OAG or guidescope??

[simon84]

Which one to use. I just dont really know. I've read plenty of articles on each one but no one seems to say if one is better than the other.

Looking at it cost wise a decent OAG is probably more expensive than a decent guide scope.

Weight wise I could quite happily use a guide scope as I only have the 80ED+camera on an EQ6.

So has anyone here tried both methods?

[ottUp]

So has anyone here tried both methods?

Yes. Both have their place and both can be made to work well. In your situation, I would choose a guide scope - because you have a mount that can handle it and are using a short focal length imaging scope. A guide scope has several advantages when you can use it:

• Can be pointed independently, so far easier to find guide star
  
• Can be focused independently, simplifies optical chain
  
• Keeps main imaging chain to camera shorter, easier to keep all the bits collimated
  
• Extra weight is on RA axis, generally easier to counterbalance than extra weight on DEC axis that results from making the imaging chain longer.
  

I use a guide scope at shorter focal lengths (300 - 1200mm) and an OAG when imaging at higher focal lengths (1500 - 2400 mm) because guide scopes also have some disadvantages

• Differential flexure: minor shifts of the guide scope in its mount can spoil guiding at high magnifications (not a problem with secure mounting at short focal lengths such as your ED80, any shifting will be too small to affect the image)
  
• OAG has guide camera seeing similar focal length to main camera, allowing guide errors to be detected more quickly
  

- Richard

[Dangerous-Dave]

I've tried both and found the OAG option difficult to implement. My main issue was getting the guide cam and imaging cam parfocal. Admittedly I was using a cheap Chinese version, designed for film use, and modern OAGs have adjusters to make this easier, but you would still be left with the issues of star shape and limited choice of guidestars. If you do a lot of long exposure imaging through a SCT I would go down the OAG route, to avoid mirror flop problems, otherwise stick to a guidescope as it is a lot easier to get right, especially if you are a beginner.

[BlueAstra]

For interest, this was my experience with an OAG:

http://stargazerslounge.com/equipment-discussion/90916-orion-oag-first-light.html

I have an ED80 piggy-backed on the C8 on an EQ6. The OAG is on the C8 and the Canon 1000d goes on either the C8/OAG or the ED80. A daylight setup was essential for par-focus setup. Then at night focussing the camera automatically gets the OAG in focus.

[Psychobilly]

The TS one is a nightmare to get fine focus adjustment .. some people are resorting to using Helical focusers to overcome its shortcomings... which adds yet more money to what is already a very expensive OAG...unfortunately its the only slimine OAG solution out there so for some applications your almost "driven" into using it

I have both setups available using a 500mm f8 telephoto lens with a DSI IIc or a DSI Ic on the 8" with the OAG......

Peter...

[MartinB]

I use both. The pros and cons have been well summed up. If you are going to use a seperate guide scope you should seriously over engineer the hardware, especially as you increase focal length. Don't rely on pushing and pulling your scope to decide whether the set up is stiff enough. OAG set ups are lighter and neater but have more complexity.

[steppenwolf]

I have used both as well and the pro v con info. above is in line with my own findings. I much prefer ta guide 'scope though as the limitations of an OAG can make spoil your mood for an evening's imaging. The idea of an OAG is great, especially for avoiding differential flexure (DF) - although they are NOT immune! - but they are such a pain to set up! When someone brings one out that has sensible helical focussing on it at a sensible price, I might be tempted to have another go as I do suffer from DF but I'd still have to contend with a very narrow selection of guide stars and life is just too short!

[MartinB]

Worth bearing in mind that guide star focusing doesn't need to be that precise, a slightly defocused guide star will give a very good, if not better, centroid. When using OAG with my 10"LX200ACF I am generally working at 1700-1800mm focal length. Even with my Lodestar that isn't a big FOV. That can be a problem when attempting to use active optics (which needs very rapid guidestar exposures, preferably 0.1 secs or less) however with conventional OAG it isn't such a huge issue if your mount will allow 3-4 second guide star exposures. A bit of preparation can help as well. Once you know the guide chip off set it is relatively easy to find a suitable guidestar in advance using planetarium software and FOV indicators (The Sky is brilliant in this respect showing your main and OAG guide chip in a single, rotatable FOV indicator).