JSnuff1
super member
Reged: 12/29/04
Posts: 198
Loc: NY
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Im in the marker for a guidscope but I am confused as to what to buy because I dont know how to calulate power with imagers. Normaly you divide the focal length by the eyepeice, but there is no EP on the scope when imaging, just a camera. I have a 10inch Lx200 (2500mm FL) so what kind of magnification does this provide when a camera is attached to it? The guidescope will need to have at least twice the magnification to be accurate correct? Lastly what kind of guidscopes are other people with this telescope using? Thanks much.
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William Optics 110 FLT (TEC version)
SBIG ST-4000XCM
Gemini GM-8
Digital Rebel
JGS Observatory Website
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Nodda Duma
professor emeritus
Reged: 11/02/04
Posts: 511
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Magnification is kind of meaningless when imaging because you can zoom in or out as you please when you look at the picture on the computer..what you are really interested when imaging is the field of view (FOV) that your camera sees.
FOV = 2*arctan[d / (2*FL)]
Where
d = width of your camera's image plane
FL = focal length of scope
as an example, 35mm film has dimensions of 36mm x 24mm. This will of course give you two different fields of view, one for the long dimension and one for the short dimension. Therefore, doing the calculation for your 2500mm scope...a 35mm frame will image a field of view of 0.825 x 0.55 degrees at 2500mm focal length. You'll want to look at the specs for your particular camera to figure out how big your frame is (it will be smaller than film if you have a digital imager).
I can't comment on the rule of thumb about the focal length of the guidescope vs. the focal length of the imaging scope. I can say that I use an off-axis guider and thus do all my guiding at the same focal length that I'm imaging at. This gives acceptable pictures, so I think guiding at twice the focal length is probably too stringent. I think it really depends on how you are guiding (manually or auto), and if autoguiding..how good the guiding algorithms are.
Hope this helps,
Jason
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TeamGS
Post Laureate
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Reged: 01/20/04
Posts: 3073
Loc: Elk Grove, CA
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The easiest way is to just look at the focal lengths of the scopes. Don't worry about EP's as none are going to be used on either scope when imaging. While a longer fl guidescope is preferred, it doesn't need to be double the focal length of the imaging scope, though that wouldn't necessarily hurt. I haven't seen any hard and fast rules anymore regarding this issue, and there are many people guiding with scopes that are only half the fl of the imaging scope. The new software packages out there are capable of sub-pixel accuracy.
Most likely, you won't be imaging with your scope at f/10, due to the difficulties related to the mount's accuracy. You will really want a focal reducer; either an f/6.3 or f/3.3 from Meade or Celestron will be fine for starters. With the f/6.3, your fl will be 1575mm. I would think that a guidescope with an fl > 750mm would suffice. You can always add a barlow if desired. I use the Orion 80mm f/11.3 guidescope package (fl=900mm)for my 8" LX200 @ f/6.3. (fl=1260mm)
Are you planning to use the N6006 as your imaging camera? If so, you won't want the Meade/Celestron f/3.3 reducer, as they don't work well with large format cameras due to coma at the outer edges.
Hope this helps!
Regards,
Gary
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Celestron 80ED
Losmandy G11
NexStar 80
Starlight Xpress SXV-H9
SXV guidehead, ToUcam 840
http://www.teamgs.org/astrophotography.htm
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2821
Loc: Fayetteville, NC
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You have a few choices when it comes to guiding esp with an lx200
you can use an off axis guider- with an eyepiece or auto guider like a 201xt.
you can add a small guide-scope on top and use an eyepiece or camera, 201xt, or lpi st-4, webcam etc.
I tend to lean toward the latter method as manual guiding would be tiring, and off axis guiders can be tricky to get a guide star in the field...
just food for thought
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group scope pic
my refractors
LX200 10"-St120
LX200 8" f6.3-Orion 80ed
LX200 8" f10-Orion 90 Mak setup pic
Meade 102ED LXD650
Sky Watcher 100 ED Triplet prototype
Nexstar 5
etx125
etx70's
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JSnuff1
super member
Reged: 12/29/04
Posts: 198
Loc: NY
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TeamGS, I cant understand why a guidescope with a smaller FL than your imaging scope will be of any help. The guide scope must 'see' the image shift before the imaging scope does, or else the unguided picture will be imaged before the correction is made.
