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Good barlow for Tele Vue Paracorr Type-2

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#1 Tubuskahusk

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Posted 17 January 2022 - 09:08 PM

I just bought a Tele Vue Paracorr Type 2. I was wondering what barlow's you guys like to use with it. I have seen in a video review that if you don't use a Tele Vue barlow you need to be extra carefull on setting due to the length of the barlow. So if you guys could give me some honest options on what you think and like to use and why. I would greatly appreciate it.
Also if you use a barlow dose that change the eyepiece setting on the Paracorr Type 2?
If you guys use any eyepiece or barlow other than Tele Vue in any combination with the Paracorr 2 feel free to let me know. Thank you so much in advance.

#2 CrazyPanda

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Posted 17 January 2022 - 10:04 PM

According to Tele Vue, they recommend setting the Paracorr to the highest setting when using a 1.25" barlow. This is to avoid it accidentally bottoming out and hitting the lens. However, I have no idea what that does to coma correction.

My understanding with the Paracorr is that it's important for the Paracorr lens assembly to be positioned with respect to the telescope's focal plane. Once it is at the right position, then the tunable top just acts like a focuser for all other eyepieces. That's why it's best to not think of the tunable top as letting you find the right setting for a given eyepiece, it's so that you have a reference point so you know the right position for the Paracorr for your telescope. The distance between the Paracorr and eyepiece is irrelevant for anything other than focus.

 

But as soon as you introduce a 1.25" barlow, it seems all bets are off. You have to then use your telescope's focuser to reach focus since it might be too dangerous to use the tunable top on the Paracorr. But then that breaks that optimal Paracorr position with respect to the telescopes' optics.

 

If you have a 2x PowerMate, Tele Vue actually recommends putting it before the Paracorr, so that it simply acts as if your telescope has double the focal length. Then the Paracorr can behave as normal. I wouldn't do this with a normal 2" barlow though, because the long light path through the Paracorr would result in a much higher multiplication factor.

 

In my mind, the only amplifier that is truly compatible with the Paracorr and allows it to function as intended, is the 2x PowerMate.


Edited by CrazyPanda, 17 January 2022 - 10:06 PM.

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#3 Jon Isaacs

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Posted 18 January 2022 - 02:49 AM

My two cents:

 

A Barlow lens is actually part of the eyepiece and with a Paracorr can be treated as such. 

 

One must be careful to make sure a 1.25 inch Barlow is not too long, my 1.25 inch TeleVue Barlow is not too long with the Paracorr 2 at it's lowest setting but with a filter it would be.. The 1.25 inch GSO 2x Shorty Barlow is shorter than the TV 2x.. 

 

The TeleVue 1.25 inch 2X Barlow is parfocal, very little focusing is needed when inserting the Barlow so the Paracorr settings remain the same.  If the Barlow is not Parfocal, then when inserting the Barlow, do the coarse focusing with the Tunable Top.

 

With a 2 inch Barlow things get more complicated.  I wanted a 2X 2 inch Barlow for a very specific purpose, I wanted to double the magnification of the 13mm, 10mm and 8mm Ethos. That was it.   For this purpose, I purchased the GSO 2 inch 2X "ED" Barlow lens.  

 

The 13mm and 10 mm Ethos use the H position and with the 2 inch GSO Barlow, focus is achieved at the A position with about 0.075 inches of additional inward focus, that's less than one step (0.10 inches) and I find the coma correction to be uncompromised. The 8mm Ethos focuses even further out so if I remember correctly, it's somewhere around B or C setting.

 

I did a quick drift time and determined the magnification factor with the 13mm Ethos was about 1.95x.  I also purchased a shorter 30mm  tube, the GSO Barlow lens can be removed, and has filter threads.  It provided a 1.75x magnification factor and focus was somewhere between A and B with the 13mm Ethos, if I remember correctly. 

 

Regarding the Powermate:  The 2x Powermate doubles the focal length and the focal ratio.  A coma corrector adds reverse coma based on the focal ratio.. It doesn't seem right to me to double the focal ratio that the Paracorr is seeing.

