17 replies to this topic

[John Miele]

I'm planning to put together a short fl widefield imaging setup. I currently use a Celestron OAG with my MN190 scope which has a fl of 1000mm. My new setup will be a small refractor maybe 65 to 80mm running around at a fl of around 300-400mm. Whenever I look at similar setups, almost everyone seems to use a small guidescope instead of an OAG. I was just going to use my current OAG. But I wondered if maybe using a guidescope was actually better when shooting at a short fl. I'm thinking less weight on the focuser would be nice and a guidescope should guide perfectly fine at these short fl's. Just planning my new purchases and not sure if I should also plan on a guidescope and bracket or just repurpose my current OAG.

 

Thanks!

 

cs...John

[AtlantaAstro]

I rock an OAG on my AT72EDII. Reduces it has a focal length of 344mm it can be done and guiding for me usually is around .5-.8 RMS. 

[Ken Sturrock]

You could certainly go either way, but I've always used OAG.

 

While you won't experience the necessity of an OAG on a small wide-field refractor, you also won't experience many of the traditional OAG headaches either. A small refractor gives you guide scope-like FOV and brightness. Some would even consider it a guide-scope. Since you already know how to set an OAG up, I'd stick with it - the sampling will be better (than a typical finder/guider) and the rig will be more compact.

 

My smallest OTA was a 70mm Stellarvue, here (now departed).

[SilverLitz]

I run an OAG on all my OTAs.  I have used on my reduced Esprit 100 (413mm) and even Vixen FL55ss (237mm).  If I can get an OAG in the BF distance, I will use it.

[DirtyRod]

I guess I'll be different. I have to use the OAG on my C8 because of the focal length but the guide scope on my 350mm refractor is just dirt simple. I can't remember the last time I had to focus it, it simplifies my image train, and the FOV is enormous by comparison to my OAG. 

 

About to order a 115mm refractor and it will definitely have a guide scope on it. 

[John Miele]

Thanks for the replies. Sounds like OAGs are still used at short fl's by a lot of you. I am comfortable using my OAG so I guess I will stick with it as long as the BF allows it. Can always change it later if needed. And one less new item to buy!

 

cs...John

[avarakin]

I use OAGs on all of my scopes - from 81mm refractor to 10" RC

[andysea]

+1 for OAG. I have no other option but to use the OAG with my current imaging setup. However even if I didn't have to I would still standardize on the OAG for simplicity.

[TonyCaf]

Not to change the subject here, but I’m thinking that an OAG on a Newtonian reflector might put a lot of weight on the focuser if one wanted to use on other setups.

[AaronH]

Not to change the subject here, but I’m thinking that an OAG on a Newtonian reflector might put a lot of weight on the focuser if one wanted to use on other setups.

It depends on your OAG. My ZWO OAG isn’t particularly heavy, nor is the guide cam. I have no issues using it on any OTA.

 

My OAG, filter drawer and camera stay as one unit, whether they’re used in a Newt or refractor. Together they consume 55mm of backspacing, which is perfect for my coma corrector. With my refractor I have extra room to play with, so I add a tilt adapter, but the camera/filter/OAG stay together.

Edited by AaronH, 02 April 2023 - 10:36 PM.

[rgsalinger]

The Celestron OAG is too heavy for some refractors. It depends on the focuser. My QHY-Medium OAG's weigh about 6 ounces and are a much better choice. One less dew heater is nice as well as the other advantages mentioned earlier in the thread. 

 

Rgrds-Ross

[mgutierrez]

another oag user on a medium FL ota (550mm). Very happy. I don't miss the tube guide at all

Edited by mgutierrez, 04 April 2023 - 04:37 PM.

[FiveByEagle]

I have used both on my GT81 setup with the EQ6R. Guiding has been identical performance on either.

[AaronH]

I have used both on my GT81 setup with the EQ6R. Guiding has been identical performance on either.

 

As long as there's no differential flexure (or mirror flop/shift in reflectors), either will do the job.

 

However, differential flexure won't show up in the guide log/statistics. If the guidescope is flexing in relation to the main scope, your mount may be doing a perfect job of guiding its guide scope, but that guidescope may not stay perfectly aligned with the imaging scope.

 

So it's worth checking FWHM and eccentricity in your subframes as well when making performance comparisons between an OAG and guidescope. I've had issues in the past where my guidescope was being guided with wonderful accuracy, producing a textbook perfect guide log, but I still had tracking-related star eccentricity because the guidescope was shifting ever so slightly in relation to the imaging scope.

[TheRock]

There is a Flexure Correction plugin for NINA that has been tried by a few folks with very good results, but it has not gotten a lot of press. If it does what it is intended to do, then hopefully, bye-bye OAGs (and good riddance):

 

https://www.cloudyni...ection-in-nina/

[rgsalinger]

I've been using an OAG for 13 years now. It takes about an hour for me to set one up from scratch. Then I keep it with the camera and the wheel all connected. I have moved the same exact OAG/Wheel/Camera combo from scopes as short as 350mm to almost 5 meters by simply adding the correct adapter. Since all three of my refractors have M54 connections on the focusers, I just plug and play these days. 

 

OAG's have other advantages as well. With a guide scope the setup gets heavier and heavier as you go longer and longer. With an OAG that's not true. You can't bump it and send it 5 degrees off target. You need one less dew heater and because it's usually within a few inches of the camera, the cable is shorter. Oh, and there's also the differential flexure caused by the change in tube width and length/width between the guide scope and the camera. 

 

You can't determine the effectiveness of a guide scope versus an OAG by looking at the guiding RMS. That's because differential flexure will not affect the guiding RMS, just the resulting star shapes. Having said that, my occasional experiments using short (under 700mm) refractors with guide scope has always given me pretty much the same results as using an OAG. When I get out beyond one meter, there's no comparison with the quality but that's using my equipment. 

 

Rgrds-Ross

[Kevin_A]

I am on the fence still with my zwo oag due to star shapes. My guide scopes produce very good but small stars compared to the oag. But… my oag did not work very well with my bigger scope flattener reducer setup and the stars were looking more birds with wings than tight round dots. I have still not tried the oag on a different scope since moving over to my zwo AM5 mount as everything works well already. I might use it on my Askar FRA300 with my 183mc pro since I can put it further into the center with the small 183 sensor.

[FiveByEagle]

I've been using an OAG for 13 years now. It takes about an hour for me to set one up from scratch. Then I keep it with the camera and the wheel all connected. I have moved the same exact OAG/Wheel/Camera combo from scopes as short as 350mm to almost 5 meters by simply adding the correct adapter. Since all three of my refractors have M54 connections on the focusers, I just plug and play these days.

OAG's have other advantages as well. With a guide scope the setup gets heavier and heavier as you go longer and longer. With an OAG that's not true. You can't bump it and send it 5 degrees off target. You need one less dew heater and because it's usually within a few inches of the camera, the cable is shorter. Oh, and there's also the differential flexure caused by the change in tube width and length/width between the guide scope and the camera.

You can't determine the effectiveness of a guide scope versus an OAG by looking at the guiding RMS. That's because differential flexure will not affect the guiding RMS, just the resulting star shapes. Having said that, my occasional experiments using short (under 700mm) refractors with guide scope has always given me pretty much the same results as using an OAG. When I get out beyond one meter, there's no comparison with the quality but that's using my equipment.

Rgrds-Ross

I probably should have elaborated- Equal performance as far as FWHM and eccentricity too.

but your point is correct. in the past I have used low quality guiding scopes that have had horrible flexure and had low phd2 number's but eggy stars. The uni-guide is a solid attachment and THAT has been equal to my old OAG.