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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
wsDK - weak secondary Dall Kirkham telescope
      #5644789 - 01/26/13 05:32 PM Attachment (67 downloads)

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-physics.com/index.htm?products/accessories/software/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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ed_turco
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Reged: 08/29/09

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5644799 - 01/26/13 05:41 PM

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

Sorry.


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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5644816 - 01/26/13 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.


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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: ed_turco]
      #5644832 - 01/26/13 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.


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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5644870 - 01/26/13 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.


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Ajohn
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5644888 - 01/26/13 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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The bear
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Loc: rushville, indiana
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ajohn]
      #5644944 - 01/26/13 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


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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: The bear]
      #5644964 - 01/26/13 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... It may be, but getting there wasn't.


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siriusandthepup
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Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645098 - 01/26/13 08:41 PM Attachment (58 downloads)

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

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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645290 - 01/26/13 11:38 PM Attachment (42 downloads)

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

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5645300 - 01/26/13 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'?

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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: GlennLeDrew]
      #5645358 - 01/27/13 01:23 AM

Quote:

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


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siriusandthepup
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Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5645396 - 01/27/13 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.


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Ajohn
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Reged: 12/03/07

Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645579 - 01/27/13 07:59 AM Attachment (27 downloads)

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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Ajohn
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ajohn]
      #5645595 - 01/27/13 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-optics.net/images/central_obstruction0.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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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645599 - 01/27/13 08:23 AM Attachment (36 downloads)

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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kfrederick
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5645638 - 01/27/13 09:00 AM Attachment (35 downloads)

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

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MKV
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5645663 - 01/27/13 09:24 AM Attachment (26 downloads)

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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siriusandthepup
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5645754 - 01/27/13 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.


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Mike I. Jones
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645800 - 01/27/13 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


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siriusandthepup
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Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5645801 - 01/27/13 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".


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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: kfrederick]
      #5645830 - 01/27/13 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.


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siriusandthepup
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Reged: 02/14/06

Loc: Central Texas, USA
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ajohn]
      #5645849 - 01/27/13 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.


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Ajohn
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5645856 - 01/27/13 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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MKV
Carpal Tunnel
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5646016 - 01/27/13 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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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5646395 - 01/27/13 04:37 PM Attachment (24 downloads)

Quote:

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.





When I worked on the 16"DK (f/6.4 with a 40% obstruction btw) I was told by a professional optician that it would need a specially made corrector to suit that particular scope and that an off the shelf corrector would not work.
The owner of the scope uses it visually as well as for imaging with a Baader MPCC.
Here's part of an e-mail he recently sent:-
"Got a great view of Jupiter 2 nights ago. Io was approaching and I saw the moon and its shadow transit the planet and following the shadow was the GRS in pink with a ring around it.
Compared to the refractor the 16in was superior. The globe sizes are similar using 5mm Nagler in the refractor and 15mm Superview in the cass."
I also include a pic of M22 taken through it. I don't think coma is the biggest issue.
It may be poorly designed and less than 1/4 wave because of the large central obtsruction but it is definitely usable.
Your 25" should be better still.

Dave


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The bear
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5646492 - 01/27/13 05:26 PM

so cool thanks for the drawing i have plans within plans so to say.
doc


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The bear
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5646494 - 01/27/13 05:27 PM

on the secondary the 3.5 one is there any particular wave front needed like say 1/10 or better?
doc


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siriusandthepup
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5646587 - 01/27/13 06:12 PM

Thanks for sharing that photo Dave - beautiful! Do you happen to recall the primary f ratio on that 16" f/6.4?

Do you know which refractor he was referring to bye the way?


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siriusandthepup
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: The bear]
      #5646642 - 01/27/13 06:34 PM

Doc,

Considerations for your optics:
Pick the best wavefront quality with the best coating, especially when you are in big project and nervous anyway. You sure don't want to have to wonder if the diagonal is impacting the wavefront if your project is something new and a bit out of the ordinary.

The 25" primary has 96% coatings from OMI.
The secondary will have enhanced coatings from Royce.
The tertiary is a Galaxy Optics 3.5" diagonal with their 97% C2 coating. John Hudek selected the best one out of his current stock for me and its rating are very good - no worries there. I'm sure he would do the same for you.

With three reflections (and a big secondary obstruction ) in the DK I was concerned about keeping my overall reflectance high so that deep sky would not suffer.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5646801 - 01/27/13 07:51 PM Attachment (33 downloads)

Hi Ed.
The focal length is 1230 so f. ratio is 3.075.
I didn't ask about the refractor but he previously has mentioned a 100mm ED. Not a fair comparason, I know.
It's probably the one in this pic.

