapter for the Type-II Paracorr.
I try to support our sponsor, but they are $10 higher for the shipped item.

This adapter screws directly to the Type-II optical assembly when the tunable top is removed. It presents a standard T-Ring thread which mates with my Nikon T-adapter.

As it turns out, the Nikon sensor distance + T-ring + TRG-1072 is almost the perfect back focus distance.
The native prime focus (no Powermate) calculates as pretty close to the FOV seen by the Pan27 on my scope.

I already have the Powermate PTR-1250 adapter that mates the 1.25" Powermates to standard t-adapters.
This is surprising high magnification, so I'm thinking the unamplified prime focus might be more useful.
Either way, I get the full benefit of the Type-II optics for the final image.

Overall, the Type-II really impresses me with its very high quality construction and workmanship.
The TV gear is definitely worth the extra $$.

I run the Pan27 at the recommended setting, and the views are just fabulous.. all the way to the edge.


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rprice
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Re: List of Paracorr settings for all brands of EP new [Re: bgavin]
      #4653092 - 06/22/11 03:48 PM

I posted this to a thread in the reflector section but I will repeat it here as this seems a more appropriate place.

I have seen a few posts from time to time asking what the Paracorr setting is for a particular eyepiece, and of course there is the lead post at the top of this section on Paracorr settings for different brands of eyepieces. However, the Paracorrs are easy to setup or calibrate on any scope and you can determine the correct setting for any eyepiece easily yourself - and you do not need a Televue eyepiece to do it.

The Paracorr is designed to be placed in the scope's light path at a position that achieves a specific distance from the Paracorr's last lens vertex to the focal plane; this is often called bfl or back focal length. The Paracorr is setup in your scope just as if you were going to do imaging with a camera or CCD; instead of placing a sensor at the focal plane you simply look at the focal plane with an eyepiece.

This specific distance is defined in the Televue literature several places. It is listed as 55 or 56 mm +/-4 mm for the PC1 and 55 or 57mm for the PC2. The tolerance of +/- 4 mm is generous so lets just say its 56 mm (2.20").

So, find a plastic soda straw or swizzel stick and make a mark on the side of the straw with a pen or sharpie 56mm (2.20") from one end. Loosen the hold down screw for the tunable top and place the PC on a table with its axis vertical. Carefully place the end of the straw or stick into the PC until the end just touches the center of the last lens. Adjust the tunable top until its top surface is even with the mark that you made on the straw. A flat item, such as a short plastic ruler laid across the top of the tunable top makes this easy.

Your PC is now calibrated to have the focal plane coincident with the top of the tunable top. Now all you do is place a strip of Scotch Magic tape (or other translucent cellophane tape) across the top of the tunable top; make sure the tape is taught, not sagging across the top. Put the PC in your scope, line up on a bright star and move the focuser until the star is sharply focused on the tape. Then, lock the focuser, or be sure to not touch the focuser knob.

You are all set to find the PC setting for any eyepiece - just put in your eyepiece and adjust the tunable top (not the focuser knob) until a star is focussed and record the setting. Repeat for any other eyepieces in question. Keep in mind that there is no guarantee that the PC will be able to focus your eyepiece, especially the older PC1. Many eyepieces (especially 1.25" ones) need more "in" travel to be properly set; you may need to find a different 2" to 1.25" adapter than the one that came with the PC1 with a thinner flange to allow the eyepiece to go further into the PC. I had to machine a special thin flange adapter to allow several of my eyepieces to focus.

But let's go back a few steps and look at the position of the tunable top after we calibrated it with our plastic straw "gauge". You will likely find that the PC1 tunable top is set right at or very close to the middle (3) mark; if you did the calibration on a PC2 you will find that the tunable top is set at the middle or "E" position. This is no coincidence. The older PC1 literature did not say much but the new PC2 literature shows clearly that the focal plane of the telescope is located at the top of the tunable top when at the "E" position without the 1.25" adapter, and the focal plane is located at the top of the inserted 1.25" adapter when at the "A" position.

