OAG for short focal length?

[Richie092]

I'm trying to decide if this is a good idea or not so any help or advice would be appreciated.

I currently have the W/O star71 (348mm focal length) guided with a ZWO 120mm on top using a guide scope. All good there but I am putting together a wide field rig with the Samyang 135 and I'm thinking of putting this guiding set up on that and going OAG on the Star71 for the following reasons...

1. It will leave the top of the scope free to mount the RPI/AsIAir Pro.

2. Lower the overall profile of the set up so I can close the lid on the peli case without removing guide scope each time.

3. Lower the overall weight a bit. Can't hurt.

4. Save £30 on another dew strap.

5. Whilst I love my Star71 it is so short and the tube rings are so close together that I need "something" between rings and guide scope mount to fit it that makes it just a little ungainly/untidy.

 

I have the following questions though.

1. Is it OK to guide a short focal length scope with an OAG?

2. I have to buy a 2nd guide camera anyway but using the OAG will I need to buy better one than the ZWO 120?

3. Is it harder to guide OAG than guide scope?

4. Am I driving myself mad and should I just by a cheap guide scope and another ZWO 120 for the Samyang?

 

Would love to hear your thoughts...

 

 

[vlaiv]

18 minutes ago, Richie092 said:

1. Is it OK to guide a short focal length scope with an OAG?

It's ok,j Just make sure you don't stop down light to your OAG by placing it far from primary sensor. Short FL scopes tend to be fast scopes and with fast scopes - you need to place pick off prism close to main sensor to get full amount of light. Say you image at F/5 and prism is 9mm wide - in order to "fit" F/5 beam into 9mm aperture - you need to place it 5 * 9 = 45mm away from the sensor (in reality - a bit closer to get at least 2-3mm full illumination).

That is only issue with using OAG with fast scopes - everything else works fine.

20 minutes ago, Richie092 said:

2. I have to buy a 2nd guide camera anyway but using the OAG will I need to buy better one than the ZWO 120?

If you pay attention to above advice, then no - ASI120 will work perfectly fine at F/5. People guide with ST80 - which is F/5 400mm FL scope - that is very similar to your OAG setup

22 minutes ago, Richie092 said:

3. Is it harder to guide OAG than guide scope?

To me - it's easier. No differential flexure and setup is lighter. It also gives better guiding results.

Only issue is focusing OAG - but it's not hard and you also have to do it with guide scope - so no real advantage there. Once you dial in OAG / main sensor distance - focusing is done at the same time when focusing main camera - so nice bonus that you focus only one scope.

24 minutes ago, Richie092 said:

4. Am I driving myself mad and should I just by a cheap guide scope and another ZWO 120 for the Samyang?

Don't know really - to me, OAG is actually better and cheaper solution than guide scope, but some people still prefer small guide scopes.

[Richie092]

Thank you for your response, it is really helpful.

I'm imaging at 4.9 so in theory, 4.9 * 9 = 44.1mm max. I'm looking at half the depth of the OAG so 7.5 + distance to sensor 17.5 = 25 so potentially need a 20mm spacer?

Is that right?

 

[vlaiv]

13 minutes ago, Richie092 said:

Thank you for your response, it is really helpful.

I'm imaging at 4.9 so in theory, 4.9 * 9 = 44.1mm max. I'm looking at half the depth of the OAG so 7.5 + distance to sensor 17.5 = 25 so potentially need a 20mm spacer?

Is that right?

 

No need to add spacer - 44mm is max distance before prism starts to act as aperture stop - if you can place OAG closer - just place it closer. Only thing that really needs to be closer to sensor would be filters and next thing can be OAG.

[Richie092]

Great, I'm shooting OSC so all good there.

Last question (probably) How important is focus on the guide cam? I set my first one up on the guide scope so stars are slightly out of focus, this was the advice five years ago when I first tried AP, is that the right thing to do for OAG?

Cheers

Richie

[vlaiv]

1 minute ago, Richie092 said:

Great, I'm shooting OSC so all good there.

Last question (probably) How important is focus on the guide cam? I set my first one up on the guide scope so stars are slightly out of focus, this was the advice five years ago when I first tried AP, is that the right thing to do for OAG?