Dave
Edit; there have been a few mods to the main scope since this pic was taken.

Edited by dave brock (01/27/13 07:55 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5646914 - 01/27/13 08:49 PM

Dave,

That 16 is sure a compact beauty! It has the look of serious business. Thanks for the pic.

F/6.4 system, primary f/3.075 - that makes the secondary 2.08x magnification. It certainly seems to do a fine job on the photography. And, from your friend's description, a nice job on planetary observing. Operating with just a Baader MPCC. A lot of folks would love a scope like that.

Thanks again.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5648250 - 01/28/13 01:53 PM

Apology most certainly accepted; it was very gracious of you.


Ed


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5648971 - 01/28/13 06:53 PM Attachment (24 downloads)

With all due respect the picture of M22 does not reflect what theory says it should look like. M22 subtends 17.3 arcmin and the whole field is about 2 degrees across. The coma expected at the edge of the field would clearly be visible.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5648981 - 01/28/13 06:59 PM

Could the Baader coma corrector be making that much difference?

That pic sure looks good...


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5649422 - 01/28/13 10:48 PM Attachment (19 downloads)

Well, yeah, now that you mention the coma corrector. I missed that part. A dedicated two-element corrector can easily correct a DK Cassegrain. Then it's called a DKC (corrected Dall Kirkham). A commercial coma corrector will substantially reduce coma, but won't eliminate it. You can see it if you enlarge the photograph's upper left corner. For some reason the same effect is not visible in the opposite corner.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5649443 - 01/28/13 11:03 PM Attachment (27 downloads)

Here is an example of a 14.5" f/6 DKC. I forgot who published it here on CN. As you can see all you need is a 2-element corrector which, if dedicated to the system will do quite well in removing coma. But the field is half of what the photograph shows. For an f/6 it's difficult to get pin point star images all the way the edge over 2 degrees. Considering that, the Badder corrector has done an excellent job.

Edited by MKV (01/28/13 11:07 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5649550 - 01/29/13 12:14 AM

Quote:

A commercial coma corrector will substantially reduce coma, but won't eliminate it. You can see it if you enlarge the photograph's upper left corner. For some reason the same effect is not visible in the opposite corner.




The reason it's not visible in the opposite corner is because it wasn't at best collimation, which if it was, would have improved the coma you showed in the top left.
Since the photo was taken the scope has been modified to reduce the distance between the secondary mirror and the spider which was giving trouble.
The point I'm trying to make is that a scope can sometimes give reasonable results even though it's not made to a professional standard. For example, on a typical night is a paracorr's performance change noticable if it's used at the wrong setting of the tunable top for a particular eyepiece?

Dave


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5649629 - 01/29/13 01:59 AM

Well, its not an RC and I'm no Photo Expert, but color me impressed with what can be achieved with a 16" f/6 range DK and a simple commercially available coma corrector.

MKV, those spots you present for the 14.5" f/6 DKC are very tight too.

If I were into the photography aspect of the hobby, either of those scopes would put a smile on my face.

Edited by siriusandthepup (01/29/13 02:20 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5650067 - 01/29/13 11:09 AM

Dave, you're absolutely right. As I said, I missed the corrector part in my initial comment, which is why I was a little puzzled by the quality. No doubt, it makes a hopelessly unfit telescope into a decent photographic instrument.

It would be nice if you took a picture of M22 without the corrector to show the folks here what an f/6 DK image would like like! Only then will they really appreciate the magic you obtained with the Baader corrector.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5650126 - 01/29/13 11:43 AM Attachment (31 downloads)

Ed, the correction of the 14-inch f/6.5 is indeed good but over a limited field of only 0.5 degree off axis (1 degree FOV). So, Dave got a much better result using his Baader commercial corrector. I am sure it has to do with different glass types. The glass use dis ordinary BK7, so a more exotic glass choice would do better. Also, you could achieve a much flatter field, which is the main reason the example I gave is limited to 0.5 degrees off axis.

Most commercial DKC are sold at about f/8, similar to RCs, and in fact are used as competitors. Much better correction is possible if you place the corrector closer to the secondary, which of course requires very accurately machined and assembled components to maintain perfect collimation.

The results are stunning, but still over a limited image field, which is why the correction with the Baader seems so unreal, especially considering that it's an off-the-shelf, "generic" corrector. Here is an example of an 9-inch f/8 DKC, with a dedicated corrector. Note the field is still only 1 degree (30 mm) wide.