So you do not even have to go through the "calibration" steps using the plastic straw we discussed above, unless you just want to prove it to yourself. Simply set the PC as follows:

PC1 - "3" setting (middle position)
PC2 - "E" setting for 2" mode
PC2 - "A" setting for 1 1/4" mode

Put the magic tape across the top, focus a star on the tape and you are ready to go as described above. Use your straw or stick to enjoy your favorite beverage....

Clear skies,

Ron


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: rprice]
      #4659559 - 06/26/11 11:42 AM

Ron,

Thanks for the info. I have the PC1. If I were to use setting "3" and a piece of clear tape to set the focal position of my scope, should I take off the 1.25"-2" adapter? Should I place the tape directly over the top of the Paracorr without the adapter? That's what I would think, but I like to remove all ambiguity.

Also, if anyone were to go through the calibration process, they should use a Q-tip or something similar that has a soft end rather than a straw, to avoid scratching the Paracor lens.

It is true that there are many 1.25" eyepieces that will not come to focus in the PC1. I just tried to determine the Paracorr settings for eleven 1.25" eyepieces. Seven out of eleven eyepieces would not come to focus. There was not enough in-focus. I was using the TV 1.25"-2" adapter that comes with the PC1. I might try other adapters.

I'm definitely not buying the PC2. Forgetaboutit. I'm either going to make the PC1 work for me or I'm going to sell it. So far this gizmo is not worth all the trouble.

Mike


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4659599 - 06/26/11 11:54 AM

Last night I went to a dark site. The predictions on three different websites were for a clear sky. Wrong! The sky was cloudy all night. But I could see some bright stars. So the night would not be a total waste, I took out my PC1 and started determining settings for some of my eyepieces. For all these eyepieces I used the 1.25"-2" adapter supplied with the PC1.

Here are the results:

ES 4.7 82deg: 5 (This matches thewheel's finding.)

Meade 5k UWA 6.7: Not enough in focus

Vixen LVW 8: Not enough in focus

AT Paradigm Dual ED 12: Not enough in focus

ES 14 82deg: 5

Faworski Super Abbe Ortho 16.8: Not enough in focus

Orion Epic ED-2 22: Not enough in focus

Meade 5k SWA 24: Not enough in focus

UO Abbe Ortho 25: 2.75

Brandon 32mm: 3.75

Baader Mark-II Zoom (1.25" mode): Not enough in focus

Mike


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star drop
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4659806 - 06/26/11 01:49 PM

The Paracorr 2/1.25" adapter has a thick flange. I have an adapter that came with my Tectron telescope that has a ~1/32" flange. Using something like that might give you ~3/8" more in focus.

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Starman1
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Re: List of Paracorr settings for all brands of EP new [Re: star drop]
      #4659867 - 06/26/11 02:24 PM

And AstroSystems has an adapter on their website that actually allows the eyepiece to drop BELOW the lip by 1/2".
So, depending on which adapter is used, you should be able to achieve enough inward movement of the tunable top for most 1.25" eyepieces.
2" eyepieces? Well, it depends. I think that the 31 Nagler and 21 and 17 Ethos eyepieces, which necessitate a setting lower than the lowest setting on the Paracorr 1, were, in addition to the shorter f/ratios becoming more common, a reason for the creation of the Paracorr 2, which has lower settings than the Paracorr 1.
For 1.25" eyepieces, though, various adapters seem to suffice.
Most 1.25" eyepieces use setting 4, some use setting 5, but if the tunable top needs to go lower than setting 5, a shorter adapter is all you can do. And if that's still not perfect, then use the PC on setting 5. Like the 31 Nagler, it's not going to be perfect, but it will be a lot better than no Paracorr at all.


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: Starman1]
      #4660247 - 06/26/11 06:33 PM

I have several 1.25"-2" adapters here that I could use. I'll compare them with the TV adapter. Just by a quick glance, I know that my Orion "Precision Centering Adapter" is a little lower than the TV one.

