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oldfruit

Paracorr Type 2

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Hi,

 

I am currently using an f/4 dob and as part of the purchase a Televue Paracorr type 2 was included. I have not used the Paracorr yet and I am noticing greater distortion than when I was using an F/5. Naturally I want to get the most out of my observing.

As I am not currently using Televue eyepieces, is there a guide out there somewhere as to which settings would be used for different eyepieces? My eyepiece collection is mainly Skywatcher Nirvana`s.

Thanks.

Mark

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I have seen a list of eyepieces that have been tested with it but can’t find it at the moment.

However, this should work. Focus an eyepiece as best you can. Then adjust the tuneable top until the coma is corrected as much as possible. When you have a good view, switch eyepieces without altering your focus. Then focus by turning the tuneable top, not the focuser. Once you’ve achieved focus that will be the setting for that eyepiece.

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Thanks Richard, In the absence of a definitive list that sounds like a good method to try.

Mark

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It might seem obvious but start with your longest focal length eyepiece. The coma won’t be so noticeable on the more powerful ones. 

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1 hour ago, Scooot said:

It might seem obvious but start with your longest focal length eyepiece. The coma won’t be so noticeable on the more powerful ones. 

It depends.

The angular coma free region is constant across magnification, although coma is often perceived at low power rather than high power.

I believe the reason is due to how coma is revealed, as the star is a point source - brightness increases with mag -, whereas the comatic blur is more like an extended object - brightness decreases after a certain mag -.

For bright stars and planets, coma is well noticeable at high power, unfortunately.

Edited by Piero
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7 hours ago, oldfruit said:

Thanks Richard, In the absence of a definitive list that sounds like a good method to try.

Mark

After thinking about it a bit more I’ve realised finding the setting for the first eyepiece will be more difficult than I implied. Once you’ve focused it, it will move out of focus when you turn the tuneable top. So unless you can find a known setting for an eyepiece, or can calculate it, you’ll need to use trial and error on each setting for the first eyepiece. You can find the settings for other eyepieces as per my first post once you’ve worked out one. 

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9 hours ago, oldfruit said:

Hi,

 

I am currently using an f/4 dob and as part of the purchase a Televue Paracorr type 2 was included. I have not used the Paracorr yet and I am noticing greater distortion than when I was using an F/5. Naturally I want to get the most out of my observing.

As I am not currently using Televue eyepieces, is there a guide out there somewhere as to which settings would be used for different eyepieces? My eyepiece collection is mainly Skywatcher Nirvana`s.

Thanks.

Mark

I hope you get this sorted out Mark!

my motives are insideous though.... as I'm in the same boat lol! 😀

What distortion? just coma or other stuff?

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28 minutes ago, jetstream said:

I hope you get this sorted out Mark!

my motives are insideous though.... as I'm in the same boat lol! 😀

What distortion? just coma or other stuff?

Hi Gerry,

It is just the coma. In my f5 the Nirvanas performed reasonably well and coma was limited was not too much of a distraction and I could live with it. Having started using an f4 I can immediately see the difference. The middle of the field of view is lovely and bright and sharp, however coma is now really noticeable in the outer parts of the fov and whilst I do not find it too bothersome, having the Paracoor can potentially enhance my experience at the eyepiece. I am sure that with all the great advice given I will find the improved observing experience reasonably easy to achieve. (Fingers crossed)

Mark

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If you have any eyepieces that someone with a paracorr also has they might be able to give you a setting for one and then you will have a starting point. I know how the focal positions of most of my non TV eyepieces compare to my TVs and so know their rough paracorr settings.

Or if you do try to calibrate it yourself the double cluster is a helpful target as there will be lots of reasonably bright stars from edge to edge.

It is true you would have to reset your focus every time you change the paracorr setting but each setting will only have one correct focus position so if you are methodical you should be able to run through the range and not have to go round in circles.

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On 08/09/2019 at 01:45, oldfruit said:

Hi,

 

I am currently using an f/4 dob and as part of the purchase a Televue Paracorr type 2 was included. I have not used the Paracorr yet and I am noticing greater distortion than when I was using an F/5. Naturally I want to get the most out of my observing.

As I am not currently using Televue eyepieces, is there a guide out there somewhere as to which settings would be used for different eyepieces? My eyepiece collection is mainly Skywatcher Nirvana`s.

Thanks.

Mark

There is an empirical way to find the right setting for an eyepiece.  You only need to do this for 1 eyepiece.  For every other eyepiece you own or will own, you simply insert the other eyepiece and focus using the Tunable Top.