Which again brings me to the the image of M22. The cluster is some 17 arcminutes wide, or or about 0.28 degrees. If you measure its core, it's easy to see that the whole picture subtends about 2 degrees of the sky. Two degrees at 96 inches of focal length (almost 2.5 meters) would be 85 mm across (3.3 inches). That wouldn't even fit on a 60X60 large camera format film, let alone CMOS/CCD processors which for most SLRs are barely 15 by 24 mm frames and 50x50 mm processors are prohibitively expensive. So, unless I am mistaken about the M22 size, and I have checked, it is truly amazing that this picture was taken with a 16 inch f/6 scope, even if the totla image field covers 1 degree in all (which would require a 50 mm CCD processor. That's hardly "amateur" stuff.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5650174 - 01/29/13 12:10 PM Attachment (27 downloads)

Mladen, the design layout you show for the Baader does not resemble the design I have that was posted here years ago by someone I don't recall. I still don't know if that one is correct either, but here is what I get without/with the Baader design I have for the 16" f/4.6 DK shown in the CassDesign output you posted. There just isn't that much difference in the coma with or without it, out to the limiting FOV that passes through the Baader without vignetting.

We should compare Baader designs. There's still a chance that neither of ours are right.
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5650195 - 01/29/13 12:21 PM Attachment (22 downloads)

These are the spots I get without (left side) and with (right side) the Baader corrector (design I have) in the path of this 16" DK. Better, but not by much.

If you use the Baader design I have with a 16" f/6 paraboloid, it does beautifully. But it's even better if you allow the primary conic constant to optimize to the value of -1.09338.

Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5650602 - 01/29/13 03:36 PM

Mike, the correctors I show are not the Baader correctors. I beieve the fhe first configuration came from the CN. I thought that was yours design. The 9.5 inch is something I designed based on similar confiugurtions, but by no means my own invention.

At any rate, your analysis only confirms that the correction seen on the image of M22 is better than either of our examples.

It is also puzzling to me that a 16 inch f/6 would cover such a wide and flat field (> 1), which would require a CCD of at least 50X50 mm, if not larger.

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5650793 - 01/29/13 05:30 PM Attachment (27 downloads)

Here is one option. This is a 25 inch F/8.1 corrected DK with the first element from Linos Photonics. You would need to make the meniscus second lens.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5651033 - 01/29/13 07:49 PM

The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ed Jones]
      #5651082 - 01/29/13 08:16 PM

Ed, your coma corrector design is sweet! It would sure allow some nice CCD shots with the 25. I do intend to add a Servocat just as soon as the basic scope is sorted out. That would allow some short exposures and stacking. I am not a hardcore imager, so short exposures would provide me with a lot of entertainment.

PM headed your way shortly.

thanks!


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5651090 - 01/29/13 08:21 PM

Mike, do you regularly use your coma corrector with your 16" f/6 for visual use?

Just for low mags or most all the time?

Thanks for your inputs - very interesting discussion.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5651314 - 01/29/13 10:42 PM Attachment (23 downloads)

Quote:

So, Dave got a much better result using his Baader commercial corrector..... Which again brings me to the the image of M22. The cluster is some 17 arcminutes wide, or or about 0.28 degrees. If you measure its core, it's easy to see that the whole picture subtends about 2 degrees of the sky. Two degrees at 96 inches of focal length (almost 2.5 meters) would be 85 mm across (3.3 inches). That wouldn't even fit on a 60X60 large camera format film, let alone CMOS/CCD processors which for most SLRs are barely 15 by 24 mm frames and 50x50 mm processors are prohibitively expensive. So, unless I am mistaken about the M22 size, and I have checked, it is truly amazing that this picture was taken with a 16 inch f/6 scope, even if the totla image field covers 1 degree in all (which would require a 50 mm CCD processor. That's hardly "amateur" stuff.




Hi Mladen.
First of all, the scope and corrector do not belong to me and I didn't take the image. The owner did.
I have used Skymap Pro to identify some stars in the image to determine the size of the field which I make as about 23 arcmin on the long axis.
Here is another image. This time NGC253 which was posted in the other thread I linked to. It may be a better one to show the field size. According to Skymap Pro NGC253 is 26.4' long.

Dave


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5651466 - 01/30/13 12:34 AM

26.4' long - almost a full moon... Another nice photo BTW.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5651479 - 01/30/13 12:44 AM

Photo's full credit Glenn Burgess, scope owner.(possibly watching here but may not be registered to post)

Dave


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: dave brock]
      #5652153 - 01/30/13 11:53 AM Attachment (13 downloads)

Hi Dave, I was wrong in assuming the viisble diameter of M22 was its angular diameter. Apologies. Apparently the true size of M22 is about 3 times its visible size. This can be easily ascertained from this site. The visible cluster seems to subtend only about 7.5 arminutes. The reference star shown below is 10 arcmin from the visible center, which makes Glenn's entire frame 0.5 degrees across.

