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# How To Use The Bartel Diagonal Calculator with a Coma Corrector

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### #26 Vic Menard

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Posted 23 March 2019 - 12:51 PM

The key reason I am interested in understanding this issue as precisely as I can is that I am planning a 20" F/3.7 Newtonian optimized for richest field, will be using the ES100 25mm (41mm field stop) and a Paracorr, for the widest field views, and need to decide on the secondary.

As a side note, in a scope of this FR, is there really any benefit of the Paracorr II over the original?

I did the numbers for your scope. Assuming you're using a Paracorr with the field lens positioned 11-inches away from the secondary mirror, and you use a 4-inch minor axis secondary mirror, without taking the Paracorr into consideration, you're barely using 100-percent illumination in the center of the field of view.

But if you instead design the scope to illuminate the field lens of the Paracorr, the 4-inch secondary mirror is an excellent match, illuminating the central 1-inch diameter of the 1.6-inch field lens 100-percent.

And at f/3.7, the Paracorr 2 performance increase over the original will be obvious (it's easily visible in my 22-inch f/4).

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### #27 Vic Menard

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Posted 23 March 2019 - 12:56 PM

I don't understand these numbers.  If "intercept" is the distance from the diagonal center (optical tube axis) to the FP as in my first example, the 10" is where the original FP of the f/3 lies.  With that set the SIPS bottom barrel just kissed the parallel rays for the mirror.

In example 2 "intercept" is moved in by 3.5", which puts all kinds of obstruction into the light path... Same for example 3.  And both are showing faster mirrors  than the design I was discussing.

The intercept distance is not to the focal plane, it's to the illuminated target. The mathematics are all conical geometry, the primary mirror is essentially the base of a cone. Example 2 moves the Paracorr field lens to the edge of the incoming bundle, and example 3 moves it outside of the bundle by 1-inch.

And yes, you are correct, it is indeed all kinds of obstruction, not so much on the incoming bundle (example 3), but on the focused bundle.

To clarify, the primary mirror in all 3 scenarios is the same focal ratio, but the illumination profile is sampled at a different location in each.

Edited by Vic Menard, 23 March 2019 - 12:58 PM.

### #28 careysub

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Posted 23 March 2019 - 01:37 PM

Are you planning to use it for AP so want to know the exit pupil size?

No, strictly visual. And for me 25/(3.7*1.15) = 5.88mm which is perfect.

(I am pleased to see that this issue is not a slam dunk even for people more knowledgeable than I.)

Edited by careysub, 23 March 2019 - 01:39 PM.

### #29 Starman1

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Posted 23 March 2019 - 04:59 PM

The key reason I am interested in understanding this issue as precisely as I can is that I am planning a 20" F/3.7 Newtonian optimized for richest field, will be using the ES100 25mm (41mm field stop) and a Paracorr, for the widest field views, and need to decide on the secondary.

There is an enormous price jump, and precision limitation from going from a 4" diagonal, the largest standard size, to anything larger, even 4.25", which must be special ordered with long lead times, so I am trying to determine whether 4" will suffice.

According to the very informative TelescopeOptics website anything less than 0.075 wave PV (that is, 1/13 wave) is diffraction limited, so I figure a 1/14 wave 4" (from Astrosystems, say) is effectively perfect. But for a 4.25" mirror they only offer it at 1/10 wave, and the cost is twice as much.

As a side note, in a scope of this FR, is there really any benefit of the Paracorr II over the original?

Yes, the original corrected a 40mm field completely at f/5 and then the corrected field shrank as the f/ratio shortened.

The Paracorr II corrects a 40mm field to f/3.5, so you won't see any coma in your lowest power eyepiece.  Astigmatism, yes, but not coma.

### #30 Starman1

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Posted 23 March 2019 - 05:20 PM

I disagree. Like the secondary mirror (which behaves, for all practical purposes, as a field stop), the Paracorr field lens is also a field stop. In this respect, a larger secondary mirror is already being stopped even if the Paracorr lenses are removed. Using the Bartels calculator, the eyepiece field diameter represents the illuminated diameter represented in the off-axis illumination chart. If the Paracorr field lens is illuminated 100-percent, the internal Paracorr lenses are responsible for the illumination profile at the eyepiece.

