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wsDK - weak secondary Dall Kirkham telescope

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#1 siriusandthepup

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Posted 26 January 2013 - 05:32 PM

wsDK : A Dall Kirkham Cassegrain design suitable for general purpose observing. I will define this variant as generally having a primary F ratio above f/4 and secondary magnification between 1.5x to 2x.

What makes the wsDK different from "normal" DK's?
Traditional DKs have primary mirrors with F ratios in the 2 to 4 range and secondaries with 3 to 5x magnification. They are excellent for high power narrow field viewing. Traditional Dk's Achilles heal is off axis coma. The wsDKs have weak curves on the primary and secondary which minimizes the coma. Also, the weak curves greatly enhance ease of making the optical components. The weak curves greatly enhance the tolerancing aspect of matching secondary and primary mirrors. The weak curves greatly widen the positioning tolerances required for the physical build of the telescope. The lower system F ratio, f/8 to f/10 normally, allows achieving a 5 to 6mm exit eye pupil on the low power end with a 40mm eyepiece and a 0.5mm exit pupil on the high end with a 4mm eyepiece. The weak secondary is what makes this variant work well.

Purpose: Stand on the ground observing for scopes larger than 20". No ladder please! Scalable to at least 50" with minimal tradeoffs.

My project 25":

My 25" DK will have three optical elements: a 25" primary, an 8" secondary and a 3.5" diagonal to throw the light cone out the side of the OTA about half way up. This is a perforation-less design - no hole in the primary.
My original optical design plan parameters for the 25" f/8.3 DK:
primary: 25"
primary F ratio: 5
back focus: -28
secondary amplification: 1.66
Full Illuminated Field size: 0.75"

As constructed (f/8.1):
primary: 25"
primary F ratio: 4.92
back focus: -28
secondary amplification: 1.644
Full Illuminated Field size: 0.75"

I initially used the ATMOS demo freeware available from Astro-Physics http://www.astro-phy...are/atmos/at... to explore the design and examine the aberrations.
It is a wonderful program. Thank you Astro-Physics!
Later I generated a Excel based spreadsheet to develop the final design.
I welcome cross checking from anyone based on the above parameters. Check the coma/spot diagrams with your favorite optical software please.
I massaged the design for about 4 years before finally settling on the current prescription. It is very tolerant of with respect to positioning of the optics.
It is also highly tolerant of variation in the production process. For instance, when it turned out that the primary would be a couple inches short (123" vs 125") due to the nature of the initial diamond generation of the blank, a change to the secondary design wasn't even warranted. The acceptable ROC of the secondary is of the nature of plus or minus 4 or 5 inches. I did make the spreadsheet changes to reflect the as built primary focal length and massaged the secondary magnification to get back to the same secondary ROC. This changed the required primary correction from 70.2 to 70.4% Negligible. We did, of course change the target correction during final figuring to 70.4%. I asked John Hall about the tolerance required for a really good mirror and he responded that +-1% of target would be a quite fine optic. He completed the mirror to scary good tolerance. We won't discuss that, because you wouldn't believe it anyway.

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#2 ed_turco

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Posted 26 January 2013 - 05:41 PM

The weaker the secondary, the larger it has to be.

Sorry.

#3 siriusandthepup

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Posted 26 January 2013 - 05:54 PM

Design objectives(actual numbers refer to my 25" DK - Dob project):
1) Easy and cheap to build
Primary is f/5, figured to approximately 70% of the full parabola correction. No perforation required. Any of the modern mirror vendors can do one easily.
Secondary is of 1.66x magnification. Spherical. In the range of f/11.5 test plate.
Tertiary is a standard diagonal, 3.5" in the case of my 25" scope.
Secondary holder requires no tilting adjustments.
All other components are standard Newt/Dob components.

2) Quick to setup
Uses Truss poles to hold Secondary ring.
No exotic pieces - sets up as a normal Newt Dob.

3) Reasonable optical aberrations
Spot diagrams very closely approximate those of a same primary size f/6 Newt.
Can you live with Newt aberrations of an f/6? I can. No coma corrector required for visual use. Photo requirements still not defined. I trust the community will assist with that analysis. It is beyond my expertise.

What about the Secondary Size/Obstruction?
33%, which is in the range of Celestron Schmidt Casses. I like their planetary performance a lot. A good rule of thumb:
Primary diameter - Secondary Diameter = Unobstructed Contrast equivalent.
Let's see, 25" - 8" = 17". I can live with 17" unobstructed Contrast.
Optical Quality Rules - it is MUCH more important than secondary size.
Secondary obstruction is a factor in this design. Focusing on minimizing it unduly would lead to giving up something else more important. Believe me when I say that it was not ignored. Thanks, John Hall (Pegasus Optics) for keeping me in line here and helping me with optimizing the final version of this design. John talked me down from the 10" secondary. Changes were made to accomodate an 8" secondary = a real improvement.

