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MtnGoat
Carpal Tunnel
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Reged: 02/18/07

Loc: Columbia Gorge, WA
LX200 10": Focal Reducer questions
      #5906581 - 06/06/13 06:24 PM Attachment (25 downloads)

Hi folks,
I recently aquired a pretty decent LX200 10" OTA for a darned good price, and and now working on the bits and pieces necessary for a working instrument covering most of the basics, IE some eyeball astronomy and some photography. I opened her up and cleaned some lube looking gunk off the secondary (I think it was stored upside down somewhere), and picked up a set of Bob's knobs (heavenly!), and just ordered the Peterson's focus fixer kit. Years ago I'd picked up a set of rings at a yard sale for $10 (weird, just rings nothing else) and they looked like they'd fit a 10" SCT. And now, wonder of wonders, I ran across one I could afford, and even crazier, they did fit!

I'm feeling pretty lucky actually because other than the crud coming right off the secondary first go no issues, it's in very nice shape and has extremely minimal focus shift. The other night I was out having a go at Saturn and even with a 4x barlow in front of the imager, the frame filling planet barely shifted at all with focus direction. Pretty sweet. This is my first big SCT after years with refractors of various sizes and tons of reflectors, I definitely appreciate 10" of high FL horsepower in a compact package.

Which brings me to my issue, I'm well up on the imaging learning curve after a few years with all my other toys so I'm good there, but after being spoiled with a bunch of low F ratio reflectors and schmidt newts, now I'm wondering the best route to tame the F10 on this puppy.

Ideally I'd like to hit at least F5 to match the other 10" I've got in the lineup, but since this is my first SCT I have no experience with focal reducers. In doing some reading I'm finding out the ratio is variable with spacing, and also that there are a couple routes to go.

My primary imager has been a DSI with the 1.25 format and it's been a fun gizmo I've got some good shots with. I've found a number of 0.5x reducers at Agena and Astronomics, and it's my understanding you screw these right onto the front of the imager, with exact reduction dependent on the spacing.

And, in the recent past I picked up a Canon DSLR which I've used on my SN6 and also with it's native lens, with great results, so I'm kinda itching to let it run some more deep sky with the new SCT.

In that vein, there are the larger reducers which presumably fit into the visual back (?), or the front of a diagonal or extender...are they threaded, or plug in using an OD/ID fit like an extension tube? For these I'm refering to Meades and Celestron's official 'focal reducer' devices. I'm guessing you do NOT use the diagonal during photography, I got a 2" diag and visual back with the scope.

SO basically, i'd appreciate a short what's what and some ideas on what makes the most sense from the perspective of some folks who've used them.

thanks in advance...

Edited by MtnGoat (06/06/13 06:32 PM)


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P26
Vendor - Peterson Engineering


Reged: 05/02/12

Loc: RI, USA
Re: LX200 10": Focal Reducer questions new [Re: MtnGoat]
      #5906832 - 06/06/13 08:57 PM

The 10" tube is a wonderful piece of optics. But sizing cameras to the scope can get tricky.

It's best to think of a focal reducer as an image size reducer. In other words, if you have a fully illuminated 1.5 inch diameter image at the focal plane, an f/5 reducer will result in a 3/4 inch diameter image.

To get the size of the fully illuminated (non vignetted)circle, multiply the clear aperture times the compression ratio, 1.5" X 0.50 = 0.75

If you were working with a small imaging chip this is great as you're now fitting all the available light onto the chip. But chips in DSLR cameras tend to be relatively large and at some point you'll encounter vignetting. Doc G did a write-up on vignetting more than 10 years ago and gave me permission to use it at:
http://www.petersonengineering.com/sky/vignetting.htm

With some focal reducers you can vary the amount of compression by shifting the distance of the optic to the camera chip without incurring optical distortion. And with such a focal reducer Compression ratio = CR = (FL - D)/FL (where FL is the focal length of the focal reducer and D is the distance of the focal reducer from the imaging chip).

My personal setup is documented at
http://www.petersonengineering.com/sky/ccd_setup.htm

Focal reducers can be tricky and it's recommended that you do a few calculations before buying. For my personal setup the CCD camera was actually sized for the telescope to achieve both maximum field of view without vignetting and the Nyquest sampling theory optimum of 2 arc-seconds field per pixel.