OK, look, the new focal plane, with the Paracorr, is considerably farther out than the focal plane without the Paracorr.

Why wouldn't you use the new focal plane position as the intercept distance on the scope?

Because the Paracorr moves the focal plane out, so the illumination of a 40mm circle at the focal plane is now farther out.

But, the Paracorr barrel is inserted into the light cone of the scope inside the focal plane of the scope, and it's a long tube, with about 85mm of Paracorr inside the old focal plane of the scope.

(and maybe some drawtube, if it is longer than the Paracorr).

Let's assume, for the sake of argument, that you chose a secondary size as if the Paracorr were not there.

Then you insert the Paracorr, and you discover the edge of the front end of the Paracorr intrudes into the edge of field light path, which is already dimmed 30% by the secondary mirror.

If you chart the difference, you'll see the edge of the field is dimmed slightly, but not significantly--maybe another 10%.

Could you notice that vignetting?  Probably not.  And it wouldn't be particularly important if it only occurred in the lowest power eyepiece, either.

What happens if you increase the size of the secondary?  It illuminates a larger circle, of course, and the Paracorr cuts off even more peripheral light (though it may be outside the eyepiece's field stop).

What happens if you shrink the secondary?  It illuminates a smaller circle, and the Paracorr may cut off no peripheral light at all.  Of course, the secondary not cuts off a lot more peripheral light than before.

So why in the world would you take the Paracorr into account when choosing a size for illumination?  You wouldn't do it for a Barlow, and you wouldn't do it for a long drawtubed focuser, either.

You calculate the size of the secondary to illuminate the circle on the focal plane of the scope.

Now, you could think of the Paracorr as a part of the eyepiece, a part that reduces the 36.2mm field stop of the 21mm Ethos to 31.5mm.

Or, you could think of it as a part of the scope that increases the focal length and f/ratio by 1.15x.

Either way allows correct figuring for the magnification and the true field.

But replace the Paracorr with a 1.15X Barlow.  Would you use the Barlow to calculate the size of the secondary?

### #31 Vic Menard

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Posted 23 March 2019 - 06:25 PM

...So why in the world would you take the Paracorr into account when choosing a size for illumination?  You wouldn't do it for a Barlow, and you wouldn't do it for a long drawtubed focuser, either.

You calculate the size of the secondary to illuminate the circle on the focal plane of the scope.

OK, if you're using a Barlow as an accessory, then yes, you wouldn't take it into account when choosing the size of the fully illuminated focal plane. But if the Barlow is not an accessory, like the transfer lens/Barlow in a Bird-Jones scope, and it's integral to the optical configuration, then you would take it into account. If the Paracorr is integral to the system, which it should be at f/3.7, it should also be taken into account. Why in the world would you choose to illuminate the Newtonian focal plane when it will never be used?

FWIW, if a scope is configured with a focuser drawtube that's too long, and it can't be corrected, you either choose a smaller fully illuminated image size and choose the appropriate (smaller) secondary mirror, or you use the larger secondary mirror and throw away the illumination it would have provided if the drawtube was configured correctly, and you get to keep the larger central obstruction.

Since the short focus, coma corrected Newtonian is a catadioptric scope, and since the field lens of the Paracorr is an effective field stop (just like a too long drawtube), you should choose the size of the secondary mirror based on that field stop, and if you can get near 100-percent full illumination on that field lens, all the better. Unfortunately, there is no benefit to illuminating the annulus surrounding the field lens, as this light will never reach the eyepiece or the camera.

### #32 Starman1

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Posted 23 March 2019 - 06:43 PM

The AXIAL ray' edge won't reach the focal plane, but the front end of a focuser or Paracorr won't vignette the OFF AXIS ray on the side opposite the protrusion any more than the front opening of the scope.

I don't disagree that a Paracorr in the light cone won't vignette, but it won't vignette noticeably or at any large percent.

And if you still want to illuminate the center 6-10mm of field to 100%, shrinking the secondary is the wrong thing to do, regardless of whether the front edge of the

Paracorr vignettes slightly.

Edited by Starman1, 23 March 2019 - 06:43 PM.

