OAG vs guidescope at 700mm FL

Golden rule for guiding error:
You guiding error should be at or below your primary image scale.

Those numbers do seem a little high. Despite that, are your sub stars round? Have you tried changes in aggressiveness and guide scope exposure time?

A note of comparison:

I use a conventional Askar guidescope on a 9.25 EDGE-HD, which is 2350mm. Guiding is between .30 and .70. I've seen the number for a whole sub as low as .20…so most of the larger variation is coming from the atmosphere above. Not much that can be done about that without going to adaptive optics, which have their own issues.

The 925's main scope image scale is a theoretical .335…and most of my subs are close, averaging around .45.  Good enough for me.

I would think an OAG might not lead to a better image for you.  At 700mm, your image scale is likely over 1.0. Your guiding numbers are usually at or below that.

It's still worth seeing if lower numbers are possible. The images might benefit, and your expertise in optimizing your gear will grow.  Good luck tracking it down!

So… There's all these rules and what not that people talk about about how you should always guide at at least 1/5th of the imaging scopes resolution etc.. But do you notice there is no maximum? That's because the more resolution your guider has, the sooner your guiding system can detect star movement, and then correct for them…

I use an OAG on all my set ups…

65PHQ + 0.75x reducer (312mm focal length)
65PHQ (416mm focal length)
120APO (840mm focal length)

and in the past, on my 8" RC @ 1634mm focal length and the C11 @ 2800mm focal length.

I currently image with an IMX294 sensor, that has 4.63um pixels, and guide with an IMX178 sensor, that has 2.4um pixels.

So on the Askar 120APO, I'm imaging at 1.14"/px, and guiding at 0.59"/px. I'm regularly guiding at or near 0.2" RMS, and on the other extreme, the 65PHQ at 312mm focal length is imaging at 3.05"/px and guiding at 1.49"/px… 

Because my guiding system is essentially twice as sensitive to movement as my imaging system, I never have issues with guide error… and an error would have to be MASSIVE.. as in, the guide star would need to move 2 full pixels on the guide sensor for my imaging sensor to know about it… 


The ONLY issue that can arise from having a super high resolution guiding system, is that seeing effects can be magnified… Running 1~1.5s guide exposures and using multi-star guiding can help to alleviate that, or in the worst conditions, I will bin my guide camera 2x2 and then be guiding at about the same resolution as im imaging, thus, negating some of the 'seeing induced error'

In my experience - Always use an OAG and save yourself a ton of struggles. Once you're used to using them, they are easy as pie, and I won't guide any other way.

I have a 50mm ED Doublet guidescope on my Tak FSQ106 which is 530mm focal length. works good.

I also have a 10 inch RC, which is 1484mm (reduced) and guide the RC with an 80mm Stellarvue Doublet which is 560mm focal length.

both seem to work fine.  I might replace the 80mm Stellarvue with something a bit smaller and lighter just to drop the weight a bit.

anyway if something works ,it works, and thats all that matters.

My limited experience with an OAG is remarkable.  I didn’t use the ZWO oag and I used the Askar OAG and Filter which gave me precisely 55mm.  I am using the Asi120 and my stars are tack sharp.  My RMS dropped from 1.0-1.2 to .3 to .5 .  The mirror is huge and does not cover the main sensor.  I have an unused ASI224 that has larger sensor area I am going to try on it.  That’s what makes this superior to the Zwo OAG is the real estate of the mirror is larger down the main tube.  The price is better when you figure you are getting a quality precision focuser with the OAG and the ZWO-L you have to buy it.  Got a spare Williams Optics  guide scope now.  Your mileage may vary.

What is your current setup? Main camera, guide camera, guide scope?

0.8-1.2" RMS guiding is probably a little high, but it may be within the acceptable limits of your setup.

If you're still not happy with it, you can adjust the guide settings to improve the accuracy. I had to do this to get from ~1.5" RMS average down to ~0.5" RMS average. It was a bit tedious and frustrating, but worth the effort.

I created a dark library for the guide camera and played around with the RA & Dec aggressiveness and steps until I settled on a good guiding number. Also, I found my guiding is better with 1.5s - 2s exposures vs 1s — too much turbulence at 1s. 

For reference, I'm using an 80mm f/6 main scope, 50mm f/4 guide scope, ASI2600MC main camera, and ASI120MM guide camera (both 3.76 µm pixels).

My main camera resolution is 1.62"/px, so my error shouldn't exceed that, which is how I knew bouncing around 1.5" RMS (and often over 1.62") wasn't good guiding.

You can go the OAG route and that will help the accuracy, but you will probably still need to optimize your guide settings regardless.

Tony Carro:
Forgot to mention that I used to do 5 minute subs and now doing 10 minutes.  Wish I would have done this from beginning.  Optics of main tube is so much better than from any dedicated guide scope for all the reasons that have been stated.

10min subs.. woww. I could only go 2min with my previous setup i.e. C6 SCT at 985mm FL. I think my guide scope definitely had a little flex and putting a dew heater strap on it might have worsened it. I could only do 60sec to 90sec subs with good star shape.
Thanks for sharing.

If your guiding is accurate enough you should be able to go as long as you like with sub-exposure time and get round stars. It's also important that your error be balanced. You should see about the same amount of error in both RA and DEC.

 My 533MCPro,10mm spacer, filter holder, OAG.  2nd shot shows how big mirror is to sensor.