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Planetary Imaging With Large Central Obstruction?

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#1 Tom Polakis

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Posted 06 December 2013 - 09:05 PM

Hi all,

This is more of an equipment question than anything, so mods can feel free to move it, if it's inappropriate on this forum.

Over on the Cats & Casses forum, I asked if there is such a thing as a single telescope that will produce deep-sky images over a moderate field diameter *and* give high-contrast planetary images and views. The scope would be in the 10" to 12.5" range. Just for the purposes of this discussion, I'd rather stay away from SCTs this time around. I've given them many chances. The thread is here.

Imaging Scope Thread

I've narrowed down the choices a bit, and currently favor two very different scopes: the Takahashi Mewlon 250CRS and the PlaneWave 12.5" CDK. The latter obviously has more aperture, is faster, and has larger field coverage. I suspect the planetary views aren't that great though. That's where this forum comes in. Has anybody here done lunar and planetary work with one of these Dall-Kirkham scopes with a large central obstruction? Are there some good examples of images that show these telescopes keeping up with more conventional planetary imaging scopes? Thanks.

Tom

#2 brianb11213

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Posted 07 December 2013 - 04:24 AM

Add the Orion Optics (UK) ODK scope into the mix ... f/6.8, good quality optics & a field size plenty big enough for a full frame (24x36mm) format sensor.

The planetary performance of the ODK is limited only by the seeing ... this type of scope appears to be more sensitive to seeing disturbances than some ... the secondary obstruction is around 40% which is necessary to achieve this sort of field width. The contrast in planetary images will be reduced but you should be able to get this back in post processing.

I would also consider a Newtonian around f/5. With a coma corrector for wide field imaging. With a Powermate for planetary imaging. Smaller CO than the SCT / RC / ODK type scopes & you can get pretty much perfect optics by buying from Zambuto, Royce or Orion Optics (UK) Ultra Grade.

#3 Tom Polakis

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Posted 07 December 2013 - 05:38 AM

Add the Orion Optics (UK) ODK scope into the mix ... f/6.8, good quality optics & a field size plenty big enough for a full frame (24x36mm) format sensor.

The planetary performance of the ODK is limited only by the seeing ... this type of scope appears to be more sensitive to seeing disturbances than some ... the secondary obstruction is around 40% which is necessary to achieve this sort of field width. The contrast in planetary images will be reduced but you should be able to get this back in post processing.

I would also consider a Newtonian around f/5. With a coma corrector for wide field imaging. With a Powermate for planetary imaging. Smaller CO than the SCT / RC / ODK type scopes & you can get pretty much perfect optics by buying from Zambuto, Royce or Orion Optics (UK) Ultra Grade.



Thanks. I love my Newtonians, but I should have mentioned that this telescope will be on a pier in the backyard that stands 4 feet tall. This particular telescope will have the focuser at the bottom end, so it's either a compound scope or a refractor, the latter just not having enough aperture.

Tom

#4 ToxMan

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Posted 07 December 2013 - 12:46 PM

Hi Tom,

Someone proposed the Schiefspieglers for planetary in a recent thread...don't know if you saw it.
Thread

Paul

#5 Tom Polakis

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Posted 07 December 2013 - 04:11 PM

Hi Tom,

Someone proposed the Schiefspieglers for planetary in a recent thread...don't know if you saw it.
Thread

Paul



Thanks, Paul. To be more specific, I am looking for a scope in that's f/8 or faster with a focal reducer, since this would be a single scope that does planetary and deep imaging well. It should also have good contrast for visual observing. That's why I am specifically asking about scopes such as R-C's and D-K's that traditionally have large central obstructions. The small volume of replies so far indicates that they are rarely used for planetary imaging or viewing.

Tom

#6 rg55

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Posted 07 December 2013 - 04:20 PM

I think you'll find that the Takahashi Mewlons are Dall-Kirkhams and are frequently used for planetary AP.

http://i516.photobuc...hotos/Jupite...

http://members.ozema...audio/astro.htm


#7 bunyon

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Posted 07 December 2013 - 07:27 PM

They're not frequently used, period. I suspect the loss in contrast can be recovered in processing but I don't KNOW that. I suspect you're not getting much feedback because not many people (relatively speaking) use them.

