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Migelo
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Reged: 10/03/12

Guiding Schmidt–Cassegrain 300/3000
      #5526197 - 11/18/12 01:55 PM

Hi!

What kind of guiding would be the best for a Schmidt–Cassegrain 300/3000 telescope?

OAG or a separate guiding telescope?


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5526483 - 11/18/12 05:02 PM

OAG with any moving mirror scope. Mirror locks rarely do the job. I had to modify mine beyond recognition so it no longer can be focused that way. Took help from a friend with a machine shop and several months of work. Still, when I guide I use the guider chip in the STL-1100XM.

Rick


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Migelo
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Reged: 10/03/12

Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5534055 - 11/22/12 02:37 PM

So this would be cool for a KAF 9000 sensor?

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4353_O...


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5534099 - 11/22/12 03:33 PM

That chip is 52mm diagnonal while the OAG you link to is good only to 42mm diagnonal as I read the page. It would work with a 11K or smaller chip but not those larger than that.

A couple other considerations. Can you even use a 52mm diagonal? My 14" (356mm) LX200R only supports about a 42mm diagonal chip due to its 45mm baffle tube well in front of the CCE. Larger would vignette corners more than I'd want to flat field out due to the high noise that would create in the corners. I doubt any 300mm SCT can support that chip. The 12 micron pixel is a good match however to that focal length at 0.8" per pixel. Might make up for the need to crop out part of the chip.

Meade now has out a line of SCT's (LX600) using an internal Crayford type system to move the main mirror. They claim it does fully lock the mirror from moving. I have not heard if it works as advertised. If you have one of those then you might try the separate guide scope method and see if it works. Keep in mind flex in such a system at that focal length is very difficult to control. Takes a super focuser on the guide camera even with a very light camera and/or a separate support for the camera along with very solid mounting of the guide scope. Only threaded mounting of the imaging camera will be sufficient as well.

Rick


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Migelo
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Reged: 10/03/12

Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5535105 - 11/23/12 09:28 AM Attachment (23 downloads)

Thx, seems I'll have to look out for another OAG. =)

The telescope is LX200 and I calculated that with the baffle tube radius of 22,5mm we'd waste about 4,9% of the chip. See the attached picture, full calculations are here but to understand the calculations, you should check out the attachment: Calculations

How much more would go to waste due to vignetting at the corners.

EDIT: Corrected the result.

Edited by Migelo (11/24/12 05:07 PM)


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5535604 - 11/23/12 02:15 PM

Vignetting starts at the very center of the image and increases at an ever increasing rate toward the edge suddenly getting greater at some point less than the diameter of the baffles. How far those are from the objective and the f ratio of the system has to be taken into account too in any math model. Other bottlenecks need be included.

22.5mm for the baffle radius is the same as in my 14". The corners of my 11K chip are about 42mm diagonally. I get a drop off of about one magnitude at the corners due to vignetting. If the night gives 23rd magnitude in the center it will drop to 22 at the corners, maybe a bit more. Flat fielding hides this but if you look closely you'll see the increase in signal to noise ratio. Where this becomes objectionable is a very personal thing. No hard and fast rule on how much is too much. Some might find this too much but I've rarely found it important.

Assuming that you feel the same then your scope too would handle a 42mm square out of the 52mm square which would waste about 35% of the chip (1-42^2.52^2).

You have to weight the added cost of wasted silicon of that chip over say the 11K vs the somewhat longer sub exposure time needed with the 11K's smaller pixel size to reach sky limited exposures. Once sky limited then the total exposure time is the same for the same image scale achieved after the fact with software binning. Or maybe you have another use for the chip that doesn't waste the silicon.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5535667 - 11/23/12 02:48 PM

But I could've just cut the image at the corners and get smth like the paper used in Battlestar Galactica, albeit the cut would be more noticable:


If I did that, I wouldn't lose so much of the chip.


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Migelo
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Reged: 10/03/12

Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5535928 - 11/23/12 05:39 PM

And are you sure about that information on the baffle tube?

I got this:

12" LX200:

Main baffle tube OD is 2 1/2"
Main baffle tube ID is 2 3/8"
Secondary baffle length from inner surface of the corrector is 1 -21/32"
Secondary baffle OD is a shade under 4" say 3 -63/64"


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5535968 - 11/23/12 06:05 PM

I can only speak to what I measured in my early 14" LX200R when I had it apart. The tightest bottleneck was about 42mm.

If this is just the LX200 model you will need a field flattener. Those usually reduce which would further restrict the usable field of view.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5535980 - 11/23/12 06:14 PM

This sucks! We'd be wasting a lot of that expensive chip.

This is a 12" telescope. I'll look more in this matter.


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5536543 - 11/24/12 01:12 AM

35% would be below my standards, yours may vary but your method of calculation is far too arbitrary. Vignetting varies across the field. Even 100 pixels from center it will start to drop off as that part of the chip sees the aperture slightly oval and thus smaller in area. With my LX200R the drop is about 1 magnitude at 42mm diameter circle which is invisible to the eye once flat fielded. Beyond that the eye starts to see a drop in signal to noise ratio that I find too much. Others may find that point inside or outside that circle. To get that I use a 3" focuser (my mirror is truly locked making an external one necessary) and all connections up to the camera are that size. When I use a 2" vignetting was worse. I quickly upgraded to 3". Well 3.1" to be more exact. Now the vignetting is solely inside the OTA.

