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rpg850

Guide Scope and Camera?

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My dilemma. I want try auto-guiding a wedge-mounted 8' SCT using PHD. I realize that an EQ mount is what I really need, but the budget doesn't currently allow for it. I have the Celestron Travel Scope 70 and a Celestron NexImage 5 web-camera. I do NOT have the tube rings or dovetail to mount the travel scope to the SCT. 

By the time I purchase a dovetail and tube rings to attach my travel scope, I could just as easily buy the Orion Mini 50mm Guide Scope. 

Which would be better for my 8" SCT: Travel Scope 70, or Orion Mini? 

Main Scope:

  Aperture:          203.2mm

  Focal Length:   2032mm (or 1280mm with focal reducer)

  Focal Ratio:      10 (or 6.3 with focal reducer)

Travel Scope 70

  Aperture:

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Well, I somehow accidentally posted before I was finished... continuing on...

Travel Scope 70

  Aperture:             70mm

  Focal Length:      400mm

  Focal Ratio:         5.71

Orion Mini Guide Scope

  Aperture:            50mm

  Focal Length:     162mm

  Focal Ratio:        3.2

So, which is better guiding a 8' SCT? Keep in mind that I'm using a NexImage 5 web-camera as my guide camera.

Any help would be greatly appreciated.

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I use an Orion and a Zhumell  9x50 162mm guidescope with my C8 with great success but this is with a CEM style mount.

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Hi,

I have an 8" fork mounted SCT on a wedge, and because of the long focal length, it is a must to use the f6.3 reducer, I also use a William optics ZS66 388mm refractor piggybacked to use as a guide scope also for wide field imaging with either my Atik 414ex CCD or my Canon 1100D DSL, obviously a lot wider field with the canon.

And it all works very well, and have had guided subs of 15mins, with nice round stars, the secret is, and I can't stress this enough, is having perfect balance of the rig, with a good quality weight balance system, if you don't do this then rest assured it will not guide and you could seriously damage the gears of the mount.

That said I know a lot of people use a small 50mm finder guider and that works very well too, but I have always though that a 180mm focal length guide scope is too small to be guiding a 2000mm focal length imaging scope.....but who am I to judge, people are getting very good results with that set up.

The choice is yours.....

AB

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One bit of good news is that guide rings are entirely redundant and have been for many years. They're very expensive, too. You no longer need to search for a guide star with modern guide cameras. This means that any improvised but rigid attachment of the guidescope will do the job.

Around the front and rear of the SCT there are the small 'accessory screws' (probably holding your DSLR attachment in the picture you've posted.) All you need is to run a stiff metal strip, alloy for lightness, from front to back, raised a little from the tube and drilled to take your guidescope tube rings. I often use a Vixen dovetail for this purpose. Nice and stiff.

The ideal option for guiding an SCT is an off axis guider. This will react to mirror movement which a guidescope won't and it means that the long focal length of the imager will also be used by the guider. While I've read on here that a finderguider will work with an SCT I agree with AB that guiding with such a mismatch of focal lengths is asking for trouble.

Olly

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One bit of good news is that guide rings are entirely redundant and have been for many years. They're very expensive, too. You no longer need to search for a guide star with modern guide cameras. This means that any improvised but rigid attachment of the guidescope will do the job.

Around the front and rear of the SCT there are the small 'accessory screws' (probably holding your DSLR attachment in the picture you've posted.) All you need is to run a stiff metal strip, alloy for lightness, from front to back, raised a little from the tube and drilled to take your guidescope tube rings. I often use a Vixen dovetail for this purpose. Nice and stiff.

The ideal option for guiding an SCT is an off axis guider. This will react to mirror movement which a guidescope won't and it means that the long focal length of the imager will also be used by the guider. While I've read on here that a finderguider will work with an SCT I agree with AB that guiding with such a mismatch of focal lengths is asking for trouble.

Olly

I agree, bit don't forget to add balance weights if you go for the piggybacked guide scope, believe me you have to match the weight from the kit you add on one side of the scope to weights on the other, like I said before if you don't, you will be, again to quote Olly, "asking for trouble" and potentially damage your mounts gears....I nearly found out the hard way, so I would rather pass on that info before someone lease does the same.

On the plus side if you do that, then it does guide and track very nicely, it's never going to be on par with a top quality equatorial mount, but it is fine for getting into DSO imaging, and is superb for planetary.

AB

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