#8 rmollise

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Posted 08 December 2013 - 10:57 AM

Hi all,

This is more of an equipment question than anything, so mods can feel free to move it, if it's inappropriate on this forum.

Over on the Cats & Casses forum, I asked if there is such a thing as a single telescope that will produce deep-sky images over a moderate field diameter *and* give high-contrast planetary images and views. The scope would be in the 10" to 12.5" range. Just for the purposes of this discussion, I'd rather stay away from SCTs this time around. I've given them many chances. The thread is here.

I've narrowed down the choices a bit, and currently favor two very different scopes: the Takahashi Mewlon 250CRS and the PlaneWave 12.5" CDK. The latter obviously has more aperture, is faster, and has larger field coverage. I suspect the planetary views aren't that great though. That's where this forum comes in. Has anybody here done lunar and planetary work with one of these Dall-Kirkham scopes with a large central obstruction? Are there some good examples of images that show these telescopes keeping up with more conventional planetary imaging scopes? Thanks.

Tom


Those are nice scopes. But a C11 or a C14 will do just as well for imaging. That's what Chris Go and Damian Peach use, and it would be hard to get a better recommendation than that. I don't know what your "experiences" were, but the top amateur planetary images are being done with SCTs. No brag, just fact. ;)

When folks start screaming about CENTRAL OBSTRUCTION? Run away as fast as your legs will carry you. :lol:

#9 DesertRat

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Posted 08 December 2013 - 12:45 PM

Tom,

Sorry, I'm am yet another without any experience with either of the scopes you referenced.

I would say that those two scopes are quite different. The Planewave would fall into the large obstruction category, the Mewlon less so. My own experience with scopes that cover the amateur size classes, and well beyond, is that aperture always wins in terms of imaging definition, everything else assumed in working order. So the Planewave would most likely produce better planetary images, but not by much. Many planetary imagers would choose the Mewlon 250 and for 'views' it would be superior. Personally if I were to go the DK & high resolution route I would go for a Royce DK 12-14" f/20, which is no where near where you want to go. Since you also want to do more wide field the Planewave is the clear winner.

And I may embarrassingly add that money was no object when I decided finally on the Edge14. :)

Glenn

#10 mattyfatz

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Posted 08 December 2013 - 12:53 PM

I'm no imager.. But in astrophotography isn't the CO a benefit? I mean on one end you say it's a central obstruction, but to the camera it's the illuminated field.. And that's a good thing.. Isn't it? All the best astrophotography I've seen has come from some sort of catadioptric.

#11 Bart Declercq

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Posted 08 December 2013 - 01:21 PM

I'm no imager.. But in astrophotography isn't the CO a benefit? I mean on one end you say it's a central obstruction, but to the camera it's the illuminated field.. And that's a good thing.. Isn't it? All the best astrophotography I've seen has come from some sort of catadioptric.


For planetary imaging, the illuminated field only needs to be a couple of arc minutes in size (only Venus actually gets beyond 1 arc minute in diameter) so for this type of astrophotography, the size of the illuminated field is virtually irrelevant - a bigger CO does not provide an advantage there, and provides several disadvantages (loss of light, loss of contrast) - the main question is if these disadvantages weigh up against the other advantages of catadiopters (mostly the compact size of this type of telescope for a certain aperture.)

#12 DesertRat

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Posted 08 December 2013 - 01:26 PM

A central obstruction is never good, with the possible exception of separating close equal mag stars. A CO is just a fact of design and nothing to aim for.

Glenn

#13 Tom Polakis

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Posted 08 December 2013 - 03:17 PM

...if there is such a thing as a single telescope that will produce deep-sky images over a moderate field diameter *and* give high-contrast planetary images and views.


Thanks for the further replies. For those who are recommending scopes that produce great planetary images, such as SCTs, note that I indicated in my original post that I want it to illuminate a moderate field for deep, wide images. The latter requirement means a large central obstruction. I have a lot of experience with SCT's for deep imaging, and don't want to go that route for this purchase. Probably best to leave it at that.