An adapter that allows the eyepiece to drop below the lip by 1/2" seems to be the way to go. I've seen these online, thought about buying one. This will be a good time to do it. I did want to be able to use a wide variety of 1.25" eyepieces with the Paracorr for observing planets. Hopefully the Paracorr will clean up the coma enough to improve my observations of planets even more than I've already been able to do up to now. (Too bad the Paracorr can't be used with a binoviewer.)

Mike


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: Starman1]
      #4660376 - 06/26/11 07:53 PM

Don,

Quote:

And AstroSystems has an adapter on their website that actually allows the eyepiece to drop BELOW the lip by 1/2".




I found two of these low profile adapters. One is on the AstroSystems website. They say that their adapter will "lower the profile of your 1.25" eyepiece another 0.5" below the drawtube." $29 + shipping.

Ultra low Eyepiece Adapter - 2" to 1.25"

The other low profile adapter is advertised on the ScopeStuff website. According to ScopeStuff, their "Negative Profile Eyepiece Adapter allows the eyepiece to be inset over 3/4" more than a low profile adapter." $39 including shipping.

Negative Profile Eyepiece Adapter, 1.25" to 2"

I wonder if they are the same adapter, or are essentially the same? I'm tempted to order the ScopeStuff adapter for the extra 1/4" eyepiece inset. However, judging by the two descriptions, AstroSystems and ScopeStuff might measure the amount of extra inset in different ways, so that actually they both may have the same inset?

I'm tempted to go with the ScopeStuff adapter.

Mike


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Starman1
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4660400 - 06/26/11 08:11 PM

Mike,
I think they are essentially the same adapter. One has a 1/2" drop from the lip, and the other has a 3/4" drop from a low-profile adapter (which would typically have about a 1/4" lip).
One caveat: these provide the full drop only with small diameter 1.25" eyepieces. Many 1.25" eyepieces today don't even fit into the recess because the upper sections of the eyepieces are too large. Because there is no screw above the top of the focuser, the upper lip is as thin as they get, so even without fitting into the recess, the adapter is still thinner than most adapters. You'll gain a setting, maybe two, on the Paracorr even if the eyepiece doesn't fit in the recess.


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: Starman1]
      #4660478 - 06/26/11 09:07 PM

Don,

My simple, old-school eyepieces such as the Orthos, Brandons, RKEs and Plossls should have no problem fitting into the "well" in the adapter. There may be a problem with already low eye relief getting even lower. But it's definitely worth $30 or so if it will allow many of my 1.25" eyepieces to perform better with the Paracorr.

Mike


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rprice
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4661509 - 06/27/11 02:04 PM

Mike,

Quote:

Thanks for the info. I have the PC1. If I were to use setting "3" and a piece of clear tape to set the focal position of my scope, should I take off the 1.25"-2" adapter? Should I place the tape directly over the top of the Paracorr without the adapter? That's what I would think, but I like to remove all ambiguity.




Yes, when using setting 3 the tape should be placed across the top of the PC1 without the 1.25" adapter.

Quote:

I'm definitely not buying the PC2. Forgetaboutit. I'm either going to make the PC1 work for me or I'm going to sell it. So far this gizmo is not worth all the trouble.






I will agree with you on this point. My scope is a 24" f/3.65 and I recently purchased a Paracorr2 to see how it compared to the PC1. The short answer is there is no discernable difference between the two Paracorr versions from a visual observing standpoint for my scope. I am sure that if I was CCD imaging over a large field of view I would see a slight improvement in the quality of the star images at the edge of my image with the PC2, but visually there is no difference.

I compared the PC1 and PC2 using a 22NT4 eyepiece in my scope, going back and forth several times carefully evaluating the quality of a star image at center, halfway out, 3/4 of the way from center to edge and right at the field stop. There was virtually no difference between the two Paracorrs.

There have been a few comments going around that the Paracorr2 has better coatings than the somewhat older PC2, so I evaluated limiting magnitude between the two PCs. I looked at several star fields, going back and forth between the PC1 and PC2, and I could see no difference.

