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Planet killer SCT?

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#26 Sarkikos

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Posted 25 January 2015 - 06:54 PM

 

 

I'm still waiting for a night of excellent seeing to see what my 10" SCTs can do (and LX6 and an LX200) but my trusty old LXD75 SC8 does a pretty impressive job when the seeing cooperates...

 

attachicon.gifJupiter (12-11-2011)-2j.jpg

 

12/11 23h 30m, 12/12/2011 0h 30m and 1h 30m UT
Telescope: Meade LXD75 SC8 @ f/30
Camera: Imaging Source DBK21, Meade IR (Lum) Filter
Exposures: 2min @ 30fps (~3,200 frames out of 3,600)
Software: IC Capture, Registax 6, Photoshop

 

The most memorable view I've had of Jupiter was through a classic 12.5" f/10 Newtonian. By far the best view I've ever had of any planet was Saturn through an 8" tri-Schiefspiegler.

 

These photos are about what Jupiter looks like on a night of excellent seeing through my 10" f/4.8 Dob with a 23% CO ... visually.  I don't do AP.

 

Mike

 

I've never witnessed anything like those photos visually.

 

Time to move to the east coast?

 

 

I said, "on a night of excellent seeing."  I didn't say it happens all the time.  Virtually never between Thanksgiving and Easter.  Sometimes during the warmer months.  But even then you need patience to wait for clearer moments ... and make sure your Newt is closely collimated (auto collimator close, not sight tube/Cheshire close) ... and thermally acclimated ... and using a binoviewer ... and keeping your eyes photopic with white light ... and a Baader M&SG filter might help ... then relax and hope for the best.  But yes, I've seen - not imaged - Jupiter about that good here in Maryland through my lowly Celestron C10NGT 10" f/4.8 Dob.

 

:grin:

Mike


Edited by Sarkikos, 25 January 2015 - 06:58 PM.

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#27 Jon Isaacs

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Posted 25 January 2015 - 06:56 PM

When you put a 35 % obstructed telescope next to a 20% obstructed telescope of the same diameter you will notice the difference. The telescope with larger obstruction will show more light in its diffraction pattern and this will make the instrument more susceptible for seeing effects and will simply have a reduced contrast, at least visually. When imaging, you'll be able to improve contrast with post processing and to be honest I don't know if the end result would differ that much anymore. 

 

Here's a picture of the obstruction of my 'planet killer', 200 mm F/25 Schmidt-Cassegrain (20% linear) and an image of Jupiter I took with this instrument under reasonable circumstances with an ASI120MC camera at prime focus.

 

Even (or especially) if it was only for visual use, I'd build my telescopes with an as small as possible obstruction. The views will be worth it ;-)

 

Cheers,

Rik

 

:goodjob:

 

Most scopes will give very good views of the planets if the seeing is very good.  In my mind, the difference between a scope that gives very good views and a "Planet Killer" is that the planetary telescope is optimized for getting the best possible views of the planets.  That means not only high quality optics, a small secondary, a properly mounted mirror that doesn't flop, it also means attention to detail, a fixed mirror with an external focuser so the focal plane is at the point of minimum SA, it means a tube designed to thermally equilibrate quickly and completely.

 

If one were looking for an SCT to label a Planet Killer, I think there is no need to look further than Rik's 200mm F/25 with the 20% CO.. That scope sure looks like a "Planet Killer" to me.  Maybe Rik could share a little more about his scope and post a some photos, it's a beautiful looking scope from behind..

 

I did find this image of the scope: 200mm F/25 SCT

 

.Jon


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#28 jgraham

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Posted 25 January 2015 - 07:24 PM

By far the best planetary scope that I've ever built was a 10" f/30 Gregorian that had a 2" secondary. I still have the optics tucked away in storage and I hope to put it back together one of these days. I'm also holding onto the mount from my deforked 10" LX6 to put it on. It should look pretty slick if I do it right.


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#29 rik ter horst

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Posted 26 January 2015 - 07:27 AM

Thanks Jon!  Here's a more recent image of the 200 mm SCT, entirely insulated, exept for the back of the Primary mirror. Either you cool your telescope as fast as possible (with some ugly tube current as a result, at least temporarily) or you try to cool it as slow as possible by insulation so no harmful currents can occur. It's all about internal thermal gradients; Try to minimize them as much as possible. The focuser makes no contact whatsoever with the internal optical path. For the rest I've used bad thermal conductors, like a 'plastic' baffle for instance and an epoxied MDF mirror- and corrector cell. If needed I can close the open back, to reduce the speed of mirror cooling. Of course I store the telescope in a cold place and in general it can be used immediately. Only very mild currents might be visible in the beginning of an observing session. 

