OAG or Finder Scope for guiding?

[hefyjefy]

Setup is Orion ED80T CF, with Orion Starshoot and Orion OAG

I have always found it quite difficult to get PHD to track stars consistently, sometimes it works just fine and other times the Starshoot camera is just a bunch of noise.  I wonder if this is due to the small size of the OAG prism?  My location is not ideal, quit a lot of light pollution and the tracking is noticeably worse when there's a moon.  Would I be better using a finder scope?  The Orion 60mm Guide Scope for example? https://www.telescope.com/Orion/Orion-60mm-Multi-Use-Guide-Scope-with-Helical-Focuser/rc/2160/p/114898.uts?keyword=60mm Guide Scope

Thanks for any ideas!

Geoff

[newbie alert]

I know some do use oag on refractors but I don't think there's much to gain with a 80mm and oag unless you have flex between your guidescope and imaging scope.. do you have the ability to rotate the guidescope to be able to pick a brighter star?

[ebdons]

Hi, don't know if this relevant but I use a OAG on an 80mm frac and image with that side by side with a 8" newt and I get better results than when I had just a guidescope.

[hefyjefy]

I can rotate the OAG to different positions and also tilt the prism slightly, but it doesn't seem to help.  It looks like the Starshoot Guider has some kind of AGC built in that in the absence of a detectable star cranks the gain up, which results in a bunch of grey noise.  When I first got it I thought there was something wrong with it!  Anyway I kind of answered my own question, I stuck the Starshoot straight on the end of the ED80 and things were far better, even under poor (bright moon) conditions.  I was able for the first time to get consistent calibration runs out of PHD.  I am now chasing very bad DEC backlash 🤔

[ebdons]

Sorry I didn't see the original post in full, but I have a sx lodestarx2 mono with the sx OAG and the prism isn't an issue at all, I just keep the prism perpendicular to the image line, it can go deeper but this cam picks up loads of stars and even more when binned x2 in PHD2. hope this helps. Ton

[Robindonne]

On 05/05/2020 at 14:58, newbie alert said:

I know some do use oag on refractors but I don't think there's much to gain with a 80mm and oag unless you have flex between your guidescope and imaging scope.. do you have the ability to rotate the guidescope to be able to pick a brighter star?

What is the reason behind this?   The oag? has the same small fov?   If you can explain to an amateur. Pls

[newbie alert]

1 hour ago, Robindonne said:

What is the reason behind this?   The oag? has the same small fov?   If you can explain to an amateur. Pls

A oag shares the same optical train but it uses a small pick off prism that diverts a portion of light upwards to the guidescope so if you can imagine the pick off light is at 12 o clock position but there's a brighter star in the 6 o clock position, if you can rotate the guidescope to 6 o clock you have a better star to guide on.. make sense?

 

[Robindonne]

On 05/05/2020 at 14:58, newbie alert said:

do you have the ability to rotate the guidescope to be able to pick a brighter star?

Oh I misunderstood it then.  I skipped this part of your text because you mentioned guidescope and i was focused on the oag part in combination with an 80 mm refractor.   But i think most of the times you can rotate your whole setup, and if not then only the oag.  But you, and some sg-er before you mentioned the uselessness of an oag with small refractors.  And cant find the reason behind these comments.  Does it have to do something with the fov, fl or other criteria?

[newbie alert]

3 hours ago, Robindonne said:

Oh I misunderstood it then.  I skipped this part of your text because you mentioned guidescope and i was focused on the oag part in combination with an 80 mm refractor.   But i think most of the times you can rotate your whole setup, and if not then only the oag.  But you, and some sg-er before you mentioned the uselessness of an oag with small refractors.  And cant find the reason behind these comments.  Does it have to do something with the fov, fl or other criteria?

The main point to a OAG is that the guide scope and imaging camera are using the same imaging train , so if there's any shift in the image it's reflected in both.. OAG are partically useful for sct's as they suffer with mirror shift..  

Don't get me wrong, if you have a separate guidescope and having issues with chasing flex then yes by all means use a OAG

[RyanParle]

I've used a TS 9mm OAG with an old QHY5 guide camera on an 80mm APO for many years with great success, I nearly always fond a good guide star no matter where I was pointing the scope. I actually have an instrument rotator on that scope which was intended to ensure I always framed my image to give a good guide star, I didnt really need to use it for that purpose and in the end i just used it to frame my main image sensor.

I actually found having an OAG setup was quicker to set up than a seperate guide scope, the OAG and Guide cam stayed attached to the scope, all i had to do was attach my DSLR to the bayonet and i was ready to go.