In all honesty, I really did not expect to see a difference between the two Paracorrs. If you carefully examine the performance graphs provided by Televue for the PC1 and PC2, while the PC2 performance is completely diffraction limited for my f/3.65 scope, even the older PC1 brings the off-axis spot size for a star at the edge of the 22NT4 field stop within a factor of 2 or so of diffraction limited. This is still a very tight image and its defects would not be visible at the very low power of the 22NT4; you need at least 25 to 30X per inch to start seeing details of the diffraction disc and the 22NT4 is only giving me 5X per inch.

So what's going on?? Well, in a nutshell, coma is not the problem; it is off-axis astigmatism (and a small amount of field curvature) in the long focal length, ultra-wide angle eyepieces that I use in my scope. Many people state that they see lots of coma in their fast Newtonian reflector when they do not use a Paracorr, but I submit they are not seeing coma at all; they are seeing predominantly off-axis astigmatism generated by the eyepiece due to the fast f/#. This was pointed out over 20 years ago in the book Telescope Optics by Rutten and Van Venrooij, and we all know that Newtonians have gotten much faster than the f/5 versions that were evaluated in that book.

Star images appear better when you add the Paracorr not because you have eliminated coma, but because you have slowed the f/# down by 15% reducing the off-axis astigmatism produced by the eyepiece. In my case it changes my f/3.65 scope to an f/4.2 scope. This 15% change may not sound like much, but I believe that the off-axis astigmatism generated by the eyepiece is inversely proportional to the f/# squared, so this would be a 32% difference.

This new generation of ultra-fast Newts is really catching on and I believe rightly so. I love standing on the ground half the time and only going up two short steps to look at an object near the zenith. And my scope is very compact and easy to move in and out of the garage. You just need to be aware that even the best available long focal length (> 20mm), wide angle (>65 degrees AFOV) eyepieces are going to have poor edge performance, Paracorr not withstanding. Shorter focal length / higher power eyepieces work much better because they do not exhibit near the off-axis astigmatism; Pentax XW's 10mm and shorter, Radians 14mm and shorter work quite well at f/3.65 (without PC) with only minor defects in images near the field stop. I am anxiously awaiting the new Delos line from Televue, hoping that they will perform well at these fast f/#s.

Sorry for the long winded reply.

Ron


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: rprice]
      #4661598 - 06/27/11 02:51 PM

Ron,

Thanks for the "long winded reply." I am glad to hear that apparently the only observable difference between the PC1 and PC2 is the wider range of in-focus settings for the PC2. That will probably be particulary true in my Newts, which are all f/4.8 and slower. Hopefully I can work around the not-enough-in-focus problem in my PC1 by using the negative profile adapter I ordered last night.

Unless the observer is experienced at evaluating eyepieces, it can be difficult to distinguish among coma, astigmatism, field curvature, etc. I don't presume to have the level of expertise as some of the experts on CN. If the image looks better to my eyes, I am happy. Also it should be kept in mind that at low power, the observer may start seeing whole eye astigmatism, which we cannot blame on the scope, the eyepiece or the Paracorr. FWIW, I've noticed that sometimes SCT users, who might not be accustomed to wide exit pupils, are surprised at the level of astigmatism that can be seen in Newts at low power, whether from eyepiece or eyeball or both.

Mike


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Starman1
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Re: List of Paracorr settings for all brands of EP new [Re: Sarkikos]
      #4661659 - 06/27/11 03:32 PM

I evaluated both older and newer Paracorrs in my f/5 scope.
My findings were totally expected:

31 Nagler and 21 Ethos had improved edge-of-field stars in the Type 2 because of the lower setting possible on the Type 2. The Type 1's lowest setting wasn't low enough for optimum edge correction.

I saw no other obvious differences (I didn't expect to at f/5 because f/5 was fully-corrected by the Type 1 Paracorr).