Cheers

Richie

Same thing really - you can let the star be slightly defocused - it won't affect performance (at least I think so - maybe they changed centroid algorithm lately?). In fact - seeing affects less centroid calculations if star is slightly defocused.

On the other hand - when you are working with longer focal length - star image is going to be spread over more pixels any way so no need to do it for that reason (to cover more pixels - that is only with large pixels and short focal lengths).

[Richie092]

Thanks for all your help Vlaiv. You have made up my mind.

[CraigT82]

I've just started using an OAG on my 1200mm fl newt (ZWO OAG), once set up it works very nicely with minimal fuss.

Getting it set up can be tricky though, particularly finding focus with the guide cam. I did it at nighttime and lost precious hours faffing with it, I'd definitely recommend doing it in the day by pointing scope at something far off! 

Oh and be prepared for weird star shapes in the guide cam. Mine look like boomerangs but PHD2 seems to identify the centroid correctly and guides on it nicely. I tried moving the prism further in but didn't seem to make any difference to the star shapes.

[Richie092]

Thanks Craig. I was looking at the OVL one but cant quite get my head around the T2 threads, it might be more straightforward to go with the one you are using - ZWO to match my cameras.

Are you happy with the quality?

 

[CraigT82]

Yes the attachments are a bit of a fiddle with the ZWO one too to be honest.

The back side of mine attached to the filterwheel using a T2 thread, this thread is a kind of three pronged dovetail ring (hard to describe!) which first screws onto the FW and then the body of the OAG goes onto that and secures using three thumbscrews.  The prongs of the dovetail (being the thing which the screws bear against) are about 20mm wide so you have this amount of rotational adjustment for the body of the OAG.

The front of the OAG has a 48mm thread, and it comes with a 48mm to 42mm step down adapter, which is what I use to attach to my M42-2" nosepiece (my moonlite focuser doesn't have any threads on the drawtube so I'm limited to using a 2" nosepiece).

One thing I'd say is that I made good use of a set of Baader delrin M42 thread spacers, in order to fine tune the final position of the guide cam once everything is all screwed up tight (because of the FW the guide cam could only stick out one way (as pic below)).

Oh one other thing with the ZWO unit, I had to file down the threads on both my T2 nosepiece and the M48-M42 step down ring, as when screwed up tight into the OAG the threads fouled the prism stalk.

Edited March 9, 2021 by CraigT82

[Richie092]

Thanks for the detailed explanation Craig.

Nice set up, I have just put my 383 on a Samyang 135 hence the changing around.

I have asked FLO about fitment of the OVL one to my Star71 and 533, I like ZWO stuff but the last couple of bits of theirs I have got haven't been perfect.

 

[Richie092]

So just to finish up. The OVL will screw straight on to my Star71, the other side - T thread - will screw straight on to my 533MC. I didn't know a T thread and an M42 were the same!

I have also ordered this

https://www.firstlightoptics.com/adapters/astro-essentials-low-profile-female-t-thread-to-male-c-thread-adapter.html

That will screw on top of the OAG and enable me to screw my 120MM direct to it. Should be quite a solid set up.

Lastly, I have ordered 3 clear nights with zero cloud/wind/security lights/UFO's to set it all up with.

 

[vlaiv]

12 minutes ago, Richie092 said:

I didn't know a T thread and an M42 were the same!

Not always

M42 x 0.75mm is T/T2 while M42 x 1mm is not. Later is used for camera lens. If you see M42 lens - you won't be able to screw it in T2 thread as pitch is different - 0.75mm vs 1mm.

[Richie092]

Hmm, OK. I have ordered it now so will give it a go. Have ordered from FLO so there won't be a problem returning it if I can't get it to work.

This is a very challenging hobby! I have no idea where most of us would be without the internet and forums like this.

[vlaiv]

35 minutes ago, Richie092 said:

Hmm, OK. I have ordered it now so will give it a go. Have ordered from FLO so there won't be a problem returning it if I can't get it to work.

This is a very challenging hobby! I have no idea where most of us would be without the internet and forums like this.

Oh, don't worry, I did not mean that you could get it wrong - M42/T2 in astro use is exclusively 0.75mm pitch. M42x1mm is used only with lenses and old cameras.

I just gave info if in the future you run into old M42 lens - that one won't screw in properly with astro gear.

[Richie092]

Thanks Vlaiv