Virtually all my posts here (labeled full image) use the full 42mm diagonal of the chip. Only by measuring noise in the corners or by checking limiting magnitude there do you see the vignetting after proper flat fielding. Beyond that however things go bad quickly, hence my choosing the 11K to best cover the usable field at reasonable resolution (1" at 2x2 and 0.5" at 1x1 when conditions permit -- not very common unfortunately).

Besides vignetting you have field curvature to deal with unless that is a newer model with ACF optics. Otherwise a flattener/reducer would call for a smaller chip with smaller pixels for the same resolution and true field of view. Even with those optics I have noticeable curvature in the corners so doubt it could support anything larger even without the vignetting.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5537415 - 11/24/12 03:43 PM

Well, my calculations included only the part of the chip that would get no light at all.

So, today we went to that telescope in the observatory and checked it out.

The last part that is attached to the telescope is:



It's attached to this:

Had to make it in SketchUp, all pictures were blurry as hell!

And as you said, the diameter is 45mm:


So, if we could make/buy that part just with 52mm instead of 45mm, would it be ok?

This is a newer version with ACF optics.

Thanks for helping me out.


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5537776 - 11/24/12 08:12 PM

That would be about the inside diameter of the 2" focuser that added to my vignetting. I no longer have it so can't measure it. Obviously won't fit the 9000 chip size, even 52mm would be pushing it for that chip.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5539256 - 11/25/12 06:56 PM

If we can somehow make 52mm work, we'll take it, but it seems messy.

Which camera would you suggest? With your 11k, how many times is it usable?

And to get back to the original subject, do you know of any OAG that works well and would fit this CCD or any of those you suggest.


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5539455 - 11/25/12 09:37 PM

MegaMOAG is usually the choice with large chips. http://www.astrodon.com/products/hardware/megamoag_off-axis_guider/

I use the 11K every clear night for something. Haven't been many of late, snowing right now.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5540358 - 11/26/12 01:56 PM

That thing costs 1400€ in EU.... xD

I got a reply from Meade about what we're talking about:
Quote:

Dear Mr. xxxxxx,

if you use the biggest possible rear cell thread (should be 3,25" for the 12" with a step down ring to 2") it should work. Only setback will be the inner diameter of the main mirror baffle tube, which is 2 3/8" for the 12" LX200.
Actually 53mm is quite big so we have no real experience or test photos.


kind regards,
xxxx xxxxxxx




So this means the narrowest part is 60.33 mm wide. That should be enough for the chip with a diagonal of 53mm. The part I don't get is:

Quote:

(should be 3,25" for the 12" with a step down ring to 2")



Does that mean that the narrowest part is actually 2" wide? I'm confused what a "step down ring" does or looks and what is it's purpose.


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5540503 - 11/26/12 03:20 PM

I think they mean to keep everything at 3.25" until you reach the camera. I'm at 3.1" with the 11K all the way to the camera. When I stepped down to 2" before the camera I had considerably more vignetting from the light cone as well as reflections off the walls of the 2" focuser tube. Keeping things wide prevents anything hitting something and reflecting into the camera. I still can get a reflection from the mounting hole for the 2" filters which can be a pain. As I mentioned before vignetting is only part of the problem!

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5540538 - 11/26/12 03:37 PM

Ok thx.

So the configuration for this large CCD would be:

-LX200(12")
-3.25" rear cell thread
-monster OAG (or any other large OAG)
-FLI CFW-5-7 Filter Wheel
-FLI PDF (or any other large focuser)
-PL09000 camera

This should be fine, if I'm not mistaken.
How can I calculate if I'm going to run out of backfocus? I just want to be sure, even thought I know that SCT have the most focus of all.


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Rick J
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Migelo]
      #5541415 - 11/27/12 12:50 AM

Unless you are using 3" filters they shouldn't come before the focuser. Also putting the focuser after the OAG means as you need to refocus the camera you'll need to refocus the guider as well.

Put the focuser first, then the mega MOAG then the filterwheel then camera.

Unless your main mirror is totally locked like mine why not use the normal focuser. I know several using focusmax with a RoboFocus unit on the moving mirror focuser with OAG and getting excellent results. It approaches focus from only one direction so backlash isn't an issue and the OAG compensates for all mirror slop.

I went my route simply because I don't guide at all unless imaging outside of the area of my TPoint map. That made the totally rigid mirror a necessity. If I were guiding I'd have not locked the mirror and let the far cheaper RoboFocus unit alone handle focusing.

Rick


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Migelo
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Re: Guiding Schmidt–Cassegrain 300/3000 new [Re: Rick J]
      #5542340 - 11/27/12 03:30 PM

Thanks for the positioning tip! Our filters are square 50mm so thanks again.

I just found out that FLI PDF is not made to work with OAG. (fli support told me that).

So you say RoboFocus will do a good job? (we're not going to have a totaly locked mirror on the SCT)


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