### #33 Vic Menard

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Posted 23 March 2019 - 07:10 PM

...if you still want to illuminate the center 6-10mm of field to 100%, shrinking the secondary is the wrong thing to do, regardless of whether the front edge of the Paracorr vignettes slightly.

While the front edge of the Paracorr is a field stop (so yes, it vignettes the light bundle just like the secondary mirror), it also is the cell for a negative lens which extends the focal ratio by 15-percent (effectively 5.85-inches for a 13-inch f/3). While this doesn't significantly change the placement of the focal plane, it does change the illumination profile at the new focal plane--not the same 100-percent illumination as the field lens, but optimal for a 100-percent fully illuminated field lens (a larger diameter 100-percent illumination profile at the 40mm field lens will not improve the illumination at the new focal plane).

I'm driving all day tomorrow. I'll watch this discussion after I get situated at my destination B&B.

### #34 careysub

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Posted 23 March 2019 - 08:18 PM

Now, you could think of the Paracorr as a part of the eyepiece, a part that reduces the 36.2mm field stop of the 21mm Ethos to 31.5mm.

Or, you could think of it as a part of the scope that increases the focal length and f/ratio by 1.15x.

Either way allows correct figuring for the magnification and the true field.

But replace the Paracorr with a 1.15X Barlow.  Would you use the Barlow to calculate the size of the secondary?

As Vic, says yes you would if you only ever used the telescope with the Barlow. I would never use the the F/3.7 without the Paracorr II -- I consider it (and all Newtonians significantly faster than F/5) to be catadioptrics, it would be like using an SCT, but without the front plate -- it would be an incomplete optical system.

So what you say in your first (quoted) sentence is what I proposed for evaluating the secondary configuration, and so it seems this is the correct procedure for my purposes. And in that case the 4" secondary is fine (*whew*).

### #35 mark cowan

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Posted 23 March 2019 - 09:21 PM

The key reason I am interested in understanding this issue as precisely as I can is that I am planning a 20" F/3.7 Newtonian optimized for richest field, will be using the ES100 25mm (41mm field stop) and a Paracorr, for the widest field views, and need to decide on the secondary.

There is an enormous price jump, and precision limitation from going from a 4" diagonal, the largest standard size, to anything larger, even 4.25", which must be special ordered with long lead times, so I am trying to determine whether 4" will suffice.

According to the very informative TelescopeOptics website anything less than 0.075 wave PV (that is, 1/13 wave) is diffraction limited, so I figure a 1/14 wave 4" (from Astrosystems, say) is effectively perfect. But for a 4.25" mirror they only offer it at 1/10 wave, and the cost is twice as much.

As a side note, in a scope of this FR, is there really any benefit of the Paracorr II over the original?

Yes for an f/3.7 the PII is considerably better than the original model.

And I've encountered a similar design problem, for a 20" f/3.3 thin menisicus (or close to that), which requires a 4.5" diagonal.  And the price/availability of appropriate diagonals goes way up, as you say, especially for comparable surface ratings.  But I'm looking to solve that with an (unproven as yet) method of manufacturing the mirror itself with an offset optical axis, thus reducing the required secondary size at the expense of higher coma in portions of the field, but still within the limits of the PII for correction.

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### #36 mark cowan

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Posted 23 March 2019 - 09:29 PM

And yes, you are correct, it is indeed all kinds of obstruction, not so much on the incoming bundle (example 3), but on the focused bundle.

There was  a thread not too long ago that discussed this to death, including (unfortunately ineffective) ways of modifying the obstruction pattern with various patterns of arcs to reduce the impact.  None of them worked well enough to be worth pursuing.

If I understand your method correctly it will not work, as the PII must be placed at a specific location vis-à-vis the focal plane.  This is documented here:  http://www.loptics.c...SIPS_facts.html  If I don't understand it correctly, please explain.  Diagrams would be quite helpful.

EDIT:  There is also quite a lot of information in that document (thanks Mike!) about the effect on field stops and illumination from the PII, as well as how to (this discussion!) design the scope that it goes in, as well as setting it up correctly (for the SIPS version, that is).

Edited by mark cowan, 23 March 2019 - 09:57 PM.