 

Jon


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#4 CrazyPanda

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Posted 18 January 2022 - 08:22 AM

A coma corrector adds reverse coma based on the focal ratio.. It doesn't seem right to me to double the focal ratio that the Paracorr is seeing


That implies a coma corrector is focal-ratio specific and a CC optimized for F3 could make coma worse at F6. But that’s not the case with the P2 as far as I know.

Edited by CrazyPanda, 18 January 2022 - 08:22 AM.


#5 Starman1

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Posted 18 January 2022 - 12:52 PM

Correct.  The Paracorr corrects coma all the way up to f/8.

But, I agree with Crazy Panda.  The best Barlow with the Paracorr is the 2X 2" PowerMate.

Though it could look like this:

https://stargazerslo...comment-3118485

Lest you think that is ridiculous, my lifetime-best view of Jupiter was with the 2X PowerMate + Paracorr II + 8mm Ethos.

That was 456X in perfect seeing and Jupiter looked like an image, only sharper.



#6 Jon Isaacs

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Posted 18 January 2022 - 12:54 PM

That implies a coma corrector is focal-ratio specific and a CC optimized for F3 could make coma worse at F6. But that’s not the case with the P2 as far as I know.

 

Correct.  The Paracorr corrects coma all the way up to f/8.

 

Think about this:

 

Why is that the same coma corrector can add the correct amount of negative coma at F/3 and also add the correct amount of negative coma at F/6? 

 

These are very different amounts of negative coma.

 

My hypothesis is that the amount of negative coma added must depend somehow on the focal ratio of light cone the coma corrector sees. 

 

When you insert the the 2X Powermate, it doubles the focal ratio but the reverse coma needed is that of an F/3 focal ratio and not the F/6 focal ratio it is seeing.

 

This is a hypothesis, like I said, something doesn't seem right. You are messing with the light cone the coma corrector sees.

 

Using a Barlow or Powermate as if it were part of the eyepiece, does not affect the coma correction as long as the correct spacing can be maintained.  

 

Jon



#7 25585

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Posted 18 January 2022 - 01:27 PM

Any experiences with a 4x Powermate?



#8 Starman1

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Posted 18 January 2022 - 01:36 PM

4x is usually too high for visual use:

https://www.televue....?id=53&Tab=_app

but it has an application for imaging.


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#9 Starman1

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Posted 18 January 2022 - 01:42 PM

Think about this:

 

Why is that the same coma corrector can add the correct amount of negative coma at F/3 and also add the correct amount of negative coma at F/6? 

 

These are very different amounts of negative coma.

 

My hypothesis is that the amount of negative coma added must depend somehow on the focal ratio of light cone the coma corrector sees. 

 

When you insert the the 2X Powermate, it doubles the focal ratio but the reverse coma needed is that of an F/3 focal ratio and not the F/6 focal ratio it is seeing.

 

This is a hypothesis, like I said, something doesn't seem right. You are messing with the light cone the coma corrector sees.

 

Using a Barlow or Powermate as if it were part of the eyepiece, does not affect the coma correction as long as the correct spacing can be maintained.  

 

Jon

To see what happens with different f/ratios, look at this chart:

https://www.televue....art_updated.jpg

Note that the axial spot size is increased as the f/ratio shrinks, but is always smaller than the Airy Disc (in yellow).

The reverse coma applied at the edge of a 40mm field puts the spot size within the Airy Disc down to f/3.5, but isn't successful at f/3.

It's unlikely for someone to use a 40mm field stop at f/3 anyway.

Note that the chart is in radius figures.  For diameter figures, double every value.

 

The PowerMate has no effect on Coma except, of course, it doubles the apparent size of the comatic star image while halving the true field.

Hence, no effect on the apparent size of coma.  The f/4 scope after the PowerMate has the same effect as halving the eyepiece focal length.

It does not convert the scope to f/8.


Edited by Starman1, 18 January 2022 - 01:45 PM.


#10 CrazyPanda

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Posted 18 January 2022 - 01:48 PM

Using a Barlow or Powermate as if it were part of the eyepiece, does not affect the coma correction as long as the correct spacing can be maintained.

 

Well it's really just about what comes to focus. Again, spacing between eyepiece and the Paracorr doesn't actually matter. The "spacing" settings are just focus point references intended to place the eyepiece at a point that once the whole assmebly is brought to focus using the telescope's focuser, the Paracorr is at the right distance from the telescope's plane to correct coma.

