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

183 replies to this topic

### #176 andreww71

andreww71

Mariner 2

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Posted 01 April 2019 - 07:50 PM

Ok, I never tested that.

So if one not see primary but Bartels program say ok in mag-drop ( say at 0.25 ) - how to solve that ?

To move in the L-distance ?

Now you haswjit a 4.35" around 0.12 in mag-drop at a1.6" FS.

Intresting, so ex a mag-drop at 0.25, one might not see the whole primary !

hakann,

I suggest the ATM should use multiple sources and / or calculations to verify data that is being presented. I also think that when you are designing a system with the intent on using the smallest possible secondary that you study it carefully.

To answer your question above, yes you could decrease the L dimension but in many cases it won't be enough.

Andrew

### #177 tommm

tommm

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Posted 01 April 2019 - 10:20 PM

I should have mentioned that the equation I gave is straight-forward to derive from the sketch just using similar triangles and trigonometry.

The more I think about my guess that led to the 1.465" number, the more my confidence in it wanes.  The fully illuminated portion at the P2 field lens has rays going through it from all parts of the mirror so incident at different angles and radii from the optical axis. Not clear you can just say that illumination profile at the input to the P2 is just scaled to form the output.

I also thought about replacing the P2 lens assembly with a single divergent "equivalent lens" which describes only the 1.15 barlow effect. But I quickly realized I had no idea where it should be positioned relative to prime focus.  The P2 lens assembly may consist of 2 or 3 divergent lenses and x convergent ones for all I know, so how to determine what the "equivalent distance" is for the simple "equivalent lens" modeling the barlow effect?  So I've pretty much given up.  Any equation estimating the number to use in Mel's calculator would depend on the distance of the field lens of the P2 from prime focus as well as all the other parameters in the equation for the radius of the light cone, so F, tfov angle, and D.

### #178 mark cowan

mark cowan

Vendor (Veritas Optics)

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Posted 02 April 2019 - 12:08 AM

The fully illuminated portion at the P2 field lens has rays going through it from all parts of the mirror so incident at different angles and radii from the optical axis. Not clear you can just say that illumination profile at the input to the P2 is just scaled to form the output.

### #179 Vic Menard

Vic Menard

Fly Me to the Moon

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Posted 02 April 2019 - 05:35 AM

...The fully illuminated portion at the P2 field lens has rays going through it from all parts of the mirror so incident at different angles and radii from the optical axis. Not clear you can just say that illumination profile at the input to the P2 is just scaled to form the output.

I've been saying this all along.

The illumination profile (as determined by the Bartels calculator) is only "clear" up to the entrance of the Paracorr, after that, the telescope is no longer a simple Newtonian.

### #180 hakann

hakann

Surveyor 1

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Posted 02 April 2019 - 07:06 PM

Vic,
I’m totally lost on the Bartels program vs the paracorr vs detecting the diagonal ;-)

I had it up before as a own topics vs the paracorr is inside 46 mm ( called Paul at TV )
I thought line tracing should solve it but the light do things in this one...

-Idea was a guy in Canada had a very fast ( f/2.4 ) 32” and a article ( now removed from internet ) said he used the paracorr 3 ( as I think Mel also ’was’ into first )
But he had no issue at low power ( E21 at a pupil + 6 mm and 92X ) whit the PII.

Well, I’m not a pro observer but I been whit some good ones around the Earth now that showed me and been in Mels scops and the 25”.
Mel guided me and said I had the pupil needed and I saw no issues.
If the paracorr was a limit, well, I can’t say - but should not he ? ( he has a limited diagonal plus he has the P3.

For Mel the EP is all there is ( that I know.

Edited by hakann, 02 April 2019 - 07:11 PM.

### #181 mark cowan

mark cowan

Vendor (Veritas Optics)

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Posted 02 April 2019 - 07:45 PM

P2 3" is really only for imaging.

for imaging applications by amateurs and professionals using CCD formats as large as 52mm diagonal and paraboloids as fast as f/3.