4) Works with normal eyepiece set - It's an F/8.3
Normal? Let's say 4mm to 40mm range without barlow.
Exit eye pupil - 4mm = 0.5mm, 40mm = 4.8mm, 55mm = 6.6mm
Final system f ratio = 8.3, eyepiece heaven - no exotics required.

5) I want to see the Moon
Designed for a 1/2 degree field. The field size shrinks as the aperture increases. No magic here. But it is nice to achieve comparable field size to Newts of similar aperture.
Designed to fully illuminate a focal plane field size of 0.75"

6) Scalable aka "Delusions of Grandeur"
Originally I was designing it to be a 48 or 50". This is a very linear scaling design.
Yes, you can design a stand on the ground 48" DK Dob - view right thru the center of the side bearing. (I have a 7" tertiary diagonal if someone would really like to go there.)
Time/money/reality equation and we end up with the 25" prototype. But hey, it is the prototype for the 48".
I'd say the lower practical limit for this whole idea is about 18" to 20". Anything that size or less is probably better off as a standard Newt Dob with a couple steps on a short ladder if needed.

7) Will this work for other primary F ratios?
MMMM - yes, but.....as always, there will be trade-offs. An f/5 primary really seems to be the sweet spot for this variant.
I have a design spreadsheet I will share with anyone who wants to play around with it. PM me with your email address and I will send you a copy - Excel compatible.

8) A scope I can live with
Positional tolerances and spacings are very forgiving with this design because of the weak primary and secondary curvatures.
A better than average mirror cell is planned because I do not want the primary wandering as much as it would with a sling cell. http://www.jpastrocraft.com/ - Thanks, John Pratte.
Tilt adjustments are not necessary on the secondary (it's spherical). Just make sure the secondary is installed pretty square to the upper ring. Collimation should be trivial - laser the center of the tertiary diagonal, adjust the tertiary to return the beam (after bouncing off the secondary) to the center of the laser. Now adjust the primary collimation knobs on a star to give a nice concentric pattern. Done!
Note: This relative insensitivity DOES NOT apply to a Classical Cass - whole other story.

#4 siriusandthepup

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Posted 26 January 2013 - 06:03 PM

Ed, you are absolutely correct about the secondary size.

This whole process has been about trade-offs and compromises. I originally had designed with a 10" secondary. John Hall talked me down off the ledge and I ended up with an 8" secondary. Better. I could get a smaller secondary, but at a cost of higher secondary magnification with attendant stronger aberrations and less wide design and positioning tolerances.

It's all about the trade-offs. ;)

#5 siriusandthepup

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Posted 26 January 2013 - 06:32 PM

25" DK construction status:

Primary: Figured by John Hall http://www.pegasusoptics.com/
96% Enhanced Coatings by OMI http://www.opticalmechanics.com/
Sitting in the box. :)

Secondary: not received yet - in the capable hands of master optician Robert Royce http://www.rfroyce.com/

Tertiary: 3.5" 98% C2 coating from Galaxy Optics. Thanks John Hudek. http://www.galaxyoptics.com/index.html

Primary mirror cell: completed this week, waiting on shipment. Thanks John Pratte http://www.jpastrocraft.com/

Dob building: scheduled for the 1st week of March in western Kentucky with my buddy and master woodworker - Ross Workman. Photos will follow initial construction for your viewing pleasure.

#6 Ajohn

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Posted 26 January 2013 - 06:41 PM

I've been playing around with this and I think the Takahashi scope sums it up. For general purpose use the magnification has to be lowish. Some reckon in the region of 3 to 3.5 but I believe the tak uses 4. The reason for the low mag is the fact that the 2ndry just magnifies the coma more and more which is pretty bad anyway. As is often said they are primarily planet scopes. Personally I suspect that a very low mag is a sensible option for a scope of the size mentioned for general purpose use. I would be interesting to see what the obstruction does to contrast in relation to an un obscured scope.I did see a graph for the effects of a 30 odd % obstruction. What it boils down too is little difference at the limit of the scopes resolution but around the performance of a 5in apo or worse when compared against an 8in SCT to around 2/3 of the way along the mtf curve. Go to a 10in sct and it wins all the way down the curve. This is on axis contrast of course. Things aren't too good off axis with any simple arrangement, newton, cass or dk.

I've yet to find any free ware that accounts for central obstructions. Texereau makes a point about large ones. There is so much light in the 2nd ring of the diffraction pattern it's this that dances around due to the atmosphere so it must have a bad effect on resolution in practice. Pass. The same atmosphere problems have been used to justify the tak scope - the atmosphere has more effect than the design.