 

This is what lets SIPS work - you set the lens assembly at the correct distance with respect to the telescope, and then you just use the focuser as normal. The focuser basically replaces the tunable top of the Paracorr, but they both serve the same function.

 

So the question then becomes, can you reach focus with the various barlow/eyepiece combinations you want, using the tunable top, or not? If you can, great. It will work just fine. If you can't, then you are moving the Paracorr out of the optimal position. But the flexibility to do this is why I don't think I'd ever get a SIPS. With a Paracorr, if you are unable to reach focus with the tunable top, you just use your focuser. You sacrifice some coma correction, but having slightly worse coma correction is a teeny, tiny price to pay compared to not being in focus at all. A SIPS is unforgiving. You can either reach focus, or you can't. If you can't, game over.


Edited by CrazyPanda, 18 January 2022 - 01:56 PM.


#11 25585

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Posted 18 January 2022 - 01:49 PM

4x is usually too high for visual use:

https://www.televue....?id=53&Tab=_app

but it has an application for imaging.

"Excellent with long eye-relief, 2" eyepieces. Using the 4x model, imagine the quality of a 35mm Panoptic at 8.8mm with 25mm of eye relief! The Powermate™ is NOT a Barlow so the Panoptic-Barlow Interface is not needed! The 2" Powermate™ fits all the way into 2" Star Diagonals, requiring minimal refocus compared to a 2" Barlow."

 

For 20mm & longer, the 4x seems ideal. I use a TAL 4x 1.25" Barlow with long FL eyepieces, but their FL makes small AFOVs.  



#12 CrazyPanda

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Posted 18 January 2022 - 01:54 PM

A 4x would be fine, but as per the chart that Don linked to, the multiplication factor varies more than the 2x. Might be more like 4.25x by the time you get to the eyepiece in the Paracorr.


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#13 Jon Isaacs

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Posted 18 January 2022 - 02:14 PM

To see what happens with different f/ratios, look at this chart:

https://www.televue....art_updated.jpg

Note that the axial spot size is increased as the f/ratio shrinks, but is always smaller than the Airy Disc (in yellow).

The reverse coma applied at the edge of a 40mm field puts the spot size within the Airy Disc down to f/3.5, but isn't successful at f/3.

It's unlikely for someone to use a 40mm field stop at f/3 anyway.

Note that the chart is in radius figures.  For diameter figures, double every value.

 

The PowerMate has no effect on Coma except, of course, it doubles the apparent size of the comatic star image while halving the true field.

Hence, no effect on the apparent size of coma.  The f/4 scope after the PowerMate has the same effect as halving the eyepiece focal length.

It does not convert the scope to f/8.

 

Don:

 

I have inspected the graphs.  

 

How do explain that the correct amount of reverse coma is added for a given focal ratio?  

 

If I have a coma free scope that is F/6, the Paracorr will add the correct amount of reverse coma as if were an F/6 Newtonian.  The Powermate changes the effective focal ratio from F/3 to F/6 so how is it that it adds the correct amount of reverse coma for an F/3 focal ratio?

 

I am dubious about the actual effect of the Powermate on the coma correction.  

 

Jon


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#14 Jon Isaacs

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Posted 18 January 2022 - 02:19 PM

Well it's really just about what comes to focus. Again, spacing between eyepiece and the Paracorr doesn't actually matter. The "spacing" settings are just focus point references intended to place the eyepiece at a point that once the whole assmebly is brought to focus using the telescope's focuser, the Paracorr is at the right distance from the telescope's plane to correct coma.

 

This is what lets SIPS work - you set the lens assembly at the correct distance with respect to the telescope, and then you just use the focuser as normal. The focuser basically replaces the tunable top of the Paracorr, but they both serve the same function.

 

So the question then becomes, can you reach focus with the various barlow/eyepiece combinations you want, using the tunable top, or not? If you can, great. It will work just fine. If you can't, then you are moving the Paracorr out of the optimal position. But the flexibility to do this is why I don't think I'd ever get a SIPS. With a Paracorr, if you are unable to reach focus with the tunable top, you just use your focuser. You sacrifice some coma correction, but having slightly worse coma correction is a teeny, tiny price to pay compared to not being in focus at all. A SIPS is unforgiving. You can either reach focus, or you can't. If you can't, game over.