The 46mm aperture on the normal P2 is not going to vignette, and it's designed not to.

### #182 Dale Eason

Dale Eason

Apollo

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Posted 02 April 2019 - 10:24 PM

I used Mel's calculator to design my 10 inch F3 with SIPS and chose the 3.1 inch diagonal. I did not do anything special that I recall in regard to using the SIPS for the calculations. Using that scope now.  Don't know what to look for to see any drop off.  It is a visual scope.  Just blown away by the views so far.   Will be taking it to TSP at the end of this month,

If any here will be there as well you can stop by and see anytime.  I plan to be on the middle field.

Dale

### #183 careysub

careysub

Mercury-Atlas

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Posted 10 April 2019 - 09:43 PM

]I communicated with Mel to get his advice on this, and this is what he had to say:

"Yes, design for the primary's focal plane: do not adjust it. Use the

normal focal plane distance. Do make the 15% smaller field adjustment.

Imagine that you place the eyepiece in the focuser sans coma corrector.
You'll see stars that are focused at the center but that literally are
stretched into invisibility at the edge. Putting in the P2, stars
sharpen up amazingly to the edge, and the focal plane changes and the
field shrinks by 15%. Think of a barlow lens that shifts the focal plane
to increase the focal length. In fact, as you no doubt know, the Ethos
line of eyepieces first lens set functions as a barlow to spread out the
light so that the rest of the lens sets can more effectively do their
job. The field stop is inside the eyepiece as a consequence of the
design.

Since there is an optimal position for the P2, it does not need the
focus range of an eyepiece. While the P2 shoulder strictly calls for
3/4" more inward travel, the inward travel otherwise required to
accommodate some eyepieces can be dispensed with, keeping an allowance
for near sighted people. Also important but little discussed is the need
to vary the P2 position slightly for certain eyepieces. For example, the
8mm Ethos is strongly overcorrected and can use a little P2 position
change to fix spherical aberration. I cannot see a visual degradation of
coma at the edge.

Overall, I design my focusers to position the P2 in its recommended
position and I add 5/16" more for near sighted eyes and for making
adjustments to eyepieces that change the spherical aberration strongly.
Positioning the focuser is confusing. That's why I standardize on the
dimensions that I show in my graphic. Actually the best design is for a
small adjustment focuser for the P2, then a longer range focuser for the
eyepiece (get rid of the tunable top)."

So just use the Diagonal Calculator and make the 15% adjustment, and you are (almost) done.

According to a diagram shown earlier in the thread, the distance from prime focus to the base of the P2 is 89mm.

So, with the zero intrusion distance of 10.55" (268mm) + 89mm = 357mm (14.05"), and with the 41mm FS EP, reduced to 35.7mm (1.40") we have the diagram below. This looks perfectly acceptable to me (and I could intrude into the light path a little if I chose to widen the fully illuminated zone, but doubt I could see the difference). So a 4" secondary it is!

[attachment=1244154:Screen Shot 2019-04-10 at 7.40.32 PM.png

#### Attached Thumbnails

Edited by careysub, 10 April 2019 - 09:46 PM.

### #184 Vic Menard

Vic Menard

Fly Me to the Moon

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Posted 12 April 2019 - 11:40 AM

...So, with the zero intrusion distance of 10.55" (268mm) + 89mm = 357mm (14.05"), and with the 41mm FS EP, reduced to 35.7mm (1.40") we have the diagram below. This looks perfectly acceptable to me (and I could intrude into the light path a little if I chose to widen the fully illuminated zone, but doubt I could see the difference). So a 4" secondary it is!

It works even better if you solve without the 89mm.

But I agree with your conclusion--I doubt you will see the difference with a 4-inch minor axis--in fact, if there was a difference to see, I believe you would find the field illumination profile is actually improved because of the Paracorr 2.

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