### #37 careysub

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Posted 23 March 2019 - 11:35 PM

There was  a thread not too long ago that discussed this to death, including (unfortunately ineffective) ways of modifying the obstruction pattern with various patterns of arcs to reduce the impact.  None of them worked well enough to be worth pursuing.

If I understand your method correctly it will not work, as the PII must be placed at a specific location vis-à-vis the focal plane.  This is documented here:  http://www.loptics.c...SIPS_facts.html  If I don't understand it correctly, please explain.  Diagrams would be quite helpful.

It is mostly about the SIPS, but it includes this critical piece of advice:

"If you prefer observing without corrected vision, choosing a regular Paracorr 2 instead of SIPS will maintain coma correction independent of focus position."

I had better that normal vision (still do, sort of) until my lens froze in place (focused about the distance to a computer monitor, surprise, surprise) by now need glasses to focus on infinity. I don't like wearing glasses while observing, I would much rather have the instrument present me with an (to me in-focus) image.

And I already have a Paracorr II. So this is fortunate.

I have every visually useful coma corrector available - the Paracorr I and II, the GSO CC (same as the Astrotech I think), and the HRCC, if I am missing one please tell me. There are other CCs but as far as I know they are suitable for astrophotography, not visual use.

However I do a lot of outreach, showing people stuff in a big fast dob, and focusing using my glasses makes since, so that it is in focus for everyone else.

### #38 hakann

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Posted 24 March 2019 - 02:49 AM

Mel get away use smaller diagonals in his scops as he has intrude the PII tube into primary.
He means the effect is non in image but he gets a smalller diagonal.
Less price, less weight, less obstruction.
It’s personal how one like to do.
Normally its the half field stop of the lowest power EP ( ex E21 mm = 18.1 mm = radial clearance )

To get ’your’ size if you use a TV coma corrector ( their 2” ) you get your std focus pt by use half primary diameter, FS clearance and then you and = 88.5 mm.
Say you has a 13” and f/3 that is 39” FL.
Use Mels calc.
6.5” ( 165.1 mm ) + 18.1 mm + 88.5 mm = 271.7 mm ( 10.69” )
That means a 4” diagonal and magdrop is 0.2.

The more you move in the L-didtance the more the PII tube will intrude but you get a smaller diagonal.

Ex, if you move it in 1” to 9.69 it show a 3.5” and magdrop is 0.25.
Now the tube is just little inside the primary.
Lets do what Mel does and we use another 1” inside primary to 8.69”.
Now a 3.1” is show and magdrop to 0.28.

If a wide field only I think I should use the coma corrector tube at mirrors edge, so a L-distance at 9.98”, and that mean a 3.5” and magdrop at 0.28.
But a 3.5 is really a 3.4”, so magdrop is 0.3.

Mel means his RF scops is not ’just’ a wide field low powers, as he go to E3.7 mm EP if sky is there.
Ok, in this fast scops a 3.7 mm is not real high power.
But few can use over 600X on our sky anyway very often.
( ok.. there is the PowerMate x 2 ;-))

### #39 mark cowan

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Posted 24 March 2019 - 08:51 AM

Yes, this works, but it works better for larger apertures (where the relative % of the 2" barrel of the corrector is less overall).  Unless you shroud the barrel with an fillet (section of arc) you'll see some spiking from that obstruction, although that method has been demonstrated to cause more of a hit on total obstruction than is worthwhile for the reduction from shrouding.  Easily tested though.

### #40 Starman1

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Posted 24 March 2019 - 09:14 AM

OK, per Mike's article about setting the SIPS position, and assuming a 1/2" thick focuser board, I get about 13.93" intercept distance on a 20" f/3.7 scope,

yielding a secondary of 3.8" as the minimum (which is a 4" with a lip 0.1" all the way around).

I think that dimension will put the SIPS lens front end inside the focuser board and very slightly inside the UTA.

It would only intrude a tiny bit into the light path at the very edge of the field on one side in the lowest power eyepiece, likely completely unnoticeable.

I would have to go back and recalculate for the PII, however.