 

About the optics, the Primary mirror is F/2.5 in order to reduce spherochromatism, and the secondary is a sphere, as coma will not be visible at F/25. This is what I also did with most of the 250 mm F/15 SCT's (Opticon) mentioned earlier in this thread. 

 

 

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#30 Jon Isaacs

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Posted 26 January 2015 - 03:07 PM

Rik:

 

Who did the machining, who did the optics?  Did you grind your own mirrors and corrector plate?

 

That is one optimized scope.. 

 

Jon



#31 rik ter horst

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Posted 26 January 2015 - 04:37 PM

Thanks John! The telescope is entirely homemade. I always enjoyed the images of my 200 mm F/20 Schiefspiegler a lot but I didn't like the dimensions of it. That's why I designed this 'planetary SCT' which has been further optimized over the years. This year I hope to finish a new small workshop at home where I can continue with experiments on an insulated 250 mm F/15 SCT, optically identical to those of my Opticon period but thermally more evolved. The tube and back side of the telescope are made of Carbon with a foam core, acting as an insulator. My daily work and moving house prevented me from finishing this telescope earlier....  

 

Cheers,

Rik


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#32 BillP

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Posted 26 January 2015 - 05:27 PM

In my mind, the difference between a scope that gives very good views and a "Planet Killer" is that the planetary telescope is optimized for getting the best possible views of the planets.  

 

 

That's what it means to me as well...simply that the design of the scope in question is such that it is optimized for contrast and definition.  So it that vein, one can call a Newt that has been optimized a planet killer, and a refractor and an SCT.  IMO you can't call a production off the shelf SCT a planet killer as it is not optimized in the standard configuration.  However, restrict the backfocus so it stays within SA limits of the design, flock it where it needs it and light shield that exposed front plate to eliminate all stray light, and put a cooling system in it that maintains zero thermal issues for it, and ensure you have great optics inside...now you have a planet killer version of an SCT.


Edited by BillP, 26 January 2015 - 05:28 PM.

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#33 Jon Isaacs

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Posted 26 January 2015 - 06:58 PM

 

In my mind, the difference between a scope that gives very good views and a "Planet Killer" is that the planetary telescope is optimized for getting the best possible views of the planets.  

 

 

That's what it means to me as well...simply that the design of the scope in question is such that it is optimized for contrast and definition.  So it that vein, one can call a Newt that has been optimized a planet killer, and a refractor and an SCT.  IMO you can't call a production off the shelf SCT a planet killer as it is not optimized in the standard configuration.  However, restrict the backfocus so it stays within SA limits of the design, flock it where it needs it and light shield that exposed front plate to eliminate all stray light, and put a cooling system in it that maintains zero thermal issues for it, and ensure you have great optics inside...now you have a planet killer version of an SCT.

 

 

Bill:

 

The one last criteria I would add for the Planet Killer SCT, would be a small secondary obstruction.  In terms of optimizing a design, that needs to be part of the equation..  As Rik has shown, that is possible and compared to the more typical 35% COs of commercial SCTs, Rik's 20% must allow for greater contrast.  it's not that a scope with a 35% CO cannot provide very good planetary views, it's just not optimal.  

 

Jon


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#34 A6Q6

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Posted 26 January 2015 - 08:04 PM

 

 

It seems generally accepted the size (percentage of aperture) of an SCT secondary causes some loss of contrast when viewing, especially notable on planets. In Newtonian/DOB terms there are some scopes built as "planet killers" in that they have a fairly long focal length and a fairly small secondary to address this. 

 

Has anyone made an SCT "planet killer" with a small secondary? 

 

Tom Duncan

C-14.  31.5%..  Best planetary SCT I have ever used....  Best planetary scope I have ever owned.

 

It's all about good quality clear aperture and good seeing.  The more of each you bring to the party, the better

 

I couldn't agree more. My C14 is the best planetary scope Ive looked through in my 30 years of observing...who would of thunk it....

  

Great topic, makes me want a C14.



#35 Gord

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Posted 26 January 2015 - 08:34 PM

Everyone should have a C14! :lol:

 

Its not not my best scope optically, but it does show more than any other scope I have, planets or deep sky.

 

CS,

 

 

 


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#36 vahe

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Posted 26 January 2015 - 10:40 PM

Insulating the tube definitely helps and I have been doing it for years, but it is definitely not a permanent fix. The main benefit of insulation is in slowing down the rate of heat transfer but tube currents do build up eventually, at least that has been my experience using this method.
I am currently experimenting with a different method which has proven to be superior to insulation wrap, but it is complicated and requires major internal tube modification to allow for installation of closed loop air recirculation system, but that a story for another day.