Whatever setting results is the correct setting for that eyepiece and you can pre-dial it before you put the eyepiece in in the future.

So, Insert the eyepiece with the tunable top set to its maximum out setting (H).  Focus the scope and look at stars at the edge of the field (or move a moderately bright star to the edge of the field).

Still see coma in the star image?  Then dial the top in one setting to G , focus the scope and repeat your evaluation of a star at the edge (it's easier to see coma on a brighter star, like Polaris, than a dimmer star).

Still see coma?  Keep moving in one setting until, when you focus, the star at the edge of the field displays no coma.

That is the setting you'll use for that eyepiece in the future.  For every other eyepiece, leave the focuser where it is and focus using the tunable top.  Write down the settings so you know what they are.

You can do this with any eyepiece from any company.  If the eyepiece needs more outward movement of the tunable top, you can add a parfocalizing ring to the eyepiece to raise it a bit.

If the eyepiece needs more inward travel of the focuser, you will have to settle for the inward most setting (A): example Nikon HW 17mm.

What you do by doing this is to set the Paracorr in the light cone from the primary.

 

There is one more way to do this: insert the Paracorr in the focuser in setting E.  Place translucent scotch tape across the opening of the Paracorr in an X shape where the eyepiece would normally go.

Point the scope at the moon and focus the telescope with the moon's image on the translucent tape.  When the Moon is in focus, you have found the perfect setting of the Paracorr in the light cone from the primary mirror.

Lock the focuser in this position, and insert each one of your eyepieces in sequence, focusing on the Moon with the tunable top.  The settings each eyepiece uses is the correct setting for that eyepiece.

 

I have done both and am happy to report that, at most, I was about a half setting off on an eyepiece or two that way, and a half setting is a very small error (about a mm).

 

The problem with both methods is if the observer is nearsighted.  In that case, perform the test(s) with glasses on that correct your vision.  Otherwise the settings for each eyepiece will be off.

Later, when you view without glasses, the distance between the eyepiece and Paracorr will have already been accurately determined and even if you have to move the focuser in to focus, the correction

will be approximately correct.

Edited by Don Pensack
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6 minutes ago, Don Pensack said:

There is an empirical way to find the right setting for an eyepiece.  You only need to do this for 1 eyepiece.  For every other eyepiece you own or will own, you simply insert the other eyepiece and focus using the Tunable Top.

Whatever setting results is the correct setting for that eyepiece and you can pre-dial it before you put the eyepiece in in the future.

So, Insert the eyepiece with the tunable top set to its maximum out setting (H).  Focus the scope and look at stars at the edge of the field (or move a moderately bright star to the edge of the field).

Still see coma in the star image?  Then dial the top in one setting to G , focus the scope and repeat your evaluation of a star at the edge (it's easier to see coma on a brighter star, like Polaris, than a dimmer star).

Still see coma?  Keep moving in one setting until, when you focus, the star at the edge of the field displays no coma.

That is the setting you'll use for that eyepiece in the future.  For every other eyepiece, leave the focuser where it is and focus using the tunable top.  Write down the settings so you know what they are.

You can do this with any eyepiece from any company.  If the eyepiece needs more outward movement of the tunable top, you can add a parfocalizing ring to the eyepiece to raise it a bit.

If the eyepiece needs more inward travel of the focuser, you will have to settle for the inward most setting (A): example Nikon HW 17mm.

What you do by doing this is to set the Paracorr in the light cone from the primary.

 

There is one more way to do this: insert the Paracorr in the focuser in setting E.  Place translucent scotch tape across the opening of the Paracorr in an X shape where the eyepiece would normally go.

Point the scope at the moon and focus the telescope with the moon's image on the translucent tape.  When the Moon is in focus, you have found the perfect setting of the Paracorr in the light cone from the primary mirror.

Lock the focuser in this position, and insert each one of your eyepieces in sequence, focusing on the Moon with the tunable top.  The settings each eyepiece uses is the correct setting for that eyepiece.

 

I have done both and am happy to report that, at most, I was about a half setting off on an eyepiece or two that way, and a half setting is a very small error (about a mm).

 

The problem with both methods is if the observer is nearsighted.  In that case, perform the test(s) with glasses on that correct your vision.  Otherwise the settings for each eyepiece will be off.

Later, when you view without glasses, the distance between the eyepiece and Paracorr will have already been accurately determined and even if you have to move the focuser in to focus, the correction

will be approximately correct.

Many thanks, I will try both methods.