This makes the correcton seen (absence of coma) a lot more reasonable with even commercial coma corrector, and not so surprising after all for such a narrow field. But the picture is a telling example of what simplified Cassgerain (DK) optics together with commercial coma correctors can do for imaging narrow angular field, deep sky object.
Regards,
Mladen

Edited by MKV (01/30/13 12:04 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5652198 - 01/30/13 12:15 PM Attachment (19 downloads)

Using the new field 0.5 degree size, here is again (what I beileve is) Mike Jones' version of an f/6.5 DKC, with images shown 0.25 degrees off axis. Diffraciton-limited to the edge.

Edited by MKV (01/30/13 12:16 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5656802 - 02/01/13 05:16 PM

Mladen,
well made point on the image scale anamoly.


By the way, I've never come across any precision optics which didn't have provision for adjustment. I agree sensitivity to collimation can be an issue, but more often it is only really a major problem when the resolution of the means of adjustment is too crude or sloppy to cope with the precision required - for example cheap binos and blobs of superglue on the corners of prisms.
It's when the precision required of a lens in its' cell exceeds the clearance needed for thermal changes that one really runs into trouble, ie less than a gnats wing (a few microns). Of course seeing will spread all spots into jiggling blobs.

Mark


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5656836 - 02/01/13 05:40 PM

Hello Ed,
I looked up this thread because of another reference to it - reading from the beginning I thought this scope was up and running and then came across your status information. The secondary can really only be finished/figured in combination with the primary - at the correct separation/focal distances. It is normal to do it this way by any optician - which of your two opticians will take the blame if the SA is not correct!?
25" is a high-end piece of no doubt expensive optics and that figure on the primary needs to be integrated into an overall corrected system.

Mark


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: mark1234]
      #5657091 - 02/01/13 08:30 PM

Mark,
Quote:

The secondary can really only be finished/figured in combination with the primary - at the correct separation/focal distances.




No in this design the secondary is spherical and can be checked against a test plate which can be tested with a KE. So the remaining problem is getting the airspaces and lenses right. It can't work without the lenses in place anyway. This isn't an easy scope to make at least for a beginner.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: mark1234]
      #5657125 - 02/01/13 09:02 PM

Mark, the giggling blobs are much more likely in a 25" then smaller scopes. In fact, I seriously doubt you'll ever see an Airy disc, but wou can always use a 7.5 inch mask and have an f/20 unobstructed ("off axis") Cassegrain.

Up to probably 16 inches, you can see good Airy disks on good nights. After than it becomes progressively more difficult due to speckling.

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: mark1234]
      #5657153 - 02/01/13 09:14 PM

Quote:

The secondary can really only be finished/figured in combination with the primary - at the correct separation/focal distances. It is normal to do it this way by any optician - which of your two opticians will take the blame if the SA is not correct!?
25" is a high-end piece of no doubt expensive optics and that figure on the primary needs to be integrated into an overall corrected system.

Mark



Hey Mark,
You are correct about the normal way to make a Cass. One optician carefully matches the primary and secondary. Why? Because the tolerances are so tight.

I purposely designed this scope for ease of manufacture. It is one of the prime features of the wsDK. I've run tolerancing studies to convince myself that it is ok. I did have to convince Mr. Royce that it was ok too.

The secondary has a design test plate fl of 91.7". For an 8" that's an f/11.46. So - how close does the focal length need to be to prevent an impact on the SA? Plus or minus 5". I have full faith that Mr. Royce can come closer than +-5 inches.

Who get the blame if the SA is unacceptable? That's an easy question to answer - ME.

Edited by siriusandthepup (02/01/13 09:20 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5657188 - 02/01/13 09:42 PM

Quote:

So - how close does the focal length need to be to prevent an impact on the SA? Plus or minus 5". I have full faith that Mr. Royce can come closer than +-5 inches.



That's 5.5% on the radius! Seriousyl, that's not a tight tolerance at all. When you make compound optics, tolerances are often a fraction of a percent. I don't think you have to worry...

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5657275 - 02/01/13 10:28 PM Attachment (15 downloads)

Mladen,

In the interest of full disclosure, the tolerances are actually more generous than I have stated. I was just being conservative with the +-5 inches.

Here is the tolerance from my spreadsheets. Note that the primary, which is figured at 70.4%, is expected to be of good overall system figure at plus or minus 1% tolerance for this application.