### #41 MitchAlsup

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Posted 24 March 2019 - 09:31 AM

Mel get away use smaller diagonals in his scops as he has intrude the PII tube into primary.
He means the effect is non in image but he gets a smalller diagonal.
Less price, less weight, less obstruction.
It’s personal how one like to do.
Normally its the half field stop of the lowest power EP ( ex E21 mm = 18.1 mm = radial clearance )

Mel essentially came to the conclusion that the fall-off rate was so small at F/3 and faster, than the Ideal size for the secondary is just greater than that which fully illuminates the center of the FoV.

One should also note: if you go this route, you need exceptional ability to position the secondary with the correct piston and offset in order that the center of the FoV actually is fully illuminated.

### #42 Starman1

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Posted 24 March 2019 - 09:34 AM

Let's see:

18mm tube clearance

(ignoring focuser board thickness, position thereof, focuser height, etc.) Paracorr length 75mm

5mm to allow for nearsighted people to focus

10.16mm height above bottom of Paracorr with Paracorr top at focus position (setting E)

49mm length of P-II tunable top.

That puts the minimum intercept distance at 411mm (16.19") with Paracorr II in place to keep the P-II from intruding into the tube.

If you allow the bottom of the P-II to reach the edge of the mirror for the nearsighted observer, then subtract 18mm, for 393mm (15.47")

[the P-II is 75 + 10.16 + 49mm = 134.16mm tall when its top is at the focal plane of the scope with Paracorr in place, so intercept distance

is mirror radius + tube clearance + 134.16mm + nearsighted focus adjustment.]

Assuming a Paracorr-reduced field stop on a 21mm Ethos of 31.5mm,

When I put the 411mm intercept figure in the calculator, the secondary size becomes: 120mm (4.72")

When I eliminate the 18mm tube clearance, the secondary size becomes 110mm (4.33")

Add a lip holder for the mirror, and i see a 4.5" mirror being the choice for low power with the P-II in the light path (tube clearance), and 5" out of the light path.

Or perhaps a custom size in between.

The secondary doesn't get larger if the 35.6mm reduced field stop of the 25mm 100° is used.

Edited by Starman1, 24 March 2019 - 09:43 AM.

### #43 tommm

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Posted 24 March 2019 - 11:09 AM

Let's see:

18mm tube clearance

(ignoring focuser board thickness, position thereof, focuser height, etc.) Paracorr length 75mm

5mm to allow for nearsighted people to focus

10.16mm height above bottom of Paracorr with Paracorr top at focus position (setting E)

49mm length of P-II tunable top.

That puts the minimum intercept distance at 411mm (16.19") with Paracorr II in place to keep the P-II from intruding into the tube.

If you allow the bottom of the P-II to reach the edge of the mirror for the nearsighted observer, then subtract 18mm, for 393mm (15.47")

[the P-II is 75 + 10.16 + 49mm = 134.16mm tall when its top is at the focal plane of the scope with Paracorr in place, so intercept distance

is mirror radius + tube clearance + 134.16mm + nearsighted focus adjustment.]

Assuming a Paracorr-reduced field stop on a 21mm Ethos of 31.5mm,

When I put the 411mm intercept figure in the calculator, the secondary size becomes: 120mm (4.72")

When I eliminate the 18mm tube clearance, the secondary size becomes 110mm (4.33")

Add a lip holder for the mirror, and i see a 4.5" mirror being the choice for low power with the P-II in the light path (tube clearance), and 5" out of the light path.

Or perhaps a custom size in between.

The secondary doesn't get larger if the 35.6mm reduced field stop of the 25mm 100° is used.

Looking at it this way, and using the Televue figure for the P2 I get the P2 focus (not prime focus) is at 254 + 18 + 75 + 60 + 5 = 412mm or 16.22", so about the same result as you.  And 18 mm less if you locate the P2 barrel bottom at the primary mirror radius.

So again referring to the Televue figure, with the bottom of the P2 barrel located at the primary mirror radius, prime focus is at: 412 - 18 - 60 + 14 = 348mm, or 13.7"

One caveat:  The above assumes the focuser barrel length is 75mm or less.

Edit: using Mel's calculator with 13.7" secondary mirror to prime focus distance and 41mm FS of the ES25-100 I get:

Edited by tommm, 24 March 2019 - 11:19 AM.