In response to the other questions, im planning on using a Meade DSI Pro or a digital rebel for imaging, and im looking for a autoguider solution.
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William Optics 110 FLT (TEC version)
SBIG ST-4000XCM
Gemini GM-8
Digital Rebel
JGS Observatory Website
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Shoestring
Vendor (Shoestring Astronomy)
Reged: 04/01/05
Posts: 61
Loc: central Iowa
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Webcam autoguiding works great, and assuming you already own a PC, the cost is low. The cost is even lower if you already own a webcam. I use GuideDog from www.barkosoftware.com.
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Doug Anderson
Shoestring Astronomy
www.ShoestringAstronomy.com
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Nodda Duma
professor emeritus
Reged: 11/02/04
Posts: 511
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The guide scope doesn't have to "see" the image shift before the scope does, it simply has to make corrections smaller than the size of your star images. This is an autoguider accuracy issue. Star image size will be a function of atmospheric turbulence more than anything else. TeamGS seems to indicate that current autoguiding technology allows a guidescope of about half the focal length of your imaging scope. This sounds about right, although I believe SBIG's STV autoguider is even more accurate.
You won't want to autoguide at too long a focal length because turbulence will have the guide star dancing around so much that it will be almost impossible to guide on it.
Cheers,
Jason
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Jason
Website
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2821
Loc: Fayetteville, NC
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the low focal length might allow for more guide stars to be in the field of view. .
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group scope pic
my refractors
LX200 10"-St120
LX200 8" f6.3-Orion 80ed
LX200 8" f10-Orion 90 Mak setup pic
Meade 102ED LXD650
Sky Watcher 100 ED Triplet prototype
Nexstar 5
etx125
etx70's
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2133
Loc: Newark, CA
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re: guidescope focal length
Note also that, depending upon the relative sizes of your imaging camera "chip" and your guide camera chip, you'll probably be getting a lot tighter FOV (more "magnification") from your guide chip than from your imager...
Jimbo
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Order of the Unblinking Eye
G-11 D70 ST4 8"f/5 AT66ED (Rig)
I used to shoot film.
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TeamGS
Post Laureate
Reged: 01/20/04
Posts: 3073
Loc: Elk Grove, CA
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I had a link that explained it better than I would be able, but with the new software and CCD's, you really don't need double the fl on the guidescope. Trust me, there are extremely few imagers using and SCT at, say, 1100mm, using piggybacked guidescopes at 2200mm. For an 80mm refractor, that would be f/28.
Regards,
Gary
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Celestron 80ED
Losmandy G11
NexStar 80
Starlight Xpress SXV-H9
SXV guidehead, ToUcam 840
http://www.teamgs.org/astrophotography.htm
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jay52
Carpal Tunnel
Reged: 09/26/04
Posts: 2280
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Quote:
The guide scope doesn't have to "see" the image shift before the scope does, it simply has to make corrections smaller than the size of your star images. This is an autoguider accuracy issue. Star image size will be a function of atmospheric turbulence more than anything else. TeamGS seems to indicate that current autoguiding technology allows a guidescope of about half the focal length of your imaging scope. This sounds about right, although I believe SBIG's STV autoguider is even more accurate.
You won't want to autoguide at too long a focal length because turbulence will have the guide star dancing around so much that it will be almost impossible to guide on it.
Cheers,
Jason
Ditto...well said.
The acceptable rule of thumb is that the guidescope should be at least half the focal length of the imaging scope. But in the days of accurate autoguiders that can guide on the centroid of a star, you can get by with much less. In fact, I've known STV owners who have successfully autoguided with their 100mm efinder attachments.
My recommendation for a guidescope is a small apo refractor. They barlow very well for a better image scale while autoguiding, plus they can be used as a secondary imaging scope.
