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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: wsDK - weak secondary Dall Kirkham telescope [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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Mike I. Jones
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Reged: 07/02/06

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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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Mike I. Jones
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Reged: 07/02/06

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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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MKV
Carpal Tunnel
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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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Ed Jones
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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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Mike I. Jones
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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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siriusandthepup
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Reged: 02/14/06

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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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siriusandthepup
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Reged: 02/14/06

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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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dave brock
professor emeritus


Reged: 06/06/08

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

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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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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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mark1234
member


Reged: 04/15/08

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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mark1234
member


Reged: 04/15/08

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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Ed Jones
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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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MKV
Carpal Tunnel
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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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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
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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MKV
Carpal Tunnel
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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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siriusandthepup
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Reged: 02/14/06

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Re: wsDK - weak secondary Dall Kirkham telescope [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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