Whereas Ron is mostly right about many inexpensive 2" eyepieces and astigmatism, the Nagler he compared, and my Ethos eyepiece set, has no edge-of-field astigmatism. The design is free from that problem down to f/4.

In my case, f/5 is not short enough to induce any form of astigmatism at the edge of the field in the Ethos eyepieces, yet, without the Paracorr, all my eyepieces display what is, to my eye, horrible coma. The star images are probably 10X as long, radially, as they are circumferentially. With the Paracorr, the stars at the edge are small points.

You see, coma is a linear problem that starts in the center and gradually increases toward the edge. The linear size of a comatic star depends solely on the distance from the center of the focal plane of the scope.
But the apparent size of the comatic star image depends on magnification.

So, for example, a 100 degree 21mm eyepiece will have the same magnification as a 21mm 50 degree eyepiece, but because the radius of the field will be twice as large in the 100 degree eyepiece as in the 50 degree eyepiece, the linear AND apparent size of the comatic star images at the edge of the field in the 100 degree eyepiece will be twice as big, and appear worse.

Double the magnification but keep the apparent fields the same, and the 50 degree eyepiece will see a comatic star image at the edge which has half the linear size, but at twice the magnification. Hence, the comatic images at the edge will appear the same as at half the power. The same is true of the 100 degree eyepiece. The comatic star images appear twice as wide as the 50 degree eyepiece, and it displays the same difference with the 50 degree eyepiece as at the lower power.

So the visibility of coma is related to the apparent field of the eyepiece: Wider = More coma at the edge.

And if the eyepiece is fully corrected for astigmatism at the f/ratio you are using, you will see coma, and perhaps de-focusing caused by field curvature, at the edge, but not astigmatism. Of course, field curvature, which might cause the stars to bloat, may make coma appear worse than it is.
I had an eyepiece that had horrible coma at the edge that the Paracorr corrected, leaving small, out-of-focus, star images due to that eyepiece's inherent field curvature. I dealt with it by focusing at the 50% field point and letting my vision correct for the center and edge. That worked because it was a low-power eyepiece.

I found myself surprised at how bad coma actually appeared at f/5. I thought, and still think, that a coma corrector is essential at f/5 to produce good star images over the entire field of view. Looking at the correction charts on the TeleVue site corroborates what I see.

But Ron is right that if the eyepiece has substantial astigmatism, correcting the coma will not be sufficient to correct the star images at the edge of the field. If you're trying to correct an Erfle eyepiece, for instance, the star images at the edge after coma correction will still appear non-stellar.
But, the best-corrected eyepieces of today will have star images tight enough at the edge, if corrected by a coma corrector, to see a globular cluster exit the field with the core still completely resolved. I am certain of that--I've seen M15 exit the edge of my 13 Ethos with the core still resolved into tiny pinpoint star images.
The correction at f/4 won't be as perfect, but the difference will not be that bad. The coma-corrected f/4's star images are still WAY better than the non-coma-corrected f/5's.


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rprice
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Re: List of Paracorr settings for all brands of EP new [Re: bgavin]
      #4661852 - 06/27/11 05:55 PM

Don,

Quote:

Whereas Ron is mostly right about many inexpensive 2" eyepieces and astigmatism, the Nagler he compared, and my Ethos eyepiece set, has no edge-of-field astigmatism. The design is free from that problem down to f/4.






That is an interesting statement; the word "no" is pretty absolute. I would appreciate your referring me to where this is stated in Televue literature or elsewhere.


Quote:

In my case, f/5 is not short enough to induce any form of astigmatism at the edge of the field in the Ethos eyepieces, yet, without the Paracorr, all my eyepieces display what is, to my eye, horrible coma. The star images are probably 10X as long, radially, as they are circumferentially. With the Paracorr, the stars at the edge are small points.






What you are describing does not sound like coma; the comatic blur does not have a 10 to 1 aspect ratio like that. If you place a bright star at the edge of the field and focus in and out, you will probably find that the very long radial image will cross over into a circumferential one - this is what I see at the edge of the field with long focal length /wide field eyepieces and it is astigmatism, not coma.

