39 replies to this topic

[25585]

An interesting snippet of information I came across from TV, "When using Tele Vue's Paracorr coma corrector, the 2.5x or 5x Powermate™ should be inserted into the Paracorr's 1¼" adapter (i.e., between the Paracorr and the eyepiece). For 2x and 4x 2" Powermates™, first insert the Powermate™ into the focuser, then the Paracorr into the Powermates™"

 

I have yet to try my 4x Powermate in a Dob, has anyone else used a 2" PM & a Paracorr together?

 

Reversing the optical order, makes me wonder if the optical design of the 2" PMs is different than the 1.25":PMs. So far, with 2" Barlows (not telecentric), I have put them inside the PC, perhaps that was wrong..?

 

"

[CrazyPanda]

I did a few tests once with my 2x PM and Paracorr.

P2 -> PM -> eyepiece: some minor spherical abberation at very high power

PM -> P2 -> eyepiece as recommended by TV -> almost no spherical abberation.

PM by itself: zero spherical abberation.

I think the idea behind TV’s recommendation is that it doesn’t change the spacing relationship between the P2 and eyepieces since the PM is basically converting the telescope to a longer focal length scope.

I don’t know what happens with the 1.25” PowerMates.

But the fundamental problem is the 2” PM and P2 is a long, heavy combo. It’s why I have the 2.7x coma correcting barlow in my kit now. When I need really high mag, I take out the P2 and use the barlow with my 6, 4.7, and 3.7 ethoi.

[Jon Isaacs]

 

But the fundamental problem is the 2” PM and P2 is a long, heavy combo. It’s why I have the 2.7x coma correcting barlow in my kit now. When I need really high mag, I take out the P2 and use the barlow with my 6, 4.7, and 3.7 ethoi.

 

I use the Type 6 Naglers with the 1.25 inch 2x TeleVue Barlow.  I still get the coma correction of the P2 and the package is still compact and since at high mags I'm concentrating on the object, anything past about 40° off axis is not of much use anyway..

 

Jon

[Starman1]

An interesting snippet of information I came across from TV, "When using Tele Vue's Paracorr coma corrector, the 2.5x or 5x Powermate™ should be inserted into the Paracorr's 1¼" adapter (i.e., between the Paracorr and the eyepiece). For 2x and 4x 2" Powermates™, first insert the Powermate™ into the focuser, then the Paracorr into the Powermates™"

 

I have yet to try my 4x Powermate in a Dob, has anyone else used a 2" PM & a Paracorr together?

 

Reversing the optical order, makes me wonder if the optical design of the 2" PMs is different than the 1.25":PMs. So far, with 2" Barlows (not telecentric), I have put them inside the PC, perhaps that was wrong..?

 

"

My lifetime-best view of Jupiter was with an 8mm Ethos, Paracorr, and 2X 2" PowerMate.

Other than the fact it stuck out of the focuser a long way, the image was as close to perfect as the aperture and atmosphere allowed.

2" PowerMates, and the 1.25" 2.5x have almost zero change of magnification with distance from the eyepiece, whereas the 5X PowerMate acts a lot like a Barlow and goes over 8x about 100mm out.

 

You can't put the 2" PowerMates in the Paracorr, as you will not be able to focus the eyepieces in the range of the Paracorr's top.

The reason is simply that the length of the lower barrel of the 2" PowerMates is way too long to insert there and come to focus.

You'd have to move the focuser in, which would de-optimize the eyepiece's relationship to the coma corrected focal plane.

The spherical aberration that results doesn't surprise me.

In reverse, however, though the Paracorr also does not insert fully into the top of the PowerMates, the PowerMates operate as if the image is at ~f/infinity, so the Paracorr + Eyepiece focuses as normal.

 

See:

https://stargazerslo...comment-3118485

[Jon Isaacs]

 

You can't put the 2" PowerMates in the Paracorr, as you will not be able to focus the eyepieces in the range of the Paracorr's top.

