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wsDK - weak secondary Dall Kirkham telescope

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#26 dave brock

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Posted 27 January 2013 - 04:37 PM

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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#27 The bear

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Posted 27 January 2013 - 05:26 PM

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

#28 The bear

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Posted 27 January 2013 - 05:27 PM

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

#29 siriusandthepup

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Posted 27 January 2013 - 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?

#30 siriusandthepup

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Posted 27 January 2013 - 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.

#31 dave brock

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Posted 27 January 2013 - 07:51 PM

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. :crazy:
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.

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#32 siriusandthepup

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Posted 27 January 2013 - 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.

#33 ed_turco

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Posted 28 January 2013 - 01:53 PM

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


Ed

#34 MKV

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Posted 28 January 2013 - 06:53 PM

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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#35 siriusandthepup

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Posted 28 January 2013 - 06:59 PM

Could the Baader coma corrector be making that much difference?

That pic sure looks good...

#36 MKV

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Posted 28 January 2013 - 10:48 PM

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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#37 MKV

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Posted 28 January 2013 - 11:03 PM

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.

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#38 dave brock

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Posted 29 January 2013 - 12:14 AM

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

#39 siriusandthepup

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Posted 29 January 2013 - 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.

#40 MKV

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Posted 29 January 2013 - 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.

#41 MKV

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Posted 29 January 2013 - 11:43 AM

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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#42 Mike I. Jones

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Posted 29 January 2013 - 12:10 PM

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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#43 Mike I. Jones

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Posted 29 January 2013 - 12:21 PM

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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#44 MKV

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Posted 29 January 2013 - 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

#45 Ed Jones

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Posted 29 January 2013 - 05:30 PM

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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#46 Mike I. Jones

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Posted 29 January 2013 - 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

#47 siriusandthepup

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Posted 29 January 2013 - 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!

#48 siriusandthepup

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Posted 29 January 2013 - 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.

#49 dave brock

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Posted 29 January 2013 - 10:42 PM

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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#50 siriusandthepup

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Posted 30 January 2013 - 12:34 AM

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






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