Note the change in required primary correction over the secondary range. From 81 to 100 fl on the test plate will yield an acceptable match with the primary's existing correction.

Edited by siriusandthepup (02/01/13 10:57 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5657307 - 02/01/13 10:44 PM

I guess the moral of this story is to do your primary first on a wsDK and measure its correction carefully. Then you have tons of latitude with making your secondary.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5657763 - 02/02/13 09:26 AM

Quote:

In the interest of full disclosure, the tolerances are actually more generous than I have stated. I was just being conservative with the +-5 inches.



Ok, thanks for clarifying this. But since we're on the design again, I looked at your original chart and noticed that you list your primary as having a positive curvature. Could you clarify that?'

Using standard optical annotation convention, both mirrors will have the curvatures of the same sign.

Mladen

Edited by MKV (02/02/13 01:10 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5657826 - 02/02/13 10:03 AM

Quote:

I guess the moral of this story is to do your primary first on a wsDK and measure its correction carefully. Then you have tons of latitude with making your secondary.



Ed the way it's done is:

1. make your primary as close as possible to the required figure
2. make your secondary spherical
3. assemble the telescope
4. test it at the focus (preferably by autocllimation rather than stars)
5. touch up the secondary until you have a perfect null

Hint 1: make your primary very, very accurately to the desired radius of curvature. Readjust your design parameters to match the final radius of curvature, then -- if you can -- perform a knife-edge test at the conjugate foci until you get a clean null. At that point you have the exact figure on your mirror. Otherwise get a good Foucault tester and try to get as close to the figure as possible. BW, conjugate foci testing requires lots of windless space! It's a nice theoretical method for small mirrors but not for anything over 10 inches in diameter. A 12-inch f/4 may require as much as 30 feet!

Hint 2: proceed to make the secondary to match the new adjusted figures based on the primary. Polish it and figure it spherical using a test plate. At this point the two optics should be close, which is why you still need to "tweak" the primary until you have a perfect null at the focus of the complete scope.

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5657914 - 02/02/13 11:04 AM

That's my lack of optical design experience showing. I could have easily flopped the sign. In my way of thinking (wrong BTW) a concave mirror (primary) has a positive sign and a concave mirror (secondary) a negative sign. That is reflected in my spreadsheet. I understand now that in the conventions used in the optical design software the direction of the curves is the deciding factor and since the curves of both the primary and the secondary are in the same direction left to right then the signs should be the same. Please correct my rookie mistakes and I will try to learn and readjust my thinking.

Also (again my lack of experience) the way I thought about primary mirror production is referencing a parabola at conic -1. In using the Foucalt test I was thinking that it was useful to generate target numbers reference the parabola full correction numbers. Hence 70.4% of the full parabola reference numbers for the DK ellipse correction targets.

I am listening to your comments with the intent to learn the proper way to do optical design. At least that way I can learn and be on the same page with the rest of the optical design community.

Thanks for all your help!


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5657919 - 02/02/13 11:07 AM

MAJOR correction to replied post, lest mirrors be made incorrectly!

Directly per Malacara, "Optical Shop Testing", Foucault/caustic test figuring strength is the negative of the conic constant!

Figuring strength is NOT eccentricity!

If the DK formulas calculate a primary eccentricity e=0.83904708, the conic constant is -e^2 = -0.704. Tested at COC with Foucault or caustic methods, the mirror figuring strength is 0.704, or 70.4% the correction an equivalent paraboloid would require.

The mirror correction is NOT 83.9%!!

All, please be careful when posting formulations here, and double-check with references and for accuracies and typos. A mistaken formula can mislead someone into spending a lot of time figuring a mirror incorrectly, only to find it out after expensive coatings have been applied, and during final assembly and initial star testing.

Thanks,
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5657966 - 02/02/13 11:33 AM

My primary is completed and coated at 70.4% of a parabola.

I sure hope that is right.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5658052 - 02/02/13 12:34 PM

Ok, thanks Mike. Apologies for mistaking the 70.4% meant to mean deviation from the sphere (which is what eccentricity, e, is). Just another reason to stick to conventional terminology i.e. conic constants, instead of %.

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5658083 - 02/02/13 12:49 PM

Sorry I misunderstood your terms, Ed. Your mirror has a conic constant = -0.704 and that is correct, spot on. Apparently this is what some call "correction". I just never thought of the conic constant as a simple percent of a parabola (because it's not).