 

For what it's worth:

 

One advantage of the 1.25x 2x TV Barlow + Type 6 Naglers is that the Paracorr setting is basically unchanged with the 1.25 Barlow, there are no spacing/insertion issues.

 

The GSO 2X 2 inch Barlow can be replaced with a shorter extension so it's about 1.75x and allows for proper spacing with the 13mm-10- 8mm Ethos in the 2 inch mode with the Paracorr.

 

Jon

[BGazing]

I was testing my P2 with PM 2.5 and E6 and E8 last night on Eskimo and Ghost of Jupiter to try 600x and 800x. 

Am I imagining things, or do you, after inserting PM2.5 into P2, have to again to dial in your focus position via tunable top, i.e. the setting changes? It went from B to H on my E8 and E6. At first, I tried E8 with PM2.5 for 600x at setting B and thought the view was not good, tried fine focusing it via focuser and gave up...only later, with E6, did I try using tuneable top, focused at H and the view was considerably better.

I never found info on that before, so I am questioning my own sanity. PM is not supposed to change the focusing position, and especially not so that you extend setting from B to H. Am I missing anything?

[Houdini]

From a theoretical point of view (as a designer of coma-correcting eyepieces), I do not understand how the Powermate + Paracorr order can work.

 

A Barlow or Powermate changes the cone of light that the coma corrector sees. If you start at F/4 and use a 2X Powermate, you get an F/8 light cone. The coma corrector in the Powermate sees this light cone and will only correct for F/8 coma. This means that 75% of the coma is left uncorrected.

 

My understanding is that only the Paracorr + Powermate order will work correctly. The Paracorr can only work correctly if it sees the correct light cone coming from the parabolic mirror. This is what a system like SIPS provides - it always keeps the coma corrector in the right place.

 

So I really wonder if people have looked at this critically enough.

Edited by Houdini, 29 April 2025 - 04:55 AM.

[starcanoe]

 

 

A Barlow or Powermate changes the cone of light that the coma corrector sees. If you start at F/4 and use a 2X Powermate, you get an F/8 light cone. The coma corrector in the Powermate sees this light cone and will only correct for F/8 coma. This means that 75% of the coma is left uncorrected.

 

 

 

But isn't there some difference between how a Barlow works and how a Powermate works? A Powermate is a teleconcentric lens/system (which to be honest of the top of my head don't exactly understand).

[Houdini]

With a 2-element normal Barlow, the angles of incidence of the central rays change, whereas with a 4-element telecentric Barlow they remain the same. This is important for eye relief and vignetting.

 

With a normal Barlow, a wide field eyepiece will see rays entering at unexpected angles, and the eye relief will increase, causing vignetting.

[starcanoe]

With a 2-element normal Barlow, the angles of incidence of the central rays change, whereas with a 4-element telecentric Barlow they remain the same. This is important for eye relief and vignetting.

 

With a normal Barlow, a wide field eyepiece will see rays entering at unexpected angles, and the eye relief will increase, causing vignetting.

 If that's the case doesn't that mean that a 4X telecentric doesn't really turn an f5 light cone from the objective into a virtual F20 light cone that the eyepiece "sees". Which would mean that a telecentric like a power mate should work with a coma correcting eyepiece but a Barlow won't (well it will only partially correct)?

[eblanken]

Hi Richard,

 

An interesting snippet of information I came across from TV, "When using Tele Vue's Paracorr coma corrector, the 2.5x or 5x Powermate™ should be inserted into the Paracorr's 1¼" adapter (i.e., between the Paracorr and the eyepiece). For 2x and 4x 2" Powermates™, first insert the Powermate™ into the focuser, then the Paracorr into the Powermates™"

 

I have yet to try my 4x Powermate in a Dob, has anyone else used a 2" PM & a Paracorr together?

 

Reversing the optical order, makes me wonder if the optical design of the 2" PMs is different than the 1.25":PMs. So far, with 2" Barlows (not telecentric), I have put them inside the PC, perhaps that was wrong..?

 

"

 

 

I did a few tests once with my 2x PM and Paracorr.