Clear skies,

Pete


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MtnGoat
Carpal Tunnel
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Reged: 02/18/07

Loc: Columbia Gorge, WA
Re: LX200 10": Focal Reducer questions new [Re: P26]
      #5907053 - 06/06/13 10:35 PM

Thanks for the handy details! It sounds like unless there's some fundamental issue, if I go with the 1.25" screw on reducers for the DSI I'll have not much to worry about, and the vignetting issue comes into play with the larger chip of the Canon. I'll read both articles.

I'm looking forwards to seeing how that focuser upgrade works out.

Edited by MtnGoat (06/06/13 11:31 PM)


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MtnGoat
Carpal Tunnel
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Reged: 02/18/07

Loc: Columbia Gorge, WA
Re: LX200 10": Focal Reducer questions new [Re: MtnGoat]
      #5908530 - 06/07/13 06:35 PM

After doing the reading and some thinking, I ordered up a 0.5x 1.25 reducer which should thread onto my DSI setup. The vignetting and reduction calculations don't appear to be necessary for this application, and it gets me rolling cheap and fast.

I'm still interested in applying the Canon here though, and will work through some of the math presented to get a look at how to approach this. Getting the simple reducer seemed the fastest way to have some fun with the least problems while I suss out the more advanced options.


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Geo.
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Reged: 10/01/08

Loc: Upstate NY
Re: LX200 10": Focal Reducer questions new [Re: MtnGoat]
      #5914282 - 06/10/13 11:14 PM

I like the old Lumicon Giant Easy Guider with the 3" f/5 reducer. I can put the slide rule away.

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MtnGoat
Carpal Tunnel
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Reged: 02/18/07

Loc: Columbia Gorge, WA
Re: LX200 10": Focal Reducer questions new [Re: Geo.]
      #5926273 - 06/17/13 08:21 PM

Interesting, I'll look into that.

Well, I got the 0.5x reducer but haven't tried it yet.

I also got my focuser kit, which has me a bit fussed...I went to install it and found that this OTA had already been upgraded! So a bit bummed it is as smooth as it's going to get, but I had wondered why it was already better than stock. It's because it wasn't! So I put the sticker on it and sold the rest of the kit to a buddy.


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mtnmedic
Late for dinner
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Reged: 11/09/04

Loc: Antelope, Oregon
Re: LX200 10": Focal Reducer questions new [Re: MtnGoat]
      #5928381 - 06/18/13 10:45 PM

Good thread. I have a 10" LX 200 Classic that I've been wanting to get a decent focal corrector for. I too have a Canon DSLR (though a lowly 450D) that I want to improve imaging with until I get a reasonably affordable CCD.

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MtnGoat
Carpal Tunnel
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Reged: 02/18/07

Loc: Columbia Gorge, WA
Re: LX200 10": Focal Reducer questions new [Re: mtnmedic]
      #5929520 - 06/19/13 03:19 PM Attachment (12 downloads)

Once I actually sit down to do some of the math linked too I'll post up my thoughts on it.

I was down in your neck of the woods last weekend. Went to Mitchell, prowled around for a few days. Incredible country!


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mtnmedic
Late for dinner
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Reged: 11/09/04

Loc: Antelope, Oregon
Re: LX200 10": Focal Reducer questions new [Re: MtnGoat]
      #5938725 - 06/24/13 09:20 PM

Quote:

Once I actually sit down to do some of the math linked too I'll post up my thoughts on it.

I was down in your neck of the woods last weekend. Went to Mitchell, prowled around for a few days. Incredible country!




We were out of town in Clackamas (near Portland) for the Oregon Volunteer Firefighters Association conference at the time. The conference put us up in a motel that was right by the 205 freeway (room overlooked it). Plenty of noise, lights and all sorts of other things that made the big city experience exactly that. Her allergies acted up and I got a head cold.

We've been discussing heading into the Mitchell area to do just the same thing you did, for a couple of days anyway. There's a piece of land for sale we want to casually look at out there, as well. It's very similar here only darker still over there. Can't get much better than that.

Can't wait to go to the Oregon Star Party coming up...


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