I have seen answers to the effect that the telescope you're looking for doesn't exist, which seem to be the most realistic. In this thread, I'm only trying to determine of any of the wide-field Dall-Kirkham telescopes like the ones I mentioned have been used to produce good planetary images. Thanks.

Tom

#14 Sunspot

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Posted 08 December 2013 - 09:15 PM

And then there were the "good old days" of the Parks combo Newtonian Cassigrain scopes which had a flip secondary to give you an F/4 Newtonian for DSO and F/16 for planetary...a little off topic, but I had to mention that. :lol:

#15 John Boudreau

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Posted 08 December 2013 - 10:48 PM

...if there is such a thing as a single telescope that will produce deep-sky images over a moderate field diameter *and* give high-contrast planetary images and views.


Thanks for the further replies. For those who are recommending scopes that produce great planetary images, such as SCTs, note that I indicated in my original post that I want it to illuminate a moderate field for deep, wide images. The latter requirement means a large central obstruction. I have a lot of experience with SCT's for deep imaging, and don't want to go that route for this purchase. Probably best to leave it at that.

I have seen answers to the effect that the telescope you're looking for doesn't exist, which seem to be the most realistic. In this thread, I'm only trying to determine of any of the wide-field Dall-Kirkham telescopes like the ones I mentioned have been used to produce good planetary images. Thanks.

Tom


Planewave CDKs have indeed been used to produce fine planetary images, but the typical CDK owner chooses that scope for a well-corrected field with the intent of deep sky imaging and so I don't know of anyone really dedicating much time at all to planetary imaging with one. If I were in the market for a dual purpose imaging instrument, the CDK would be at the top of my list.

While I know I've seen nice planetary CDK images from time-to-time, I found it's tough to find many with a quick web search but here are some Mars shots that at least show it has promise for that task. The first two taken with a 12.5". The last linked image was taken by the same guy who shot the image in the 2nd link, but with a 20" CDK.

http://www.skynews.c...ery/pow478_298/

http://astrophoto.de...-03-02.html#pic

http://www.astrophot...clouds_font.jpg

#16 Bart Declercq

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Posted 09 December 2013 - 05:51 AM

...if there is such a thing as a single telescope that will produce deep-sky images over a moderate field diameter *and* give high-contrast planetary images and views.


Thanks for the further replies. For those who are recommending scopes that produce great planetary images, such as SCTs, note that I indicated in my original post that I want it to illuminate a moderate field for deep, wide images. The latter requirement means a large central obstruction. I have a lot of experience with SCT's for deep imaging, and don't want to go that route for this purchase. Probably best to leave it at that.

I have seen answers to the effect that the telescope you're looking for doesn't exist, which seem to be the most realistic. In this thread, I'm only trying to determine of any of the wide-field Dall-Kirkham telescopes like the ones I mentioned have been used to produce good planetary images. Thanks.

Tom


Planewave CDKs have indeed been used to produce fine planetary images, but the typical CDK owner chooses that scope for a well-corrected field with the intent of deep sky imaging and so I don't know of anyone really dedicating much time at all to planetary imaging with one. If I were in the market for a dual purpose imaging instrument, the CDK would be at the top of my list.

While I know I've seen nice planetary CDK images from time-to-time, I found it's tough to find many with a quick web search but here are some Mars shots that at least show it has promise for that task. The first two taken with a 12.5". The last linked image was taken by the same guy who shot the image in the 2nd link, but with a 20" CDK.

http://www.skynews.c...ery/pow478_298/

http://astrophoto.de...-03-02.html#pic

http://www.astrophot...clouds_font.jpg


I've occasionally got access to a 24" Dall-Kirkham which I want to use for planetary & lunar observations. Unfortunately I haven't struck it lucky with the seeing yet but the one time I took some lunar shots in less-than-totally awful seeing, the lunar shots exceeded the best I've seen from any 16" telescope so I'm quite hopeful for better conditions.

#17 conus

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Posted 09 December 2013 - 04:34 PM

Agreed. It would be tough to find better planetary images than those captured with the C14. The only comparable photos I've seen were done with 16 and 18 inch dobs (Wes Higgins, for example) but none showed any more detail or better color.


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