P2 -> PM -> eyepiece: some minor spherical abberation at very high power

PM -> P2 -> eyepiece as recommended by TV -> almost no spherical abberation.

PM by itself: zero spherical abberation.

I think the idea behind TV’s recommendation is that it doesn’t change the spacing relationship between the P2 and eyepieces since the PM is basically converting the telescope to a longer focal length scope.

I don’t know what happens with the 1.25” PowerMates.

But the fundamental problem is the 2” PM and P2 is a long, heavy combo. It’s why I have the 2.7x coma correcting barlow in my kit now. When I need really high mag, I take out the P2 and use the barlow with my 6, 4.7, and 3.7 ethoi.

 

+1 on what Panda said . . . 2 inch PM first then P2 is what TeleVue Recommends . . . 

 

Best,

 

Ed

Edited by eblanken, 29 April 2025 - 11:03 AM.

[Starman1]

If the Telecentric sees a light cone, its negative lens expands the light cone, making the light rays more parallel, i.e. a 2X turns the angle of the f/4 rays into an f/8 light cone.

But, the positive lens that follows pulls the rays back in, returning the rays to the f/4 angles, only now the image scale has been expanded by the negative lens and the image is magnified.

 

When that precedes the Paracorr, the Paracorr sees the original f/ratio light cone, only with an expanded image scale.  So coma correction is the same as if the PowerMate weren't used.

The Paracorr may have a different position relative to the primary mirror, however, because the focal point has been moved by the PowerMate.

Because the Paracorr does not fully seat in the PowerMate, as I recall, the position of the Paracorr moves in somewhat, but I don't remember by how much.

 

Now, if the PowerMate is used after the Paracorr, it is as if the eyepiece had an additional magnifier in its base, i.e. was converted to a very long eyepiece.

But the position of its focal plane will have changed slightly, requiring a different setting of the Paracorr's top for optimum spacing. 

 

These complications made me obtain shorter focal length eyepieces and ditch the PowerMates.

 

However, germane to the issue at hand, a PowerMate should work fine with the Houdini eyepiece.  A Barlow, I don't know.

[Houdini]

To my knowledge a 4X telecentric barlow, just like a 4X standard Barlow, transforms a F5 light cone into a F20 light cone.

The difference between the two kinds of barlows is that the telecentric maintains the angles of the chief rays, whereas the Barlow modifies them.

 

For example, on some web sites you can find the ray trace diagram for the 1.5X APM telecentric barlow (coma-correcting, but irrelevant here).

I've pasted a link to this image below.

You can see on the image that the light cones on the left are 1.5 times narrower than the original on the right (please measure with a ruler to confirm this ), while the image scale is 1.5 times larger.

 

In the image below you can see how the original F/4 light cone (coming from the right) changes to F/6 after the telecentric barlow.

That's how you would expect any 1.5X barlow -- telecentric or not -- to behave.

 

Any coma corrector or eyepiece you would place on the left will only see the F/6 light cone.

If you put a coma corrector on the left, it would only correct the coma for a F/6 beam, which is only 44% of the coma in the original F/4 beam.

 

Edited by Houdini, 29 April 2025 - 11:30 AM.

[Starman1]

Robert,

I think there is something wrong with your ray trace.

Here is how I understand the PowerMate:

 

Attached Thumbnails

• 

[Houdini]

Thanks for the discussion, Don.

 

The ray trace diagram I linked to is from some vendor's website for the APM 1.5X telecentric barlow.
It looks like a real and accurate ray trace for that commercial telecentric barlow.

 

I don't think the situation in your third diagram can exist. When you increase the image scale, the light beam becomes narrower.

 

If you happen to meet someone from Televue, please ask them this question. It would be very interesting to hear their comments on the matter and if they think the recommendation on their website is still correct.

 

It would also be very interesting if someone with a sub F/4 telescope, a 2X Powermate, a Paracorr 2 and a very good wide field eyepiece could do a critical test.

Edited by Houdini, 29 April 2025 - 01:39 PM.

[Starman1]

First, a 2X Barlow changes the angle of the rays entering the eyepiece (or Paracorr), but it does not and cannot change the coma produced by the primary.