 

Vahe

 

TEC8-A.jpg


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#37 maadscientist

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Posted 26 January 2015 - 11:25 PM

For Planetary Imaging, the C14 is King......I have a 20 inch f4.3 Starmaster, but it's a draw so far..... Pic of Jupiter is with my C14...

 

JupiterGRS2014-12-27-033305DanL.jpg


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#38 Jon Isaacs

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Posted 27 January 2015 - 09:25 AM

Everyone should have a C14! :lol:

 

Its not not my best scope optically, but it does show more than any other scope I have, planets or deep sky.

 

CS,

 

Everyone should have a pair of 10x50 binoculars..  When it comes to telescopes, there are many, many options... Some that will show considerably more than a C-14.

 

Jon


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#39 bob midiri

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Posted 27 January 2015 - 09:34 AM

I think the OP stated which SCT  is a planet killer. Im sure that there are better scopes optically suited for besting a SCT in subtle planetary detail, but to me in all the years I have observed, my C14 fits the OP's description. My 16" f6.5 or my 10"F7.2 dobs are planetary killers also, but if I weigh everything, including comfort and relaxation by sitting down observing...versus climbing ladders and pushing the dob to center, then yes to me this particular C14, with its high quality optics , along with the aperture and the other things, to me is a real planet killer


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#40 Gord

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Posted 27 January 2015 - 11:32 AM

 

Everyone should have a C14! :lol:

 

Its not not my best scope optically, but it does show more than any other scope I have, planets or deep sky.

 

CS,

 

Everyone should have a pair of 10x50 binoculars..  When it comes to telescopes, there are many, many options... Some that will show considerably more than a C-14.

 

Jon

 

Jon,

 

I don't disagree with you on either point.  But as Bob has correctly focused in on, for the question of SCT's, the C14 has a lot to offer.  As an all around package, it ticks a lot of boxes and offers a lot of bang for the buck.  Especially on the planets!

 

Clear skies!



#41 Mitchell Duke

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Posted 27 January 2015 - 03:30 PM

Its should not be a question of which scope, but it should be a question of the seeing conditions. Just look at the Solar system imaging to see what scopes they use. Central obstruction might make a very very small difference, but only if seeing is perfect which might happen twice a year. As always seeing is king, and that is the bottom line.
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#42 Tom Duncan

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Posted 27 January 2015 - 04:08 PM

Wow, thanks for all the input guys, very interesting. Bottom line is there is no commerciallly available 'planet killer" (small secondary) telescope, right? 

 

Tom 



#43 TG

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Posted 27 January 2015 - 08:37 PM

For Planetary Imaging, the C14 is King......I have a 20 inch f4.3 Starmaster, but it's a draw so far..... Pic of Jupiter is with my C14...

 

attachicon.gifJupiterGRS2014-12-27-033305DanL.jpg

 

:bow: The C14 seems to be not just planet-optimized but Jupiter-optimized. :lol:

 

Tanveer.


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#44 TG

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Posted 27 January 2015 - 08:40 PM

Thanks Jon!  Here's a more recent image of the 200 mm SCT, entirely insulated, exept for the back of the Primary mirror. Either you cool your telescope as fast as possible (with some ugly tube current as a result, at least temporarily) or you try to cool it as slow as possible by insulation so no harmful currents can occur. It's all about internal thermal gradients; Try to minimize them as much as possible. The focuser makes no contact whatsoever with the internal optical path. For the rest I've used bad thermal conductors, like a 'plastic' baffle for instance and an epoxied MDF mirror- and corrector cell. If needed I can close the open back, to reduce the speed of mirror cooling. Of course I store the telescope in a cold place and in general it can be used immediately. Only very mild currents might be visible in the beginning of an observing session. 

 

About the optics, the Primary mirror is F/2.5 in order to reduce spherochromatism, and the secondary is a sphere, as coma will not be visible at F/25. This is what I also did with most of the 250 mm F/15 SCT's (Opticon) mentioned earlier in this thread. 

 

Rik, I had good results as well when wrapping my C11HD in Reflectix (aluminized bubble wrap). I think a big part of thermal trouble comes from differential cooling between the sky facing top and ground facing bottom of the Al-tube.

 

Tanveer.



#45 JMW

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Posted 27 January 2015 - 11:24 PM

With my C11 Edge HD or my Webster D14 with f/4.3 14.5 inch Zambuto mirror I am seeing limited. I have used my Webster D14 once at 524 power and had a fantastic view of Saturn. Most times I am at 366 power or less. On my C11 EdgeHD I usually am viewing planetary at 280 power or less.