John
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#7 The bear

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Posted 26 January 2013 - 07:16 PM

can you draw us out a little diagram of how you did this and plan to do the mounts? or maybe you can send me some drafts so i can get some ideas for mine that would be great.
doc

#8 siriusandthepup

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Posted 26 January 2013 - 07:27 PM

Yes Doc,
Will do - give me a few days. I will probably have to pencil sketch it and then scan it.

The mechanical design was not easy at all. I went through many redesigns until I arrived at a reasonable solution.

When you see it now though, I am guessing you will think its simple... :lol: It may be, but getting there wasn't.

#9 siriusandthepup

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Posted 26 January 2013 - 08:41 PM

OK Doc, lets see if I've got this shrunk down enough. the squares are 4".

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#10 MKV

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Posted 26 January 2013 - 11:38 PM

Ed, here is your design in OSLO. As predicted, large coma.

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#11 GlennLeDrew

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Posted 26 January 2013 - 11:50 PM

And that's only a 0.2 degree field. A 2" eyepiece would provide up to a 0.56 degree field. Might a coma corrector be profitably employed, even though designed for a paraboloid of faster aperture? Or would it 'overcorrect'?

#12 dave brock

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Posted 27 January 2013 - 01:23 AM

And that's only a 0.2 degree field. A 2" eyepiece would provide up to a 0.56 degree field. Might a coma corrector be profitably employed, even though designed for a paraboloid of faster aperture? Or would it 'overcorrect'?



The 16" DK that I refigured
here works surprisingly well with an "off the shelf" coma corrector.

Dave

#13 siriusandthepup

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Posted 27 January 2013 - 02:44 AM

Thanks MKV,

Could you do me a big favor and run the same thing through OSLO with the same field parameters for an 25" F/6 Newt?

A sanity check for me please. Thanks very much.

#14 Ajohn

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Posted 27 January 2013 - 07:59 AM

The coma correctors are aimed at F4.5 scopes usually but I did come across some one who was using a commercial one on a much faster scope for an astrograph so I suspect more coma can be corrected by varying it's position,

Looking at this I think the coma is a little worse than an F6 Newtonian.It's not as good as an F8 newtonian though. 1 Posted an F15 pure cass spot diagram in the oslo problem thread. The DK is useless for wider fields at that F ratio with a fast primary.

John
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#15 Ajohn

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Posted 27 January 2013 - 08:20 AM

This image shows the effect that the central obstruction has on the diffraction spot and MTF. It's normalised so applies to any scope. 1 on the bottom axis is the resolution presented to the scope that will results in no contrast at all. The Rayleigh limit gives about 7 1/2% contrast.

http://www.telescope...bstruction0.PNG

You can see clearly why so many scope have a central obstruction of around 30% and it's generally accepted that they shouldn't be any bigger.

John
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#16 MKV

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Posted 27 January 2013 - 08:23 AM

Sure, Ed, here is a 25-inch f/6. The physical FOV is 0.75 inches, the same as in your 25-inch f/8 DK Dob, but do realize that the angular field is about 36% larger in the f/6, namely 0.27° vs 0.19° in the DK Dob. So, I've included two sets of spots. On the left are the spots for the same field coverage (0.75 inches), and the one on the right is for the same angular coverage (0.19° degrees). Hope this helps.

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#17 kfrederick

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Posted 27 January 2013 - 09:00 AM

Here is a 24 inch chief with the eyepiece 38 inches above the primary . I have the primary finished . Ed I like your design :jump:.Should be very nice .Better than a very fast newt I would think .Blocking stray light might not be easy .

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#18 MKV

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Posted 27 January 2013 - 09:24 AM

Although this image is not to the same scale as the one for your dob, you can use the scale bar representing 0.004 in. to measure coma on both. From this, one can see that at the edge of a 0.75 inch FOV, the DK Dob has a total coma of 0.002 inches (of which 1/3, or sagittal portion of 0.0075 inches will be visible). By contrast, the f/6 over the same 0.75 inch field shows a total coma of 0.00137 inches (0.0006 in. sagittal), and for the same angular covera a coma of 0.00133 in. (0.0004 in sagittal). So, your DK Dob's coma is definitely worse than f/6 Newotnian.

Keep in mind that these are geometric optics and that the real image will include the diffraction effects as well. Nonetheless, the relative performance of the two can be assessed this way.

Give the extent and the nature of coma (being the same overcorrected type as in Newotnians) I don't see why a Newotnian coma corrector wouldn't be beneficial.

On the other hand, your central obstruction alone is very close to a 1/4 wave wavefront error, so I am not sure what this DK sytem would be good for. It's not really ideal for photography, or high power pleneteray observation or imaging due to diminished ocntrast.

If you look at the MTFs with central obstructions included, it's obvious that the DK Dob will perform ever so slightly worse than an f/6 Newtonian, but then some will say that an f/6 Newotnian is more than good enough! So, you're no worse off than f/6 Newotnian.

