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Using a Paracorr & a Powermate together..

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#26 Starman1

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Posted 30 April 2025 - 09:42 AM

It's sometimes good to read TV instructions, or re-read them. There's a joke that Slavs read instructions only when things go wrong, to see what went wrong.

 

"For Dobsonian/Newtonian owners using Tele Vue’s Visual Paracorr coma corrector and a POWERMATE

• 1¼" POWERMATES (2.5x or 5x): Insert the 1¼" POWERMATE into the Paracorr’s 2"-1¼" adapter and set the
Tunable Top at its highest position."

 

So yes, it's H position (apparently for every 1 1/4 EP).

Except the 6mm and 8mm Ethos and the 40mm Plössl.



#27 BGazing

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Posted 30 April 2025 - 09:48 AM

Except the 6mm and 8mm Ethos and the 40mm Plössl.

Wait, I thought I have figured it out! lol.gif

The way I read it, it says that every time one puts PM2.5, the setting is H, no matter what the original setting was (sans PM).

Or are you saying that this was written only with the D-setting EPs in mind?


Edited by BGazing, 30 April 2025 - 10:06 AM.


#28 Jon Isaacs

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Posted 30 April 2025 - 10:35 AM

My two cents:

 

I would think, only with eyepieces that take the D setting .

 

One nice thing about the 1.25 inch 2x (and I believe 3x Barlow) is that they're parfocal with nearly all 1.25 inch TeleVue eyepieces.  No change in the Pacorr settings 

 

Jon



#29 BGazing

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Posted 30 April 2025 - 01:47 PM

My two cents:

 

I would think, only with eyepieces that take the D setting .

 

One nice thing about the 1.25 inch 2x (and I believe 3x Barlow) is that they're parfocal with nearly all 1.25 inch TeleVue eyepieces.  No change in the Pacorr settings 

 

Jon

But what you say would mean that PM 2.5 changes the setting from D to H.


Edited by BGazing, 30 April 2025 - 01:47 PM.


#30 jrmacl

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Posted 30 April 2025 - 03:12 PM

I am about to ask such a stupid question, because obviously the answer is if it was possible it would have already been done - but is it possible to make a coma corrector that is telescope focal length specific, like one for f3.3 and another for f4? Like the way that some field flatteners are made for specific telescopes? If possible would that cut down on size and weight vs. the paracorr?



#31 Starman1

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Posted 30 April 2025 - 03:48 PM

I am about to ask such a stupid question, because obviously the answer is if it was possible it would have already been done - but is it possible to make a coma corrector that is telescope focal length specific, like one for f3.3 and another for f4? Like the way that some field flatteners are made for specific telescopes? If possible would that cut down on size and weight vs. the paracorr?

A coma corrector can be tailored to a specific f/ratio to correct a certain size of field.

But how coma correctors work is that they correct coma at all f/ratios, they merely correct certain sizes of field down to a particular f/ratio and then correct ever smaller fields below that.

In practice, people with newtonians longer than f/6 don't use coma correctors, so having a coma corrector be useful from f/3 to f/6 pretty much covers the range.

 

So if you want to correct a 40mm field to make it free of coma, the Paracorr II will do so down to f/3.5, whereas the Paracorr I will only do so down to f/5.2 (estimate).

If you, on the other hand, only need 30mm of field to be corrected, both Paracorrs will go down to even shorter f/ratios with full correction of coma in the field.

 

Some of the size and weight of the Paracorr, which, by the way, is lighter than the Explore Scientific HRCC, is due to the tunable top that allows all eyepieces to be focused (so the Paracorr stays in the same place in the light cone for all eyepieces)

and can accommodate a wide variety of eyepiece focal plane positions.  If all your eyepieces were parfocal, the top of the Paracorr could be a simple drop-in tube, like a Barlow, which would make it smaller and lighter.

If you only wanted correction to f/5, for example, it could be simpler and lighter.  If you only wanted to correct 1.25" eyepieces, it could be even smaller and lighter.

 

But, if Tele Vue is going to sell a coma corrector, it needs to be usable with their eyepieces, and a lot of other eyepieces to make it useful for people who have newtonians and sets of other brands of eyepieces.

And that constrains how it is made and how many lenses it contains, and thus the weight and size of the CC.


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#32 BGazing

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Posted 01 May 2025 - 01:56 PM

And the answer from TeleVue waytogo.gif

 

"What you didn’t notice is that when the 6 and 8mm Ethos are used alone, they rest on the bottom of their 2” skirts.  When you insert either in the 2.5x (or 5x) Powermate, they rest on the flat between the 2” skirt and 1.25” barrel.  Since they are now sitting closer to the Paracorr than when used alone in the Paracorr’s 1.25” adapter, the tunable top needs to be adjusted up further to reach the proper coma correction.  So, H is the correct setting when used with 1.25” Powermates."