Your description of coma as varying linearly with field is correct. And since coma varies linearly and if you keep the AFOV the same over a range of magnifications, then the coma will appear roughly the same at all of the different magnifications. But it is common for fast Newtonian users to employ a Paracorr with their lowest power eyepieces, but feel they do not need it with shorter focal length eyepieces.

For example, I always use the PC when using my 22NT4 or similar focal length, wide angle eyepiece. But when I switch to higher power, I don't use the PC. For example, I can use 10, 7, 5 and 3.5 XW's, or short fl Nagler T6s without a PC and the field looks sharp except for the very edge, where the image degrades somewhat. This is possible because very high quality short focal length eyepieces are well corrected for off-axis astigmatism at fast f/#s, so in this case all I am seeing is coma, and nothing else. The problem of off-axis astigmatism at fast f/#s is only present at the long focal lengths and ultra wide fields. Off-axis astigmatism scales linearly with eyepiece focal length; a 20mm eyepiece of a given design will have 4 times as much as a 5mm eyepiece of this same design.


Quote:

But, the best-corrected eyepieces of today will have star images tight enough at the edge, if corrected by a coma corrector, to see a globular cluster exit the field with the core still completely resolved. I am certain of that--I've seen M15 exit the edge of my 13 Ethos with the core still resolved into tiny pinpoint star images.
The correction at f/4 won't be as perfect, but the difference will not be that bad. The coma-corrected f/4's star images are still WAY better than the non-coma-corrected f/5's.






The first part sounds perfectly reasonable at f/5. With the PC in place the Ethos eyepiece is working at almost f/6 (f5.75), so its performance should be superb. You did not state what focal length the Ethos is, but the shorter the better. I also see a similar view when using the 10mm or 7mm XW without a PC; I let a bright globular drift across the field and it stays resolved to the core up until the last 5 degrees of apparent field.

My point in all this is that a specific group of eyepieces - 20mm and longer focal length with AFOVs of 70 degrees or more - exhibit off-axis astigmatism with sub f/4 primary mirrors. And I am not talking about war surplus Erfles, I am talking about the best eyepieces money can buy in this group. Extrapolating the performance of these long focal length ultra-wide angle eyepieces at f/5 to what happens at f/3.65 and faster (f/3.3 scopes are in production now) is simply not valid.

And I am not trying to be critical of the eyepieces that are available; the Nagler, Ethos and Pentax XW eyepieces are fantastic and better than anything that has been available to the amateur in the past for low power wide angle view through fast Newtonians. My comments are intended to be informational and to assist others that may own or intend to purchase one of these ultra-fast Newts.

Clear skies,

Ron


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Starman1
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Re: List of Paracorr settings for all brands of EP new [Re: rprice]
      #4661974 - 06/27/11 07:41 PM

Ron,
Al Nagler himself has stated that all their eyepieces are tested at f/4 to have no extra aberrations added due to the fat light cone with its oblique angles. The only design he admitted they have produced that did have astigmatism at the edge of the field were the original Widefields.
He told me that the Panoptic line was developed after the introduction of the Paracorr where, when coma was corrected, the residual astigmatism became visible in the Widefields. After that, their design goal was zero astigmatism.
[and zero angular magnification distortion, but that's another story].

The comatic blur is shaped more like a narrow V that has a faint extension beyond the V shape. It is not astigmatism because it does not change from radial to circumferential on the other side of focus. I suspect many people do not view in skies dark enough to reveal the full extent of the comatic star image. 10X as long as wide might be a slight exaggeration, but it is not unusual for the star image to be many times as wide as the Airy Disc if coma is present.
Coma does not end with the edge of the visible image--it extends to fainter than the eye can see. The camera can capture it, though. Here is a good example of what I see without a coma corrector:
http://www.astronomytelescope.net/images/astronomytelescope99.jpg Would you not describe that as 10X as long as wide when the star image is at the point?
I would.