In fact, your configuration is much better in terms of its physical size. In this respect, your DK Dob will have a conveniently located focus and will be just over 7 feet long, whereas a Newt of the same size will have the focus 11 feet off the ground (!). So, given their nearly equal performance, I think your DK dob is a better solution than a Newt. Also, it won't require a heavy mount needed for traditional Cassegrain configurations. Good job, Ed! :)

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#19 siriusandthepup

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Posted 27 January 2013 - 10:24 AM

Thanks everyone for running the spots. Wow!

First order of business - I must apologize to Ed Turco.

Ed, I apologize for saying you were wrong about the coma. You were correct. The coma is there and just because it doesn't bother me visually doesn't mean it doesn't exist.

I am learning some new things here. I'm thinking that for photo applications it appears that even an f/6 Newt could use a little help with coma correction.

Thanks Ed.

#20 Mike I. Jones

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Posted 27 January 2013 - 10:49 AM

Ed, thank you for the courtesy of your public apology here to Ed Turco. I wish all skirmishes here could be resolved with similar grace.
Mike

#21 siriusandthepup

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Posted 27 January 2013 - 10:50 AM

MKV,

I am not a big optical design person. I am very single track in pursuing an interest and here is my thought process on the DK. Ed T. was right - the coma is there.

I'm primarily a visual observer in my 50+ years in this hobby. My gold standard has always been the f/6 Newt (usually a 12.5"). I am happy as a clam observing with an f/6 focal ratio and am quite satisfied with the view.

If I can achieve that level of correction in the DK then I will be more than satisfied viewing with it.

About the secondary obstruction - Is the 25 as good as a 25" APO? No. As good as a 25" Newt? No. OK then, How good? 25 - 8 = 17" for MTF/contrast. Should seeing ever approach doing it, I would be thrilled with 17" unobstructed performance. The Celestron C14 has just as large a central obstruction and it is one of the best planetary scopes on the planet. Is it as good as a 14" Newt? No. As good as a 14" APO? No. But it is wonderful. As good as an 8" APO? Owners say Yes!

My thinking goes along these lines. The 25" will have potential planetary performance beyond the 12.5" and 16" scopes that I own now. It will have deep sky performance approximating a 24" Newt. My feet won't need to leave the ground with the focuser zenith height of 62".

#22 siriusandthepup

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Posted 27 January 2013 - 11:06 AM

Kfred,

You are on the money about the stray light / baffling situation. It's a challenge.

I bought a 3 foot wide roll of butcher paper and rolled out about 10 feet of it on the floor. Nothing like a full size diagram for analyzing the baffling. I recommend this to anyone designing baffling. Make a full size drawing of your optical system and then you can see and measure directly what you can do for baffles without vignetting your system. Its fun to do too. :)

Kfred - I am jealous of all the people who get to view through your Chief at WSP. I want an observing report from them all. :bow:

#23 siriusandthepup

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Posted 27 January 2013 - 11:16 AM

John,

Thanks for your inputs. I do have a coma corrector - TV type one - the older one optimized for f/4.5. I am going to try it on my 12.5" f/6 Dob. I am curious about it now - I've never even thought about trying it on the f/6.

Also, when the DK is completed I will try the coma corrector for visual use. And I will report the results back here to add to the knowledge base.

thanks and have fun at WSP.

#24 Ajohn

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Posted 27 January 2013 - 11:19 AM

Lot's lots of people use F4.5 newts visually and photographically without a coma corrector. Even with 2in eyepieces. For photography though the coma corrector must improve things. In fact where people have lashed out for super wide 2in rather expensive eyepieces for these I have always tried to point out that a coma corrector is probably a much better investment.

Not much of a problem for me as I stick to smaller scopes. I used an F4.2 11in dob for a while but wasn't too keen on it and never tried a coma corrector.

John
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#25 MKV

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Posted 27 January 2013 - 12:55 PM

Ed, I have a 12.5 inch f/5.6 Newtonian and I never used it with a coma corrector. Do I see coma? It depends. Visually, we concentrate on the center of the field, andunless collimaiton is off, there is no perceptible coma there.

The central obstruction simply means that you'd see less then you would a smaller one or without one. Will it be perceptible? Probaly not unless you can directly compare twos copes next to eahc other.

Your much bigger problem might be cooling time and atmospheric turbulence, which will be very a significant factor in a 25". In fact, I seriously doubt you'll ever see an Airy disk.

By the way a C-8 has a 38% obstruction. C-14 is not the best telescope but it has the aperture which is the final arbiter in resolution and the amount of light that can be seen.

Your solution is optically sufficient for what it will be used, but I your idea of a DK in a dob configuration is superb in my opinion. So, forge on, and we will all anxiously expect to read more about it and see pictures of your project. Good luck!






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