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jay
www.allaboutastro.com
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jay52
Carpal Tunnel
Reged: 09/26/04
Posts: 2280
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BTW, I often autoguide with the guidechip binned 2x2 with the Tak 106 and 3x3 with the 12.5" RCOS. This essentially reduces the focal length of the instrument in relation to the guidechip (enlarges the image scale). To guide when unbinned will often "chase" the seeing. There's a happy medium to be reached...and having too much focal length for your guidescope is perhaps worse than having too little.
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jay
www.allaboutastro.com
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JSnuff1
super member
Reged: 12/29/04
Posts: 198
Loc: NY
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Thanks for the feedback guys...Ive been looking at some of the scopes that were mentioned but they seem to be in the 500+ price range...i just burned 3000 on the LX I cant spend another 500 on just a guidescope. Can anyone recommend some good guidscopes in the sub 500 price range? Thanks again.
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William Optics 110 FLT (TEC version)
SBIG ST-4000XCM
Gemini GM-8
Digital Rebel
JGS Observatory Website
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Paul Rix
Carpal Tunnel
Reged: 11/06/04
Posts: 2924
Loc: Zanesville, Ohio,USA
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Could a regular finderscope that accepts 1.25in eyepieces be used as a start?
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Climbing the Learning Curve
Meade 10" LX200 Classic: WO 66SD.
Philips SPC900NC, DMK21AU04.AS, Meade DSI Pro and DSI Pro II.
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TeamGS
Post Laureate
Reged: 01/20/04
Posts: 3073
Loc: Elk Grove, CA
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Well, any decent achro refractor in the 60-80mm range could work. The most important parts, IMO, are the mounting rings, and the ability to lock the focuser at the point of focus. Orion makes an 80mm guidescope package for around $300. You could also get some Losmandy rings and a plate for around $200, then find a used refractor for around $100. I just saw 4 refractors on Astromart that might work, for around $130.
Regards,
Gary
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Celestron 80ED
Losmandy G11
NexStar 80
Starlight Xpress SXV-H9
SXV guidehead, ToUcam 840
http://www.teamgs.org/astrophotography.htm
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Astrosetz
sage
Reged: 10/05/03
Posts: 294
Loc: Wisconsin
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How about a "shorttube 80" with a 2x Barlow? That would be 800mm fl.
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-Astrosetz
www.astrosetz.com
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2821
Loc: Fayetteville, NC
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also think about a counter balance system
the orion is a good package as it comes with a illuminated eyepiece. but there is deal on those around.
--------------------
group scope pic
my refractors
LX200 10"-St120
LX200 8" f6.3-Orion 80ed
LX200 8" f10-Orion 90 Mak setup pic
Meade 102ED LXD650
Sky Watcher 100 ED Triplet prototype
Nexstar 5
etx125
etx70's
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c131frdave
Post Laureate
Reged: 01/17/05
Posts: 4373
Loc: Arcanum, Ohio
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Quote:
BTW, I often autoguide with the guidechip binned 2x2 with the Tak 106 and 3x3 with the 12.5" RCOS...
Man. It's good to be Jay. 
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Tak NJP
SXV-H9
Various sizes and shapes of formed glass
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jay52
Carpal Tunnel
Reged: 09/26/04
Posts: 2280
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Quote:
Quote:
BTW, I often autoguide with the guidechip binned 2x2 with the Tak 106 and 3x3 with the 12.5" RCOS...
Man. It's good to be Jay.
LOL! Sorry, Dave...didn't mean to sound like I was bragging. Perhaps I'll just say 4" refractor and 12.5" reflector for now on! 
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jay
www.allaboutastro.com
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TeamGS
Post Laureate
Reged: 01/20/04
Posts: 3073
Loc: Elk Grove, CA
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Quote:
LOL! Sorry, Dave...didn't mean to sound like I was bragging. Perhaps I'll just say 4" refractor and 12.5" reflector for now on!
I don't know if I'd be bragging, if all I had was a cheesy Tak 106 for a guide scope. Now if you had an EBay 50mm f/15 Super Achro, THEN you would have something worthy of crowing about. 
Regards,
Gary
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Celestron 80ED
Losmandy G11
NexStar 80
Starlight Xpress SXV-H9
SXV guidehead, ToUcam 840
http://www.teamgs.org/astrophotography.htm
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