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5658118 - 02/02/13 01:06 PM

Mladen,

It's just a sign of my old school upbringing. When I started this project I had no ATM friends that were into Optical Design programs. I used ATMOS demo for my initial explorations and then generated my DK design spreadsheet based on the DK equations I found in a book. Again, no training or experience on my part. I checked my spreadsheet outputs against ATMOS results and all seemed well, so I proceeded from that point. It certainly would have been nice to have the benefit of Cloudy Nights "group think" and experience back then.

I evolved this project in a virtual design knowledge and experience vacuum, hence my slow pickup on the proper terminology and conventions.

I am enjoying the interactions with folks who do know what they are doing in optical design. It's a good learning experience for me, just please be patient with my slow pickup. I am persistent and I will eventually come up to speed. I do appreciate all the criticisms, questions and suggestions from everyone. I know that they are honest attempts to help.

Thanks All!

Edited by siriusandthepup (02/02/13 01:19 PM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5658133 - 02/02/13 01:16 PM

Ed, thanks. You are very kind. It was an honest mistake. And thanks to Mike, again, for catching it in time. I altered the relevant post so as not to confuse anyone else.

Mladen.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5658607 - 02/02/13 05:53 PM

Ed, my very old, maybe 20 years old? Dall Kirkham 16cm F3.8/13.5 was done from equations in James Muirden's book from the 1970's. He pointed out that a slight oblating figuring would be needed on the spherical secondary after assembly and he was spot on. I now can ray-trace retrospectively and understand that this corrects the higher orders which the basic conic equations and a slide rule omitted. I can also confirm that the tolerances on radius and spacing are very wide, relatively speaking, for diffraction limited optics.
This small scope is still going strong on the original coatings. Tried to post the Registax png of some webcam moon images - png file too big but I think you can visualise a 6" f13.5 image at or near diffraction without chromatic abberation.

Mark


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: mark1234]
      #5658730 - 02/02/13 06:53 PM

Mark,

Do you remember the name of the Muirden book? I thought I'd read them all. I would love to know more about the touch ups not covered by the basic equations. If I remember correctly I got the equations from Stephen Tonkin's book "Amateur Telescope Making".

Also, do you have a pic of your telescope? I'm sure we'd all love to see it. Thanks!


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5659060 - 02/02/13 10:48 PM

Someone was talking regarding centering the optics.

With a DK in mind, I gave a critical examination of the secondary cell/focuser and made these conclusions.

Since there is around 1 1/2" range from the stalk the secondary rides on, that spacing is not especially critical as it would help give a good range of back focus for various things like cameras, spectrographs etc.

There is NO provision for CENTERING the secondary, only adjusting its tilt, as the secondary is a spherical surface, it has no unique center of revolution


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5659251 - 02/03/13 01:32 AM

Thanks for correcting the error, Mladen.

However, figuring strength actually can be expressed as relative to a paraboloid in percentage of correction. The negative of the conic constant >> IS << the figuring strength. Figuring strength directly and linearly gives the scale factor to be multiplied by theoretical Foucault r^2/R or r^2/2R knife-edge positions for non-paraboloidal mirrors.

An ellipsoidal primary for a Dall-Kirkham might have an eccentricity of 0.8390471, giving a conic constant of -(e^2) = -0.704 and thus a figuring strength of +0.704. This implies that if the primary were zonally tested at center of curvature, the zonal r^2/R or r^2/2R values giving theoretical KE shifts for an equivalent paraboloid would be scaled by 0.704, or 70.4% of the zonal shifts for the paraboloid. The mirror can accurately be said to be 70.4% corrected relative to a paraboloid.

Likewise, a hyperboloidal primary for a Ritchey-Chretien Cassegrain might have a conic constant of -1.076, and thus a figuring strength of +1.076. Zonal KE shifts for the equivalent paraboloid are all multiplied by 1.076, and the mirror can be zonally tested at COC in exactly the same fashion as the equivalent paraboloid, just with slightly stronger KE shifts. This RC primary could be said to be 107.6% corrected.

A Rosin hyperboloidal astrograph primary is an extreme example of figuring strength, as Rosin primaries often have conic constants of down to -1.5 and even -2.0 or less, depending on the overall system design.

A sphere is the other extreme of figuring strength. A sphere has zero eccentricity, thus both the conic constant and figuring strength are zero as well. Zonal knife edge positions for the equivalent paraboloid would all be multiplied by zero, and this is of course exactly what we know to be the case for testing a sphere.

The same principle can be applied when making the primary mirror for, say, a Wright-Schmidt telescope, which typically has an oblate spheroid figure with positive conic constant and negative figuring strength. Equivalent paraboloid zonal settings would then be multiplied by a negative figuring strength, implying that the center of the mirror should focus long relative to the edge, which is indeed the case for an oblate spheroid.