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#33 eblanken

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Posted 02 May 2025 - 01:18 AM

Hi All,

 

I am about to ask such a stupid question, because obviously the answer is if it was possible it would have already been done - but is it possible to make a coma corrector that is telescope focal length specific, like one for f3.3 and another for f4? Like the way that some field flatteners are made for specific telescopes? If possible would that cut down on size and weight vs. the paracorr?

 

 

A coma corrector can be tailored to a specific f/ratio to correct a certain size of field.

But how coma correctors work is that they correct coma at all f/ratios, they merely correct certain sizes of field down to a particular f/ratio and then correct ever smaller fields below that.

In practice, people with newtonians longer than f/6 don't use coma correctors, so having a coma corrector be useful from f/3 to f/6 pretty much covers the range.

 

So if you want to correct a 40mm field to make it free of coma, the Paracorr II will do so down to f/3.5, whereas the Paracorr I will only do so down to f/5.2 (estimate).

If you, on the other hand, only need 30mm of field to be corrected, both Paracorrs will go down to even shorter f/ratios with full correction of coma in the field.

 

Some of the size and weight of the Paracorr, which, by the way, is lighter than the Explore Scientific HRCC, is due to the tunable top that allows all eyepieces to be focused (so the Paracorr stays in the same place in the light cone for all eyepieces)

and can accommodate a wide variety of eyepiece focal plane positions.  If all your eyepieces were parfocal, the top of the Paracorr could be a simple drop-in tube, like a Barlow, which would make it smaller and lighter.

If you only wanted correction to f/5, for example, it could be simpler and lighter.  If you only wanted to correct 1.25" eyepieces, it could be even smaller and lighter.

 

But, if Tele Vue is going to sell a coma corrector, it needs to be usable with their eyepieces, and a lot of other eyepieces to make it useful for people who have newtonians and sets of other brands of eyepieces.

And that constrains how it is made and how many lenses it contains, and thus the weight and size of the CC.

 

Very interesting side-question & Well Explained Answer !!!

 

Thank you both,

 

Ed


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#34 Jon Isaacs

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Posted 02 May 2025 - 07:56 AM

 

If all your eyepieces were parfocal, the top of the Paracorr could be a simple drop-in tube, like a Barlow, which would make it smaller and lighter.

 

The original Paracorr was called the Photo-Visual and did not have a Tunable Top. I still have mine from back when.

 

Back then, the 2 inch TeleVue eyepieces were pretty much parfocal as were the 1.25 inch eyepieces. 

 

All that was necessary was the proper design of the 2 inch to 1.25 inch adapter so that the 2 inch and 1.25 inch eyepieces were parfocal.

 

That was a bit tricky since the 1.25 inch eyepieces focus inwards of the 2 inch eyepieces like the 20 mm Type 2 Nagler.  The adapter is recessed for the 1.25 inch Eyepieces and the Paracorr has a slot in the side for the thumbscrew to secure the 1.25 inch eyepieces.

 

It's very similar to the GSO 2 inch 2x Barlow design and in fact, the GSO adapter is also recessed with and identical offset, that's what I'm currently using.

 

I have a Paracorr 2 but from my backyard I'll use my 1.25 inch Type 5 and type 6 Naglers with the 30 mm UFF which is parfocal with the 20 mm Type 2. It's nice, no messing with the settings.

 

The lower section with the optics can be unscrewed and used photographically. I believe it's a standard 48 mm thread 

 

When eyepieces like the 31 Nagler came along, this scheme no longer worked. The 31 mm Nagler required much more infocus. It also required more clear aperture. The next year, the Visual Paracorr with the Tunable Top was introduced.

 

You could buy a Tunable Top to fit the Photo-Visual.

 

Jon


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#35 BGazing

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Posted 02 May 2025 - 02:35 PM

The original Paracorr was called the Photo-Visual and did not have a Tunable Top. I still have mine from back when.

 

Back then, the 2 inch TeleVue eyepieces were pretty much parfocal as were the 1.25 inch eyepieces. 

 

All that was necessary was the proper design of the 2 inch to 1.25 inch adapter so that the 2 inch and 1.25 inch eyepieces were parfocal.

 

That was a bit tricky since the 1.25 inch eyepieces focus inwards of the 2 inch eyepieces like the 20 mm Type 2 Nagler.  The adapter is recessed for the 1.25 inch Eyepieces and the Paracorr has a slot in the side for the thumbscrew to secure the 1.25 inch eyepieces.

 

It's very similar to the GSO 2 inch 2x Barlow design and in fact, the GSO adapter is also recessed with and identical offset, that's what I'm currently using.

 

I have a Paracorr 2 but from my backyard I'll use my 1.25 inch Type 5 and type 6 Naglers with the 30 mm UFF which is parfocal with the 20 mm Type 2. It's nice, no messing with the settings.

