Analysis paralysis: OAG vs guidescope

[BrendanC]

I was all ready to invest in a nice little guidescope, probably the Astro Essentials one for £59, to use with my T7C (ASI120MC clone). Then my eye was drawn to OAGs - specifically the Orion Thin OAG, because I'll be using it with my Skywatcher 130PDS and Canon EOS1000D DSLR with Skywatcher 0.9x coma corrector. It should work spacing-wise.

BUT... I cannot make my mind up, so I'm hoping someone here can make it up for me!

The way I see it:

• If the guidescope works, then I'll always be thinking 'Hmmm, maybe it would work better with an OAG?'
• If the guidescope doesn't work, then it's a cheaper mistake - but then I'd probably go ahead and get the OAG and try that instead
• If the OAG doesn't work then it's a more expensive mistake but I can fall back to try a guidescope instead
• If the OAG does work then, well, I'm a happy bunny. End of.

By 'works' or 'doesn't work', it seems to me that:

• Guidescopes are easier to set up but suffer flexure (weight isn't really an issue - I've got an NEQ6 mount)
• OAGs can be more fiddly but don't suffer flexure

I guess what it all boils down to is:

• How often is flexure a problem with guidescopes?
• How often are OAGs more effective than guidescopes?

Any takers?

[bottletopburly]

I use a qhy mini guidescope 130mm fl with a 200pds not had any problems with flexure , though Williams optics just released a mini guide scope too , maybe oag better at a longer fl  can’t see your guidescope being a problem .

[rickwayne]

The guidescope mount has a lot to do with it, or so I've read. At only 362mm with a flattener/reducer, my scope wasn't exactly in the "gotta have it" range for an OAG, but I found one inexpensively and figured I'd give it a whack. LIghter and easier to balance than the guidescope.

If you will have a rigid two-ring mount for the scope, you'll be up and running more easily with that. If you were planning to use a finder-scope shoe, yeah, an OAG might do you right instead, especially at 650mm.

An OAG can also be more fiddly to keep up if you're a nomad like me and are always boxing your scope, driving it about, and setting it up. Hard to keep the focus, the guide cam keeps getting bashed down onto the stalk. And if the spacing works out, consider a helical focuser for the guide camera, it beats trying to adjust a friction-fit fitting on a stalk in the dark with frozen fingers, where moving it in or out 2mm is enough to go from stars to vague doughnuts.

[BrendanC]

Thanks all.

Good to know flexure can be avoided with guidescopes.

I was hoping just to use the finderscope shoe but I've also come across mentions that it's not the best solution. The cheapest one rarely is!

I won't be moving the scope at all, it's permanently positioned in the garden. 

I'm still torn! I could spend £60 on a guidescope and it'll more than likely work - but then I'd always wonder whether I could get better results with an OAG. Or I could spend £110 on an OAG, and it might not work (the spacing isn't totally guaranteed apparently), then I'd be back to square one (although I guess I could sell the OAG at a small loss).

I'd just rather avoid that heart-sinking feeling of things not working, trying everything, getting more and more frustrated, which I might be more likely to get with an OAG than a guidescope. Then again, I want to avoid the heart-sinking feeling of having got everything to work with a guidescope, but always in the back of my mind wondering whether an OAG could have been a better solution.

So, still a bit stuck. But thanks for the additional considerations - two-ring mount is definitely something I should think about, which would push up the cost of the guidescope anyway.

Any more for any more?

Edited October 17, 2020 by BrendanC

[BrendanC]

This page is very good: https://astrojolo.com/gears/oag-vs-guidescope/

I'm wondering whether my camera might not be sensitive enough for an OAG?

[Seelive]

I would suspect that the largest differential flexure would come from the mass of the camera hanging off the focusing mount at different orientations, but I have no idea how much flexure there would be with that scope and camera. But if your exposure times are not excessive, then any differential flexure over the exposure time should be negligible. I use a 50mm SW Evoguide attached to my imaging system just using a finderscope mount and haven't had any problems. 

[BrendanC]

Thanks. I would be wanting to push the exposure times as far as I can! Which is why, if I used a guidescope, I would probably always have in the back of my mind whether I could have pushed harder with an OAG.

It now seems to boil down to:

• OAGs can have problems with getting a guidestar, particularly if the camera isn't mono - could be a real issue
• Guidescopes have greater FOV to get around that but it still remains that flexure could be a problem - plus the 'what if' issue.

So a guidescope is more likely to work, but an OAG, if it works, is more likely to work better.