Hope that helps,
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: mark1234]
      #5659555 - 02/03/13 09:12 AM

Quote:

Dall Kirkham 16cm F3.8/13.5 was done from equations in James Muirden's book from the 1970's. He pointed out that a slight oblating figuring would be needed on the spherical secondary after assembly and he was spot on.



Mark, if you undercorrect the primary, then slight oblating of the secondary would indeed correct the figure. However, if the primary is overcorrected, just the opposite would be true, you'd be creating a slight ellipsoid on the secondary.

That may seem like an easy fix mathematically, but from the shop technique point of view it's much easier to make all the corrections on the primary and leave the spherical secondary alone.

Spherical secondary also makes DKs easier to collimate and maintain collimation, so why throw that advantage away by aspherzying it when all the correction can be on the already aspherized primary?

Mladen


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5659631 - 02/03/13 09:53 AM

Quote:

The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.
Mike




Telescope services do one Mike which they say will work down to F3 but it's rather expensive and long so would only be of use on an astrography with the camera on axis.

http://www.teleskop-express.de/shop/product_info.php/info/p5903_TS-Optics-2-5...

There is also a Bader to F3.5 as well as the other one they do
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p486_Ba...

There is a syntra / skywatcher one to F4 around on there as well. Long again which means huge central obstruction I suspect.

John
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ajohn]
      #5659898 - 02/03/13 12:30 PM Attachment (33 downloads)

I came across this corrected DK by Wynne. He refers to it as a wide field cassegrain, spherical 2ndry plus corrector with a parabolic main mirror. UV to IR using ubk7 plus a rather flat field. Nice even spot size too.

It's as per his prescription other than I have left it in mms rather than cms and have set the 2ndry dia for 0 field angle which oddly according to oslo doesn't cause any light loss across the field.

John
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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Ajohn]
      #5904857 - 06/05/13 10:22 PM

Any updates with this thread? I have been looking at this design with interest. My calculations with cassegrain calculators also show low coma for such a design. I have a 25" F/4.7 mirror that would make a good candidate for this DK system. Not using a ladder would be a HUGE plus.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: coinboy1]
      #5905519 - 06/06/13 09:14 AM Attachment (12 downloads)

Here's one possible design for your 25" f/4.7 primary. 0.38-0.9m spectral band, 1.4" circular FOV. The 2-element corrector is made from affordable glasses (no high-$$$ fluorocrown glasses).

Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5905521 - 06/06/13 09:15 AM Attachment (17 downloads)

Here's what it looks like, complete with baffles.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5905524 - 06/06/13 09:17 AM Attachment (25 downloads)

and here are the spots. The circle is the first Airy dark ring, so the images are extremely sharp.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5905566 - 06/06/13 09:35 AM Attachment (38 downloads)

Here's the OSLO-EDU file for this Cass, including the baffle tube apertures.
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: Mike I. Jones]
      #5907587 - 06/07/13 09:38 AM

Wow! Thanks mike!! What's the system f/ratio? I have OSLO but I'm not too great with it but I will defintley load the file into it. Thanks for looking into it

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: coinboy1]
      #5908306 - 06/07/13 04:28 PM

Nevermind I see that it is an F/14 system. I just loaded it into OSLO EDU and looking at the design now. Would it be possible to scale this down to an optimized F/8 with a corrector lens. I would be interested in that even if I have to make the mirror a different F/ratio and corrector lenses. I have not any experience making lenses but willing to try!

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: coinboy1]
      #5908341 - 06/07/13 04:45 PM

I think an f/8 is do-able, but the secondary obscuration with your f/4.7 primary would be enormous. If you can speed the primary up to f/4 or even f/3.6 or so, the obscuration would be reasonable. The secondary would still be spherical and the primary an ellipsoid. I'm away for now but will look at it when I get back.
Mike


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987165 - 07/24/13 12:12 AM Attachment (14 downloads)

Hi Gang,

Here is a little update on the 25" wsDK project. Ross Workman aka "The Planetman" and I have completed some woodwork for the upper ring (actually three 1/2" baltic birch rings now glued together).

The mirror box is also complete, my photos are too big and I must reload software to shrink them before I can post the mirror box construction pics. That will have to wait a while, but I do have some small pics to post so you can at least see it and get an idea of where this project is going.