 

Mark

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I am happy to report that I managed a short observing session the other night and wow, what a difference a Paracorr makes. Using a 28mm Skywatcher Nirvana I used the middle setting on the Paracorr and with a slight tweak it was great, flat field almost to the edge with minimal coma on the very edges of the FOV.

I can see that my observing experience will be much improved in the coming months.

Thank you all for the help and advice.

Mark

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I'm ordering a Paracorr myself and am happy , very happy to say that my Astrosystems dob has enough room-3 1/2"- for the Paracorr without hitting my filter slide...this is great news for me as I love the filter slide.

I'm going to try the 30ES 82 as my wide neb eyepiece and see how it is.

On 22/09/2019 at 13:54, oldfruit said:

wow, what a difference a Paracorr makes.

Great that it is working well for you Mark!

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4 hours ago, jetstream said:

I'm ordering a Paracorr myself and am happy , very happy to say that my Astrosystems dob has enough room-3 1/2"- for the Paracorr without hitting my filter slide...this is great news for me as I love the filter slide.

I'm going to try the 30ES 82 as my wide neb eyepiece and see how it is.

Great that it is working well for you Mark!

Curious to know how you will find it, Gerry.

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On 22/09/2019 at 19:54, oldfruit said:

I am happy to report that I managed a short observing session the other night and wow, what a difference a Paracorr makes. Using a 28mm Skywatcher Nirvana I used the middle setting on the Paracorr and with a slight tweak it was great, flat field almost to the edge with minimal coma on the very edges of the FOV.

I can see that my observing experience will be much improved in the coming months.

Thank you all for the help and advice.

Mark

Was that with the F/4 20 inch Mark ?

Are you going to use the Paracorr with your 12" Revelation F/5 ?

It would be interesting to see how much the slightly slower focal ratio benefits from coma correction.

 

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5 hours ago, Piero said:

Curious to know how you will find it, Gerry.

Were going find out if it "disappears" when viewing or if faint objects do when using it lol!

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I pretty much use it on setting 4 for 1.25" and 3 for 2". Works for me.Might not be perfect but saves all the messing about.

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3 hours ago, jetstream said:

Were going find out if it "disappears" when viewing or if faint objects do when using it lol!

I think it could lose a tiny bit on axis, but the improvement across the field should be huge at f4. You will have the whole field within diffraction limited threshold. The increase in magnification could also help you spot some other faints..

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3 minutes ago, Piero said:

I think it could lose a tiny bit on axis, but the improvement across the field should be huge at f4. You will have the whole field within diffraction limited threshold. The increase in magnification could also help you spot some other faints..

Were going to see where the Paracorr fits into Suiters wobbly stack for sure and I agree I might lose a bit on axis to induced aberration. My scopes are well above the diffraction limit so hopefully there is some wiggle room across the board.

A main interest is to see if extended objects contrast is increased near the edge of view resulting from decreased comatic smear. My experience says "you never know" lol!

A side note, but interesting : from TV's site. It seems low obstuction also helps with exit pupil flexibility.

 

"Myth #2: Exit pupils larger than 7 mm waste light and resolution.

With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power.

image...
 

As explained in the text, there is no practical limit to the low magnification that can be used with a refractor. But the secondary obstruction found on most reflectors does set limits, because the shadow spot it forms in the exit pupil grows as the magnification is reduced. Consider this extreme example of an exit pupil formed by an 8-inch Schmidt-Cassegrain with a central obstruction equal to 43 percent of the aperture's diameter. A telecompressor lens and long-focal-length eyepiece give 14x magnification. While the central shadow remains 43 percent of the exit pupil's diameter, it is now 6.2 millimeters in diameter and would nearly fill the 7-mm pupil diameter of the dark-adapted eye."

 

 

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3 hours ago, jetstream said:

@oldfruit I hope sidetracking your thread a bit is OK, I don't mean to de rail your paracorr purchase/experience.

Not at all.

 

The more input and information the better.

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11 hours ago, John said:

Was that with the F/4 20 inch Mark ?

Are you going to use the Paracorr with your 12" Revelation F/5 ?

It would be interesting to see how much the slightly slower focal ratio benefits from coma correction.

 

Yes it was with the 20 inch, not used it in the 12 inch yet but at F5 coma was much less of an issue, though still present.

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Well, beyond approx 25 Deg AFOV, coma is visible in a F5 telescope. Not that it isn't visible within, it's just that beyond 25 deg the diffraction disk becomes larger than what it should be with the diffraction limited optics. 

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I see very little, if any, coma with my F/5.3 12 inch, even with the Ethos eyepieces. I guess it's there but at a level that is not significant, at least to my eye ?

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