### #44 careysub

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Posted 24 March 2019 - 11:23 AM

It appears then the use of the Paracorr effectively extends the intercept distance by about 5" (134.16mm) beyond a normal EP configuration (although reducing the effective field stop)?

Let's see:

18mm tube clearance

(ignoring focuser board thickness, position thereof, focuser height, etc.) Paracorr length 75mm

5mm to allow for nearsighted people to focus

10.16mm height above bottom of Paracorr with Paracorr top at focus position (setting E)

49mm length of P-II tunable top.

That puts the minimum intercept distance at 411mm (16.19") with Paracorr II in place to keep the P-II from intruding into the tube.

If you allow the bottom of the P-II to reach the edge of the mirror for the nearsighted observer, then subtract 18mm, for 393mm (15.47")

[the P-II is 75 + 10.16 + 49mm = 134.16mm tall when its top is at the focal plane of the scope with Paracorr in place, so intercept distance

is mirror radius + tube clearance + 134.16mm + nearsighted focus adjustment.]

Assuming a Paracorr-reduced field stop on a 21mm Ethos of 31.5mm,

When I put the 411mm intercept figure in the calculator, the secondary size becomes: 120mm (4.72")

When I eliminate the 18mm tube clearance, the secondary size becomes 110mm (4.33")

Add a lip holder for the mirror, and i see a 4.5" mirror being the choice for low power with the P-II in the light path (tube clearance), and 5" out of the light path.

Or perhaps a custom size in between.

The secondary doesn't get larger if the 35.6mm reduced field stop of the 25mm 100° is used.

So using the Paracorr adds 134.16mm 5.28") to the required intercept distance?

Mel Bartel has this to say:

"The TeleVue P2 2 inch coma corrector requires an additional 3/4 inch inward travel and moves the focal plane outward 1 13/16 inches.

I place the primary mirror focal plane 1 15/64 inches above focuser block top and since the focuser's compression ring is 3/8 inch thick, the primary mirror focal plane is positioned 55/64 inch above focuser's compression ring top. This results in the primary mirror focal plane being positioned 35/64 inch above coma corrector barrel bottom.

I use 5/16 inch in-travel for observers who are strongly near sighted."

Mel Bartel seems to suggest that it adds 1.81" (1 13/16").

He also says "Placing the focuser can be confusing with a coma corrector." Indeed.

### #45 careysub

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Posted 24 March 2019 - 11:27 AM

So again referring to the Televue figure, with the bottom of the P2 barrel located at the primary mirror radius, prime focus is at: 412 - 18 - 60 + 14 = 348mm, or 13.7"

This is the same calculation I made, with the same answer, and the illumination looks acceptable.

Edited by careysub, 24 March 2019 - 11:27 AM.

### #46 tommm

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Posted 24 March 2019 - 11:37 AM

Yes, 10mm to 12mm fully illuminated fov is roughly as good as most do I think.  With the 13.95" I gave earlier its pretty marginal, around 5mm, but still above the 0.4 magn drop at the edge, so ok by Mel's standard, and visually it likely wouldn't be noticed as he says.

### #47 Starman1

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Posted 24 March 2019 - 01:20 PM

It appears then the use of the Paracorr effectively extends the intercept distance by about 5" (134.16mm) beyond a normal EP configuration (although reducing the effective field stop)?

So using the Paracorr adds 134.16mm 5.28") to the required intercept distance?

Mel Bartel has this to say:

"The TeleVue P2 2 inch coma corrector requires an additional 3/4 inch inward travel and moves the focal plane outward 1 13/16 inches.

I place the primary mirror focal plane 1 15/64 inches above focuser block top and since the focuser's compression ring is 3/8 inch thick, the primary mirror focal plane is positioned 55/64 inch above focuser's compression ring top. This results in the primary mirror focal plane being positioned 35/64 inch above coma corrector barrel bottom.

I use 5/16 inch in-travel for observers who are strongly near sighted."

Mel Bartel seems to suggest that it adds 1.81" (1 13/16").

He also says "Placing the focuser can be confusing with a coma corrector." Indeed.

No, the length of the Paracorr when the top is set to setting E (where the focal point of the Paracorr is) is 134.16mm.