 

I live just east of 10,000+ foot Sierra Nevada range and often am under the jet stream. I also need to actively cool my scopes with fans to keep up with rapidly dropping temperatures through out the night.



#46 rik ter horst

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Posted 28 January 2015 - 03:57 AM

Looking good Vahe! I'm curious about your own experience regarding closed loop air recirculation. From what I've experienced this really makes sense.The earlier 250 mm Schmidt-Cassegrains I've made were thermally not the best and for a long time I've been wondering how I could improve their thermal behavior. At the end I concluded that it's 'only' a matter of removing thermal gradients and this can be done either by good insulation or by fans, circulating/mixing the inner air mass (so no fans that bring cold air inside the tube, although that can help as well).  I used four Tempest fans and placed them behind the primary mirror (there was sufficient space), no trouble with vibrations whatsoever.

Tanveer ( :)) , you're entirely right about this and thats why (internal) fans can do such a good job. 

 

Great image Maadscientist!!

 

Cheers,

Rik


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#47 John P

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Posted 28 January 2015 - 06:43 AM

Very interesting thread, somehow more interesting than the more classic, on-going discussions about central obstructions.  Also interesting that there is no mention of the traditional favorite planet killer, the Celestron 9.25" SCT.

 

If I had the money and the time, I'd be interested in the Orion Optics (UK) 800mm Maksutov-Cassegrain, I believe it's at f/20, as a planet killer, lunar telescope. I understand it's back in production and rarely comes up in the classified sections.  I'd love one.


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#48 Jon Isaacs

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Posted 28 January 2015 - 07:20 AM

 

 

Everyone should have a C14! :lol:

 

Its not not my best scope optically, but it does show more than any other scope I have, planets or deep sky.

 

CS,

 

Everyone should have a pair of 10x50 binoculars..  When it comes to telescopes, there are many, many options... Some that will show considerably more than a C-14.

 

Jon

 

Jon,

 

I don't disagree with you on either point.  But as Bob has correctly focused in on, for the question of SCT's, the C14 has a lot to offer.  As an all around package, it ticks a lot of boxes and offers a lot of bang for the buck.  Especially on the planets!

 

Clear skies!

 

 

 

 

Everyone should have a C14! :lol:

 

Its not not my best scope optically, but it does show more than any other scope I have, planets or deep sky.

 

CS,

 

Everyone should have a pair of 10x50 binoculars..  When it comes to telescopes, there are many, many options... Some that will show considerably more than a C-14.

 

Jon

 

Jon,

 

I don't disagree with you on either point.  But as Bob has correctly focused in on, for the question of SCT's, the C14 has a lot to offer.  As an all around package, it ticks a lot of boxes and offers a lot of bang for the buck.  Especially on the planets!

 

Clear skies!

 

 

 

There is no doubt that for some observers, maybe even many observers, a C-14 is a must have scope.  But it's big, awkward, and long of focal length.  Certainly not an all around performer that would suit everyone's needs.

 

:shrug:

 

Jon


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#49 Sarkikos

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Posted 28 January 2015 - 08:24 AM

 

There is no doubt that for some observers, maybe even many observers, a C-14 is a must have scope.  But it's big, awkward, and long of focal length.  Certainly not an all around performer that would suit everyone's needs.

 

:shrug:

 

Jon

 

 

I would pass.  The only way I would have a C14 if I it were permanently mounted in an observatory where I lived.  It would also help if the observatory were in a dark zone.  But then because of the big f/number, you would not have capability for wide field deep sky.  You better use it for planets and other small stuff.  So I suppose an observatory in the suburbs would be adequate.

 

Personally, I would probably never have an SCT larger than 11", and that's pushing it.  I'm starting to think that an 8" SCT hits the sweet spot.  Larger than that, you ought to be thinking about a Dob.

 

 A 14" truss Dob makes more sense to me.   Much more portable, probably easier to acclimate to ambient temps and can achieve wider field views compared to a C14.  And also capable of being a planet killer.

 

Mike


Edited by Sarkikos, 28 January 2015 - 08:30 AM.

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#50 bob midiri

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Posted 28 January 2015 - 10:34 AM

Thats what makes this such a great hobby, everyone has their own preferences...and there is more then enough available goods to go around. There never was and never will be one telescope that fills everyones niche!! What may be my preference..is mine alone, some may agree, most will not, and forums like this make it possible to share our agreements and disagreements with each other in a civil manner!!.


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