 

Yes, that is what I said. 

 

If there is a Tunable Top setting that does not require refocusing, then the Barlow between the Paracorr and the eyepiece works just fine.  In the case of the 10mm and 13mm Ethos using the 2X 2 inch GSO Barlow, it requires about 0.075" of inward focuser travel which has an insignificant effect on the coma correction.  

 

Jon


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#15 CrazyPanda

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Posted 18 January 2022 - 02:27 PM

Yes, that is what I said.

 

If there is a Tunable Top setting that does not require refocusing, then the Barlow between the Paracorr and the eyepiece works just fine.  In the case of the 10mm and 13mm Ethos using the 2X 2 inch GSO Barlow, it requires about 0.075" of inward focuser travel which has an insignificant effect on the coma correction. 

 

Jon

 

But being a 2" barlow, you can safely swing the tunable top through its full range. A 1.25" barlow might be risky to do that with. Hence my "all bets are off" comment. Not all eyepiece/barlow combinations are going to result in such minor adjustments to the Paracorr's own position.



#16 Jon Isaacs

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Posted 18 January 2022 - 02:35 PM

But being a 2" barlow, you can safely swing the tunable top through its full range. A 1.25" barlow might be risky to do that with. Hence my "all bets are off" comment. Not all eyepiece/barlow combinations are going to result in such minor adjustments to the Paracorr's own position.

 

As I had said, it depends on the 1.25 Barlow.  My 1.25 inch 2X TeleVue clears the lens but not by much. If someone plans to use this Barlow, they should check it.  My 1.25 inch GSO 2x shorty clears by a lot as does my Celestron 1.25 inch Xcel LX 3 X Barlow.  

 

The adjustment for the Barlow depends on the Barlow, no doubt about that.  For my purposes, the 2 inch 2X GSO Barlow works with the three eyepieces I wanted it to work with, the 13mm, 10mm and 8mm Ethos's but there are plenty of eyepieces it would not work with.  I just wanted to double the magnification of those three eyepieces to increase the maximim magnification from 350x to 700x in my big scope.  

 

I have the Type 6 Naglers down to the 3.5mm so I am set with the 2X TV Barlow.. 

 

Jon


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#17 Tubuskahusk

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Posted 18 January 2022 - 03:53 PM

Thank all of you for your time and advice. It has been very helpful. If more people want to chime in please do.

Edited by Tubuskahusk, 18 January 2022 - 03:53 PM.


#18 MitchAlsup

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Posted 18 January 2022 - 04:03 PM

"radially" is not quite the correct word in what follows--it is close to the right word::

Why is that the same coma corrector can add the correct amount of negative coma at F/3 and also add the correct amount of negative coma at F/6? 

 

These are very different amounts of negative coma.

 

My hypothesis is that the amount of negative coma added must depend somehow on the focal ratio of light cone the coma corrector sees. 

The coma has already moved the converging light cones based on where (radially) they reflected off the primary.

The PowerMate only doubles the radial position of the displaced cones, 

So the Paracorr sees the cones at the radial position of F/3 even though they are now only converging at F/6.

And thus adds the proper amount of negative coma.



#19 Starman1

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Posted 18 January 2022 - 11:06 PM

Don:

 

I have inspected the graphs.  

 

How do explain that the correct amount of reverse coma is added for a given focal ratio?  

 

If I have a coma free scope that is F/6, the Paracorr will add the correct amount of reverse coma as if were an F/6 Newtonian.  The Powermate changes the effective focal ratio from F/3 to F/6 so how is it that it adds the correct amount of reverse coma for an F/3 focal ratio?

 

I am dubious about the actual effect of the Powermate on the coma correction.  

 

Jon

It adds the same correction at all focal ratios but it has less effect the shorter the f/ratio.

The PowerMate expands the image scale but does not change the f/ratio from the primary.


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#20 turtle86

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Posted 19 January 2022 - 05:56 PM

Thank all of you for your time and advice. It has been very helpful. If more people want to chime in please do.

 

I like using a 2x Powermate with the Paracorr 2.  It's unwieldy, and will likely cause balancing issues, but works very well.