 

The lower section with the optics can be unscrewed and used photographically. I believe it's a standard 48 mm thread 

 

When eyepieces like the 31 Nagler came along, this scheme no longer worked. The 31 mm Nagler required much more infocus. It also required more clear aperture. The next year, the Visual Paracorr with the Tunable Top was introduced.

 

You could buy a Tunable Top to fit the Photo-Visual.

 

Jon

I wonder why 1.25 Paracorr was discontinued. Some degree of coma-correction for small ultraportable dobs, surely there is a market.



#36 Starman1

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Posted 02 May 2025 - 03:14 PM

I wonder why 1.25 Paracorr was discontinued. Some degree of coma-correction for small ultraportable dobs, surely there is a market.

In this era, the number of scopes that still have just 1.25" focusers is small.

Even the 6' Celestron Omni reflector went to a 2" focuser before it was discontinued.

And those smaller scopes typically cost less than a 1.25" Paracorr would cost.

And would likely not balance with that much extra weight in the focuser.

And might not come to focus with the extra in-travel required at the focuser.

 

There might be a market, but it would be a very very small market.  A company like Tele Vue might take years to sell just one production run.

And making it would take production time away from the factory making something else that sells quickly.

 

Last, Robert (Houdini on CN) is soon coming out with 1.25" coma-correcting eyepieces. No CC required, then.

Patience.


Edited by Starman1, 02 May 2025 - 03:15 PM.


#37 Olimad

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Posted 02 May 2025 - 03:28 PM

There’s also the APM 2.7x coma-correcting Barlow, but its high magnification factor limits what these kinds of scopes are best suited for: a wide true field of view (TFoV).

 

It seems Houdini will also release a 2x coma-correcting focal extender, which I assume will also be 1.25". That should allow for a wider TFoV than the APM.

 

If they could produce a Barlow or focal extender with a lower magnification factor, it would be excellent.

 

Even better would be a 1.25" coma-corrected eyepiece designed to achieve the maximum possible TFoV — likely with a smaller apparent field of view (AFoV) than current Houdini eyepieces.



#38 jrmacl

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Posted 02 May 2025 - 03:57 PM

There’s also the APM 2.7x coma-correcting Barlow, but its high magnification factor limits what these kinds of scopes are best suited for: a wide true field of view (TFoV).

 

It seems Houdini will also release a 2x coma-correcting focal extender, which I assume will also be 1.25". That should allow for a wider TFoV than the APM.

 

If they could produce a Barlow or focal extender with a lower magnification factor, it would be excellent.

 

Even better would be a 1.25" coma-corrected eyepiece designed to achieve the maximum possible TFoV — likely with a smaller apparent field of view (AFoV) than current Houdini eyepieces.

Yeah, I always thought that Nikon got it right with their 1.6x Barlow, it seems to make more sense with a 2x Barlow overlapping focal lengths too often. I never really use my Barlow, but when I do it's almost always just the 1.5x screw on lens part.



#39 Houdini

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Posted 03 May 2025 - 03:40 PM

But I'm not assuming anything, I'm running optical analyses with ray tracing of telecentric barlows and coma-correcting eyepieces. The results do not depend on any assumptions on my part, just like the ray trace graphic of the 1.5X APM barlow above is not a matter of assumption, it's a matter of optical science.

 

Maybe something is missing, and that's what I would like to find out.

But maybe I'm right, and we see next year a small addendum to the Televue web site saying that "the coma correction in this configuration is not complete. We recommend using the 2 inch Powermate with the SIPS system".

 

So I'm looking forward to this critical observation with a sub F/4 telescope.

 

I think we're getting closer to solving the mystery with Paracorr in the 2" Powermate.

Our CN friend Sixela wrote me the following message (translated from his original Dutch):

 

The Powermate 2x and 4x with a Paracorr in it do indeed correct coma well.

But that is because with a Paracorr in a Powermate (if I remember it even sticks out a bit) the focal plane is so far from the shoulder of the PM that the Powermate itself will correct part of the coma (just like the APM 1.5x). That PM namely only zeroes coma with the focal plane on the shoulder of its eyepiece holder.

This sounds plausible.

It would mean that at the distance the Paracorr is set in the 2" Powermate, the Powermate effectively acts partially as a coma-correcting barlow.

 

Thanks, Alexis!


Edited by Houdini, 03 May 2025 - 03:40 PM.


#40 Jon Isaacs

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Posted 03 May 2025 - 09:05 PM

I wonder why 1.25 Paracorr was discontinued. Some degree of coma-correction for small ultraportable dobs, surely there is a market.

 

My first coma corrector was the 1.25 inch Paracorr. I bought it used. They're very rare.  If you're going to invest in a coma corrector, your going to also invest in a 2 inch focuser..

 

Jon




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