Guide Scope or Off Axis Guider

[centroid]

Back in my previous astro imaging days (pre 2014), I initially used a separate guide scope and an SX Lodestar, but switched over to using an Off Axis Guider (OAG). This worked very well, and I never went back to using a guide scope.

Now, In many of the pictures of peoples setups that Isee, its nearly always a separate guide scope being used. Have OAGs gone gone out of favour?

What do you use, Guide Scsope or OAG, and what camera. I always used an SX Lodestar, which worked very well indeed.

There are a lot of CMOS guide cams on the market now, which in general, are cheaper than the Lodestar.

I ask because having now returned now to astro imaging, I need to decide which direction to go in.

[malc-c]

Personally I don't think it's going out of favour... just that a lot of those getting into imaging for the first time see all the images with secondary scopes added and assume that is the "standard" way to guide.  Sometimes it can be budgetary.  I started with the good old tried and tested ST80 as a guidescope, but wanted to reduce the weight of the rig.  I looked at an OAG for my canon D400 and 200P combo, and needed to get one of the very slim offerings form Germany...The cost was three figures, so I kept putting that off, and then found a small cheap (sub £30) conversion ring that would allow the camera I have to attach to standard SW 9 x 50 finder, and that works for me equally as well.  I would like to try OAG, but when money is tight its a lot to shell out, especially if there would be little gained.

[centroid]

8 minutes ago, malc-c said:

Personally I don't think it's going out of favour... just that a lot of those getting into imaging for the first time see all the images with secondary scopes added and assume that is the "standard" way to guide.  Sometimes it can be budgetary.  I started with the good old tried and tested ST80 as a guidescope, but wanted to reduce the weight of the rig.  I looked at an OAG for my canon D400 and 200P combo, and needed to get one of the very slim offerings form Germany...The cost was three figures, so I kept putting that off, and then found a small cheap (sub £30) conversion ring that would allow the camera I have to attach to standard SW 9 x 50 finder, and that works for me equally as well.  I would like to try OAG, but when money is tight its a lot to shell out, especially if there would be little gained.

I found that using an OAG was a very convenient, and effective way of guiding. My last set up was an 8" Meade ACF SCT, and a WO 110mm Triplet refractor, mounted side-by-side on an NEQ6. This would have meant adding, say a 60mm guide scope, or using the SCT as a guide scope for the 110, or the 110 as a guide scope for the SCT, quite some 'overkill', and expensive guide scopes 🙂. So, the OAG was the perfect solution. I note that FLO has one at £129 (https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html), or an ZWO one on special offer.  I now have a 115mm Triplet, and an HEQ5 Pro syn scan mount, and don't really want to mount 60mm guide scope on top. Especially as unlike the days when I had a permanent setup in an observatory, I now have to deploy the gear in the garden, when I want to use it.

[vlaiv]

Had 60mm F/4 guide scope and eventually sold it in favor of OAG. It costed about the same - maybe even a bit less OAG and I now exclusively use that for main imaging scopes.

I still have one tiny 32mm guide scope to use with lens and AzGTI in EQ mode - simply because OAG can't fit in that combination.

It is important to do a bit of calculations for OAG. Prisms are rather small and when people use them with fast scopes - this can lead to OAG prism being aperture stop for guide camera. Prism is 8mm and that means one needs to place OAG fairly close on fast systems. Say you image with F/6 scope - 8mm x 6 = 48mm. Prism needs to be closer than 48mm to imaging sensor in order to avoid it being aperture stop for guide sensor.

Another thing is guide resolution - in most cases you don't need that much resolution as OAG provides and you can safely bin x2 to improve SNR on guide star.

[centroid]

As I said earlier Vlaiv, an OAG was my preferred method of guiding, so that is the way I will be heading again, I think. I had to do the spacing calculations with my previous set up which included a filter wheel. Albeit the OAG used to sit ahead of the filter wheel. I will be imaging mainly at the scopes native f/7, but may on occasions add 0.8x reducer, I'll see how that works out.  Going with a colour camera this time, for convenience, so no filter wheel.

Edited August 21, 2021 by centroid

[newbie alert]

Depends what youre trying to guide,fl wise..id say up around the 1000mm fl and beyond id go oag.. cant see a benefit from a shorter fl

Personally i use a oag on my sct (1318mmm) and a separate guidescope on my esprit 80(400mm)

[centroid]

23 minutes ago, newbie alert said:

Depends what youre trying to guide,fl wise..id say up around the 1000mm fl and beyond id go oag.. cant see a benefit from a shorter fl

Personally i use a oag on my sct (1318mmm) and a separate guidescope on my esprit 80(400mm)

Convenience really, neat and tidy setup, no need to buy a guide scope plus dovetail bar, and 'piggy back' on the 115.

[david_taurus83]

OAG is best. Best guiding, no flexure, image drift etc. 

[centroid]

12 minutes ago, david_taurus83 said:

OAG is best. Best guiding, no flexure, image drift etc. 

Yep, that's how I found it 🙂

[Craig a]

I agree with above comments, I think OAG is a must with any scope with mirrors, I had issues with stars in my images with a guidescope on my 1000mm f4 Newtonian switched to OAG and boom all sorted, differential flexure was eliminated, I’d never go back to a guidescope again