As for why newtonian users do not employ coma correction at higher powers, I am mystified. I see the exact same coma with my 8mm Ethos barlowed as with my 21 Ethos by itself. I have not see a high enough magnification that the coma was not present. Which is why I use a coma corrector at all magnifications and with all eyepieces. I see it in f/6 scopes, too. It's a matter of expectations: I want center-of-field images at the very edge. I don't get that, but it's close at f/5 with a coma corrector.

I am very familiar with aberrations in eyepieces, and I've owned over 300 of them. I know exactly what astigmatism looks like. One of the ironies of eyepiece design is that leaving positive angular magnification in the eyepiece and solving for rectilinear distortion, as is done on many eyepieces designed for terrestrial use, results in a smaller apparent size for the comatic blur at the edge of the field. That's the case for the Pentax XWs, which have less RD, but as a result, more AMD. Excessive AMD (on a wider design) is probably one of the reasons Pentax did not go wider than the 70 degree field (that, and the fact their spotting scopes don't illuminate a field much wider than that).

One thing I have experienced is that a lot of longer focal length eyepieces have more field curvature than shorter focal lengths (the Pentax XWs are an exception--it is the 20mm and 14mm that have the most field curvature). This tends to bloat the star images at the edge of the field, and exaggerate both coma and astigmatism. The worst "coma" I've seen in a scope was an f/4.3 newtonian without coma correction, using such an eyepiece. The scene in Star Wars when they go to superluminal speeds and you are looking at the stars from the bridge of the Millenium Falcon is only a *slight* exaggeration of what I saw (and some small scopes have star images that look like that all the time).


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rprice
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Re: List of Paracorr settings for all brands of EP new [Re: bgavin]
      #4662439 - 06/28/11 01:32 AM

Hi Don,

Your conversation with Al Nagler is interesting, but I am not able to dismiss the studies and evaluations that Rutten and Van Venrooij did in their book, and there are others that are more recent. And more importantly, the notion that Nagler and Ethos eyepieces are "perfectly" corrected for f/4 and faster cones is difficult to believe when I see something different in the eyepiece. It would be wonderful if Televue would provide performance graphs or spot diagrams for the long focal length Naglers and Ethos eyepieces showing how they performed at incremental f/ratios from f/5 to f/3. I commend them for providing the performance graphs for the PC1 and PC2.

Regarding your description and picture of coma, that is all quite reasonable. However, what I see at the edge of the field in my 24" f/3.65 with the 22NT4 and PC2 is not coma. It is a blur that goes from radial to circumferential in appearance and is predominantly astigmatism. Without the PC2, I see basically the same thing, only more severe.

As others have pointed out in some of the "fast scope" discussion threads, many have jumped onto the sub f/4 Newtonian bandwagon before all of the details and nuances of these scopes have been proven out, myself included. I am convinced, though, after using my new scope over a hundred times this past year that it is a resounding success and will give me a lifetime of viewing pleasure. But I am also convinced that the unique issues of eyepiece performance in f/3.65 and faster scopes is still being 'shaken out', especially the lowest power, ultra wide field eyepieces.

The fact of the matter is probably that neither the PC2/Nagler or PC2/Ethos combination performs "perfectly" with the sub f/4 Newtonians, but I believe these two are the best choices you have right now at any price. I, too, want center-of-field images at the edge. So, one has to decide if it is worth spending $1000 to $1300 for a low power wide field combo that gives you significantly improved but not "perfect" performance, or wait until something better comes along.

Thanks for your comments.

Clear skies,

Ron


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photiost
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Re: List of Paracorr settings for all brands of EP new [Re: rprice]
      #5062881 - 02/08/12 08:08 PM


This being a super long thread ..

I would just like to ask if anyone has finally compiled a table showing the Paracorr setting(s) for different eyepieces.


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Sarkikos
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Re: List of Paracorr settings for all brands of EP new [Re: photiost]
      #5087418 - 02/23/12 02:37 PM

I am slowing determining the settings for my eyepieces. A few days ago I found that the setting for an ES 82 deg 30mm eyepiece is about 2.25. This was in my Paracorr I. I was using a 10" f/4.7 Dob. I primed the Paracorr and focuser with my ES 100 deg 9mm at setting 2.

