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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: 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
sage
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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
sage
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Reged: 12/03/07

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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Reged: 01/20/11

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
Pooh-Bah
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Reged: 02/11/08

Loc: rushville, indiana
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
Pooh-Bah
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Reged: 02/11/08

Loc: rushville, indiana
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
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
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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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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MKV
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Reged: 01/20/11

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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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
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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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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