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#21 TayM57

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Posted 23 January 2022 - 02:45 AM

I like using a 2x Powermate with the Paracorr 2.  It's unwieldy, and will likely cause balancing issues, but works very well.

2x PM + P2 + EP is quite a stack. The focuser needs to be able to handle that kind of weight. Check the drawtube alignment every once in a while. To do this, use a laser collimator and adjust the secondary so that the dot is in the center of the collimating circle on the primary. Than rack the focuser in/out to see if the dot moves. After using my ES92's with the powermates frequently, I had to tighten the tension by adjusting two set screws on my 10" f/5.6 Moonlite focuser.



#22 Piero DP

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Posted 23 January 2022 - 09:00 AM

I use a Baader VIP Barlow with my PC2. I don't think the 1.25" standard (=as the VIP is sold) configuration has enough back travel and the 2" standard configuration cannot certainly be used as it will hit the lens of the PC2.

I haven't used the VIP in the standard configuration much due to these large changes in focus position anyway..

However, if the 1.25" nosepiece is replaced with this Baader adapter: https://www.baader-p...takahashi).html (the VIP cell must be unscrewed from the 1.25" nosepiece and screwed onto the adapter above - part: 2458199), the resulting modded VIP Barlow can be used in 2" mode and will reach focus with the PC2 without any issues.

Mine is used with the Docter and 3 Delos eyepieces frequently.
Optically, the VIP Barlow is outstanding.

Edited by Piero DP, 23 January 2022 - 09:02 AM.


#23 sixela

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Posted 23 January 2022 - 09:07 AM

If you have a Baader VIP you need to get rid of the 1.25" nose and buy either this:

https://www.baader-p...takahashi).html

[already mentioned by Piero DP]

or

https://www.pierro-a...ader-vip_detail

 

and a ring from M48-> 2" (unless you plan to use it on Baader 2"-1.25" adapters only).

 

This screws onto 2" eyepieces or 2"-1.25" adapters (including the one that comes with the Baader VIP).

 

Other barlows used regularly (i.e. my own -- I tested other barlows but didn't use them regularly):

-2.5x TeleVue Powermate. Nothing special needed: it doesn't insert too deeply and most eyepieces can still be positioned properly with respect to the Paracorr.

-2x Zeiss barlow. I put it into a Baader PushFix adapter extended so that you can screw 2" filters in front of the barlow and that it cannot hit the Paracorr lenses

-1.8x TMB barlow. Absolutely safe, but you need a really short 2"-1.25" adapter or some eyepieces won't come close enough to the Paracorr even with the top on position A (again, I used a Baader Pushfix).

-Celestron Ultima/Orion Shorty Plus. Nothing special needed. Low insertion depth, no issues in placing eyepieces correctly (the latter IIRC, not sure if I was using it with a PushFix or not).

-Astro-Physics BarAdv: can be screwed onto 2"-1.25" adapters or 2" eyepieces. Cannot hit the Paracorr lenses.


Edited by sixela, 23 January 2022 - 09:14 AM.


#24 Piero DP

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Posted 23 January 2022 - 09:09 AM

VIP cell mounted onto the adapter mentioned in my previous post.
IMG_20200227_122001.jpg
 
VIP Barlow from the bottom:
IMG_20211121_142148.jpg
 
VIP Barlow attached to my zoom using an additional 15mm spacer (I don't use this spacer when the VIP is used with the PC2):
IMG_20211121_142036.jpg

 



#25 Starman1

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Posted 23 January 2022 - 10:45 AM

I use a Baader VIP Barlow with my PC2. I don't think the 1.25" standard (=as the VIP is sold) configuration has enough back travel and the 2" standard configuration cannot certainly be used as it will hit the lens of the PC2.

I haven't used the VIP in the standard configuration much due to these large changes in focus position anyway..

However, if the 1.25" nosepiece is replaced with this Baader adapter: https://www.baader-p...takahashi).html (the VIP cell must be unscrewed from the 1.25" nosepiece and screwed onto the adapter above - part: 2458199), the resulting modded VIP Barlow can be used in 2" mode and will reach focus with the PC2 without any issues.

Mine is used with the Docter and 3 Delos eyepieces frequently.
Optically, the VIP Barlow is outstanding.

It's been out of production for quite a while.  Have you heard that Baader will ever bring it back?




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