First, the mirror box - 1/2" baltic birch, 60" tall. I was concerned for stiffness using 1/2", but also didn't want too much weight. So we built the box "model airplane style" with inner reinforcing baffles - also 1/2" BB. It turned out nice, approximate weight 80 lbs for just the box without hardware. It went together without a single glitch and it all fit perfectly and the box is totally square in all directions. How often does that happen?

Edited by siriusandthepup (07/24/13 01:28 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987168 - 07/24/13 12:14 AM Attachment (12 downloads)

And the inside pic.

Edited by siriusandthepup (07/24/13 12:15 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987178 - 07/24/13 12:22 AM

This one shows how the spider/tertiary mirror will be placed. Some mounting blocks are being fabricated to attach/tension the spider vane tips.

This pic will have to wait also - too big to post.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987192 - 07/24/13 12:32 AM Attachment (10 downloads)

So.... on to the upper ring construction. Most of these pics are smaller. First we cut out 34" x 34" squares from Baltic Birch (BB). Outer ring diameter 33.5". A sandwich construction with outer rings being 2" wide and the inner ring 3" wide. That brings the inner ring diameter to 27.5". A spiral solid Carbide 1/4" router bit was used and punched through the 1/2 BB nicely in about 3 rounds (one stop in the middle for vacuuming out the sawdust) without any bogging or burning - a new favorite bit for this job.

Edited by siriusandthepup (07/24/13 01:14 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987194 - 07/24/13 12:35 AM Attachment (11 downloads)

The inner cut - Remember sports fans - always cut the outer diameter first else you loose your center for your router circle cutter.

Edited by siriusandthepup (07/24/13 12:36 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987198 - 07/24/13 12:40 AM Attachment (10 downloads)

Doing this with a helper is highly recommended for a number of reasons. The trusty helper (Me) here is in charge of vacuuming and router cord management during the actual routing process.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987205 - 07/24/13 12:48 AM Attachment (10 downloads)

Notching the outer rings with a 1" Fostner bit to allow clearance for the vane tip screws. They will probably be changed to Allen heads in the final form. No fingers were harmed in the process. The pic does not show the ample clearance between digits and the bit.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987207 - 07/24/13 12:49 AM Attachment (10 downloads)

Hula Hoop anyone?

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987218 - 07/24/13 12:55 AM Attachment (10 downloads)

Applying glue to one of the outside rings. After spreading the glue evenly with a brush, the outer ring was inverted and placed on top of the inside ring. After carefully measuring the position with respect to the center ring the outside ring was secured in place with a few pins from the pin nailer. Rinse and repeat for the other side and you now have a sandwich ready for clamps.

Edited by siriusandthepup (07/24/13 12:55 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987221 - 07/24/13 12:57 AM Attachment (11 downloads)

Glued and clamped to dry overnight.

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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987229 - 07/24/13 01:04 AM Attachment (14 downloads)

Checking the spider fit. Looks good for now. 27" vane edge to vane edge - NOT counting the the little tip thingies at the ends. 27.5" inner ring diameter. So that leaves 1/4" each side, vane to inner ring clearance - perfect.

This upper ring will be supporting the 8" spherical wsDK Cass secondary. The ring alone with no hardware or secondary included weighted 8 lbs on the bathroom scales.

After the glue dried the outer edges of the three glued together rings were given a good belt sander sanding with 120 grit (80 grit would have been good too) until the outer edge was nice and smooth all the way around.


Edited by siriusandthepup (07/24/13 01:22 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987256 - 07/24/13 01:33 AM Attachment (18 downloads)

And lastly for tonight a close up (minus the aluminum mounting blocks for the spider tips) showing the approximate fitting and positioning.

Edited by siriusandthepup (07/24/13 01:34 AM)


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: siriusandthepup]
      #5987401 - 07/24/13 06:30 AM

Quote:

This upper ring will be supporting the 8" spherical wsDK Cass secondary. The ring alone with no hardware or secondary included weighted 8 lbs on the bathroom scales.




That spider seems awful thin to me to support an 8-inch mirror at 90 degrees without flexure or vibration.


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Re: wsDK - weak secondary Dall Kirkham telescope new [Re: MKV]
      #5987452 - 07/24/13 07:51 AM

I understand that concern. The spider is an Astrosystems Super Duty II Spiders - Style 4 Astrosystems spider page. Vane thickness is .048" and we will see how well it works. The secondary will only weigh 3.5 lbs. when Mr. Royce gets that conical blank finished. We will just have to cross our fingers. If necessary I can always talk to Randy at Astrosystems about making me a custom heavy duty spider. The spider vanes feel fairly stiff on their own, so I'm optimistic at this point.

Edited by siriusandthepup (07/24/13 08:20 AM)


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