A setting of 2.25 for the ES 82 30mm produced sharp stars across the FOV. The Double Cluster looked especially nice. I recall seeing a little grouping of reddish stars within the following cluster. Very sharp, good color rendition.

Mike


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Richard Low
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Re: List of Paracorr settings for all brands of EP new [Re: photiost]
      #5213464 - 05/09/12 10:20 AM

Quote:


This being a super long thread ..

I would just like to ask if anyone has finally compiled a table showing the Paracorr setting(s) for different eyepieces.





Here is a working list, perhaps someone could review on its accuracy. Kindly add or edit where necessary.

Paracorr setting 1
Meade QX 36mm
Meade Series 4000 UWA 14mm
Pentax XW 14mm, 20mm
Speers-Waler variable 5-8mm
Siebert Observatory Series 34mm
Televue Ethos 10mm, 13mm in 2" mode
Televue Nagler T5 20mm, 31mm
Televue Nagler T2 20mm
Televue Nagler 13mm (original) in 2” mode
Televue Panoptic 41mm, 35mm, 27mm,
Televue Panoptic 22mm in 2” mode
Televue Widefield 32mm

Paracorr setting 2
Explore Scientific 100° 9 mm, 14 mm
Meade 5K 34mm SWA
University Optics 40mm MK-70

Paracorr setting 3
1rpd 30mm
Pentax XL 40mm
WO UWAN 28mm
Televue Nagler T4 17mm
Televue Nagler T5 26mm

Paracorr setting 4
Televue Ethos 3.7mm, 6mm, 8mm in 1.25" mode
Televue Nagler 1.25” originals, T5s, T6s – all in this series
Televue Radians – all in this series
Televue Panoptic 15mm, 19mm, 24mm
Televue Zooms – all in this series
Televue Plossls 8-32mm

Paracorr setting 5
Explore Scientific 100° 20mm
Explore Scientific 82° 4.7mm, 14mm
Meade Series 4000 UWA 8.8mm in 2” mode
Televue Ethos 17mm, 21mm
Televue Ethos 10mm, 13mm in 1.25" mode with TV Hi-Hat 2”-1.25” Adaptor
Televue Nagler T5 31mm
Televue Nagler T4 22mm
Televue Nagler T4 12mm in 1.25" mode
Televue Nagler T2 12mm, 16mm in 1.25" mode
Televue Nagler 9mm (original) in 1.25" mode

----------------

Paracorr Type 2

Paracorr setting A
Pentax XW – all in this series
Televue Ethos 17mm, 21mm
Televue Nagler T5 31mm
Zeiss Abbe Orthos – all in this series

Paracorr setting B
TV Ethos 6mm, 8mm in 1.25" mode
Televue Nagler T4 22mm

Paracorr setting C
Explore Scientific 100° 20mm

Paracorr setting D
Televue Ethos 3.7mm in 1.25” mode
Televue Nagler T6s – all in this series
Televue Nagler T4 12mm
Televue Nagler T5 16mm
Televue Radians – all in this series
Televue Panoptic 19 & 24mm
Televue Nagler Zoom 2-4mm, 3-6mm
Televue Plossls 8-32mm

Paracorr setting E
na

Paracorr setting F
Televue Nagler T4 17mm
Televue Nagler T5 26mm

Paracorr setting G
Televue Nagler T5 20mm
Televue Panoptic 35mm

Paracorr setting H
Televue Ethos 3.7mm, 6mm, 8mm in 2” mode
Televue Panoptic 41mm, 27mm
Televue Plossl 40mm


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nevy
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Re: List of Paracorr settings for all brands of EP new [Re: Richard Low]
      #5328241 - 07/21/12 12:52 PM

Does anyone know the best setting for a tv 55mm plossl ( smoothside) for the Paracorr type2 ?

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