In the case of an f/8 light cone (after the fact), the coma is still that of the f/4 primary mirror.

 

Second, perhaps it is best to think of a Barlow as part of the eyepiece.  And what happens when you use such a negative lens in the eyepiece with a positive upper section?

It expands the image, but it doesn't change the aberrations from the primary.  The upper positive section is now looking at an expanded image.

In essence, this is what happens in a PowerMate--the negative lens expands the image and the positive lens looks at the expanded image and brings the light rays back to the same angle they entered with.

It performs the same function as making the eyepiece have half the focal length, but see the same f/ratio.

[Houdini]

What you suggest -- doubling the image scale while keeping the same f-number -- would violate the laws of optics, as far as I know.

In a telescope, you cannot make a 1.5X change in image scale without changing the f-number by the same amount.

 

But nothing I say is going to make you change your mind.

 

So like I said, feel free to discuss this with Televue to find out if their recommendation about the 2" Powermate and Paracorr has a scientific basis, or if they wrote it because the other way is physically impossible (the Powermate being too long).

[Starman1]

Perhaps we are just looking at it differently.

You see the f/4 scope + 2X Barlow as an f/8 scope, while I see the f/4 scope + 2X Barlow as an f/4 scope with 2x the focal length.

Since coma is unchanged by the Barlow, I think doubling the focal length is a better way to think of it.

[CrazyPanda]

You see the f/4 scope + 2X Barlow as an f/8 scope, while I see the f/4 scope + 2X Barlow as an f/4 scope with 2x the focal length.

 

Well it has to be an F/8 telescope. You can put a simple Plossl into the PowerMate and see the improved edge correction, not to mention of course the exit pupil behaves the same (e.g. 16mm eyepiece = 2mm exit pupil just like in a native F/8 scope).

 

It may be more accurate to describe it as an F/4 telescope with an eyepiece that is 1/2 as long.

Edited by CrazyPanda, 29 April 2025 - 03:19 PM.

[Starman1]

Well it has to be an F/8 telescope. You can put a simple Plossl into the PowerMate and see the improved edge correction, not to mention of course the exit pupil behaves the same (e.g. 16mm eyepiece = 2mm exit pupil just like in a native F/8 scope).

 

It may be more accurate to describe it as an F/4 telescope with an eyepiece that is 1/2 as long.

Agreed.   That's the way to think of it, because it keeps the f/ratio the same.

[Houdini]

That's correct if it's just 2X Powermate + Eyepiece, you could look at it as if this was an eyepiece with half the FL.

 

But we were talking about adding a Paracorr in the order Powermate + Paracorr + Eyepiece.

 

You could consider that the "Paracorr + Eyepiece" becomes a coma-corrected eyepiece. But this eyepiece will not provide the correct level of coma correction when it's put in the Powermate. In a 2X Powermate the combo will only provide 25% of the coma correction that is needed.

Edited by Houdini, 29 April 2025 - 03:46 PM.

[BGazing]

 

But the position of its focal plane will have changed slightly, requiring a different setting of the Paracorr's top for optimum spacing. 

 

 

Thank you, I thought I was imagining things.

[Starman1]

That's correct if it's just 2X Powermate + Eyepiece, you could look at it as if this was an eyepiece with half the FL.

 

But we were talking about adding a Paracorr in the order Powermate + Paracorr + Eyepiece.

 

You could consider that the "Paracorr + Eyepiece" becomes a coma-corrected eyepiece. But this eyepiece will not provide the correct level of coma correction when it's put in the Powermate. In a 2X Powermate the combo will only provide 25% of the coma correction that is needed.

Well, I can tell you that is not true at all.  I have use Ethos eyepieces in a Paracorr in a PowerMate and the coma correction was just as good as it was without the PowerMate.

So something is missing in your analysis, and I think it is that you are assuming that coma diminishes to the longer f/ratio when a PowerMate is used, and it does not.

[Houdini]

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.

[BGazing]

Thank you, I thought I was imagining things.

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).