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Computing a secondary mirror for a Dalll Kirkham primary

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#1 JohnH

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Posted 22 March 2019 - 01:20 PM

Who was an uptick in my General Health in recent months, and now find myself interested in getting some of my old projects we started. One of them is 24 in 5.6 primary mirror from a Dall Kirkham

I know that it was figured correctly for this use and formula, but I no longer have information regarding the secondary mirror. For those of you out there that have the appropriate software for this problem, what sorts of tolerances do I have regarding the spherical secondary. I know that the final F ratio of the completed telescope was f11 and given the quality of the primary mirror and measuring it with a spirometer of good known quality, I know that the f-ratio on the primary is 5.6. Do I need to test the primary first to see if the shadow Graham actually matches what it should look like for a properly corrected ellipsoid mirror for a Dall Kirkham

#2 gregj888

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Posted 22 March 2019 - 02:13 PM

John,

 

Need a little more information.  Best is the figure for the primary with that there may be several solutions.  Mirror spacing and or back focus is also helpful.



#3 Garyth64

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Posted 22 March 2019 - 03:09 PM

I started playing with the math, and in the middle of it, I noticed you said the final f ratio was 11.

 

That means with the primary at f/5.6, the amp of the system is only 1.96x.  That is going to give a very large secondary of about 9".  (but it may have a RC of about 204".

 

Is the secondary already made? Or do you need to make one?


Edited by Garyth64, 22 March 2019 - 03:37 PM.

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#4 TOMDEY

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Posted 22 March 2019 - 03:25 PM

If the PM by itself is F/5.6 and the final system is to be F/11... that would make for a huge spherical secondary mirror (and consequent huge central obstruction). You're sure those are what the system design calls for? The only other 1st-order parameter would be the back-focus, relative to the PM.

 

Given that you already have the (ostensibly) entirely finished PM in-hand, your very 1st step should be to scrupulously characterize it, even if that means sending it out to a shop that can map the wavefront... in particular, report back to you the radius, conic constant and wavefront quality, when used in an optimized Dahl configuration. While they're at it, they will also back-solve for the size, radius and spacing to the spherical SM, and even correct back-focus. There could be a family of those, but will very likely converge on a highly-preferred ~standard~ lay-out.

 

What I'm saying, being that you have the mirror, but lack the certification or intended design... let a capable shop tell you what you actually have there and how to configure it. Then proceed with confidence.

 

Otherwise, it's just another unknown big piece of glass!

 

[We did a lot of similar exercises at work, over the decades. Some heritage PM comes out of a crate without certification. So we measure it and then see what use it might be, hopefully as-is or re-worked to make it useful. PS: Usable as-is is extremely rare.]    Tom


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#5 TOMDEY

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Posted 22 March 2019 - 03:34 PM

Oh! Here's a similar medium-sized Dahl. In this case, it's a 30-inch downward-pointing (exquisite!) test-set collimator, in one of the optics labs. I was designing and building several parfocal light sources for it... they came out good, and we then proceeded to use the thing. (As-shown I was confirming that my sources were performing as intended.)   Tom

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  • 24 certifying Toms collimator sources 80.jpg


#6 gregj888

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Posted 22 March 2019 - 03:43 PM

Tom, you have some of the coolest toys... :-)



#7 lylver

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Posted 23 March 2019 - 05:57 AM

Hello you should keep magnification provided by the secondary mirror over x2 for wide angle astrograph (~42% obstruction) and x3.2 for planetary (30%)

Try this http://www.astrophot...ain_e_main.html

and enhance a bit the secondary diameter (in the web page, the diameter is given for axial diameter, excluding desired field)

 

Mirror spacing for a DK is very tolerant, I saw a more than 10mm modification of mirror spacing on a 13"f15 recently.



#8 JohnH

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Posted 23 March 2019 - 08:23 AM

Hello you should keep magnification provided by the secondary mirror over x2 for wide angle astrograph (~42% obstruction) and x3.2 for planetary (30%)
Try this http://www.astrophot...ain_e_main.html
and enhance a bit the secondary diameter (in the web page, the diameter is given for axial diameter, excluding desired field)

Mirror spacing for a DK is very tolerant, I saw a more than 10mm modification of mirror spacing on a 13"f15 recently.


Given I already have a completed mirror from an old telescope that worked and I have the secondary assembly which consists of a mirror holder for an 11 inch mirror you were correct all of you that this thing does have a huge Central obstruction and also that focusing on this scope was achieved by using a motorized secondary carrier that has an apparent range of about 2 inch of movement running on a couple of Spur Gears with cutoff switches at the end of travel limits on either end.
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#9 JohnH

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Posted 23 March 2019 - 10:34 AM

Hello you should keep magnification provided by the secondary mirror over x2 for wide angle astrograph (~42% obstruction) and x3.2 for planetary (30%)

Try this http://www.astrophot...ain_e_main.html

and enhance a bit the secondary diameter (in the web page, the diameter is given for axial diameter, excluding desired field)

 

Mirror spacing for a DK is very tolerant, I saw a more than 10mm modification of mirror spacing on a 13"f15 recently.

I recall reading in Telescope Technology Today on the various types of Cassegrains, and that the DK has some advantages like spherical secondary, good range of accomodation for back focus, but the big ones were larger CO for faster designs and large coma.

 

One testing method is this primary has TWO foci, being an ellipsoid mirror, some one has a light source and one focus and tests at the other.

 

How does one compute what distance that is?



#10 Vla

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Posted 23 March 2019 - 11:25 AM

That's easy: near focus is at R/(1+e), far focus at R/(1-e) from mirror vertex, R being the mirror r.o.c. and "e" the eccentricity, which can be written as e=-sgrt(K), where K is the mirror conic. As for the minimum secondary diameter, it is given with a simple relation: k=(1+B)/(M+1), in units of the aperture, with B being the back f.l. in units of the primary mirror f.l. and M the secondary magnification. So, if we take for B in this case 0.15 and M=2 (for simplicity), the minimum secondary size is 1.15/3=0.38 of the primary mirror's effective aperture. To get the needed parameters, including conics, we need to know the back focal length B, i.e. at what distance behind the primary the two mirrors form the best focus.


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