Still undecided! Argh!

Edited October 17, 2020 by BrendanC

[BrendanC]

OK, I've decided: I want to go the OAG route, but I need to know if the T7C is sensitive enough.

Given that it's essentially an ASI120MC, I need to know if anyone has successfully guided with one through an OAG.

So, any takers on this? I'll leave it a couple of days, if none, then I'll post again but specifically asking this question.

Thanks for all the help so far.

[alacant]

27 minutes ago, BrendanC said:

T7C

Hi

We use a T7m @ bin2 with an oag on a 10" f5 without problems and with much better stars than using a guide telescope. Not sure about the t7c. Is it a lot less sensitive? I'm sure you'll be fine with the 130.

HTH

Edited October 17, 2020 by alacant

[BrendanC]

That's precisely my point - a mono camera is more sensitive, so I just need confirmation that the colour version is sufficiently sensitive. I have absolutely no idea whether or not it is, but would be happier if someone had used an OAG with an ASI120MC because that's pretty much exactly what I'd be using.

Great to know your setup gets better stars than a guidescope, which is really the attraction for me.

Edited October 17, 2020 by BrendanC

[alacant]

55 minutes ago, BrendanC said:

the attraction

Certainly over 1200mm focal length, not only are the stars better, but you get to use every frame. Over 650mm certainly the latter, even if the former is marginal.

Unfortunately, hardly anyone uses nor has heard of the t7. Let's hope someone can chip in. Maybe ask in the cameras section of the forum with a link back here?

Cheers

Edited October 17, 2020 by alacant

[BrendanC]

Exactly, which is why I'm asking about the ASI120MC, because it has the same sensor.

If someone has used one with an OAG, that would be a very good indicator that the T7C would work too.

Good idea to ask in that section but link to this post, to avoid cross-posting.

[ollypenrice]

1 hour ago, alacant said:

Certainly over 1200mm focal length, not only are the stars better, but you get to use every frame. Over 650mm certainly the latter, even if the former is marginal.

Unfortunately, hardly anyone uses nor has heard of the t7. Let's hope someone can chip in. Maybe ask in the cameras section of the forum with a link back here?

Cheers

Surely focal length is only half the story. The other half is pixel size, since resolution derives from focal length and pixel size. What matters is your resolution in arcseconds per pîxel, not your focal length.

These images were shot at 0.89"PP using a guidescope (ST80 with Lodestar Mk 1.)

https://www.astrobin.com/419975/?nc=user

https://www.astrobin.com/393219/?nc=user

https://www.astrobin.com/335042/?image_list_page=2&nc=&nce=

I really don't know what problem anyone is trying to solve with a guidescope and a refractor. Once you move to a reflector the need to avoid pinching the mirror does argue in favour of an OAG which will 'see' mirror movement. 

Olly

[alacant]

25 minutes ago, ollypenrice said:

focal length is only half the story

Both the OP and I are using DSLR cameras. With Newtonian reflectors. 

HTH

[BrendanC]

So, I've had confirmation that the T7C should be fine with the OAG.

I'm going to do it.

Wish me luck... I'm going in...

[rickwayne]

I'm using a 120MC with my OAG. I can't say that I have the finest performance yet but I do get one- or sub-second guiding when things go right.

[Viktiste]

I also use a Zwo 120MC with a Zwo OAG on a SW Esprit 80ED.  

I used to guide with the stock finder scope in it's flimsy mounting arrangement. But when I bought my gear I added an OAG to the order. So I had to test out the OAG of course, and I have stuck with it.

It works fine, there are always a few stars to guide on. Not a lot of stars to choose from, but so far it has worked very good.

It was fiddley to set up focus the first time - it definitely helped to do that during daylight. 

For my small scope, guiding is good both with the guide scope and the OAG. But now there is one less scope that needs a dew heater.

Edited October 23, 2020 by Viktiste

[Viktiste]

On 17/10/2020 at 19:28, ollypenrice said:

I really don't know what problem anyone is trying to solve with a guidescope and a refractor

OAG and refractor you mean?

Two things: No need to customize a mounting arrangement  for a guide scope. No need to dew heat the guide scope.

But for guiding performance an OAG is probably overkill with a refractor.

[ollypenrice]

3 hours ago, Viktiste said:

OAG and refractor you mean?

Two things: No need to customize a mounting arrangement  for a guide scope. No need to dew heat the guide scope.

But for guiding performance an OAG is probably overkill with a refractor.

No, I agree an OAG is vastly to be preferred for a reflector.

Olly