If one ignores the focuser board, inset of the focuser board, height of the focuser, then 134.16mm is added to the radius of the mirror and the mirror clearance to get to the minimum possible distance for a Paracorred focal plane from the secondary.  That's where I got 411mm.  That's the distance to the Paracorr focal plane, not the mirror's.

I would add 5mm (included in the 411mm figure) so your friends who are nearsighted could also use it.

Now, since the tunable top is 49mm long and it sits 10.16mm above the stop, that is 59.16mm.  The normal focal point of the mirror is 9mm above the lens in the Paracorr, so the Paracorr extends the focal point by 58.16-9mm = 49.16mm.  That's 1.93"  That makes the un-Paracorred focal point intercept distance 411- 49.16 = 362mm approx.

That's about right.  Somewhere around 14" plus a little bit.

If we do it the other way: mirror radius + clearance + focuser board + focuser height + 25mm (includes near-sighted allowance), we get 355mm, which is in the ballpark of 14"

### #48 mark cowan

mark cowan

Vendor (Veritas Optics)

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Posted 24 March 2019 - 03:45 PM

If you allow the bottom of the P-II to reach the edge of the mirror for the nearsighted observer, then subtract 18mm, for 393mm (15.47")

You just require the nearsighted observer to wear their glasses and pocket that 5mm...

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### #49 hakann

hakann

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Posted 24 March 2019 - 04:19 PM

I did CAD up this 20" and the FS at 41 mm. ( clearance = 20.5 mm )
( UTA ring at 56 mm wide )
Focuserboard = 18 mm plus a 1 mm plate between it and focuser.

I added a filter at 4.5 mm on end of the PII tube so the filter is now on edge of the clearance ( no intrude in light path )
It's a Starlight 2" focuser drawed in and it's 6 mm untill the PII bottoms.
That get a L-distance at 367.58 mm or 14.47".

Paracorr is I said before is whit the 9 mm above lens to std focus pt is = 88.58 mm ( whit filter add 4.5 mm )

In Mel's calc a 3.9" ( 4" ) diagonal is bearly make it, and magdrop is at 0.3.

If a shorter L-distance the paracorr will intrude in the light path. ( wrong, maybe not, just saying )

#### Attached Thumbnails

Edited by hakann, 25 March 2019 - 05:04 AM.

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### #50 careysub

careysub

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Posted 26 March 2019 - 03:42 PM

Now, since the tunable top is 49mm long and it sits 10.16mm above the stop, that is 59.16mm.  The normal focal point of the mirror is 9mm above the lens in the Paracorr, so the Paracorr extends the focal point by 58.16-9mm = 49.16mm.  That's 1.93"  That makes the un-Paracorred focal point intercept distance 411- 49.16 = 362mm approx.

That's about right.  Somewhere around 14" plus a little bit.

If we do it the other way: mirror radius + clearance + focuser board + focuser height + 25mm (includes near-sighted allowance), we get 355mm, which is in the ballpark of 14"

The way I estimate it is: optical cone radius 10.5" (266.7mm), 2" focuser tube length (tube just touches the cone, 50.8mm), 25mm above the tube racked in position = 342.5mm (13.48"). Tube walls, focuser board thickness, ring widths, and focuser heights are all irrelevant if we just consider the focuser tube position and length, as long as some choice of low profile focuser and UTA construction can put it there (and it can). Using a 1.5" tube will cut 12.7mm off of this, for an absolute minimum (with no light cone intrusion) of 329.8mm.

If the Paracorr adds 49.2mm to this then the numbers are 391.7mm (15.4") and 379.0mm (14.9"), and reduces the effective field stop from 41mm to 35.65mm (1.40").

With 14.9" even the 4.25" is only marginally acceptable (0.4" diameter field virtually even, 0.15 mag edge drop relative to center of field), but if I have to make that jump I might as well go to the 4.5" (0.6" even field, ~0.11 mag edge drop). I probably could not tell the difference between these two, but the CO ratio is only slightly greater (21.25% vs 22.5%) and either is not bad at all.

However if I wanted to go with the 2" tube only the 4.5" performs adequately.

So I guess 4.5" is it, to give me some lee way.

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