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Pros vs Cons of an OAG

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#1 matt_astro_tx

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Posted 08 April 2021 - 12:42 PM

I've got an ASI183MC arriving this weekend, and an AT60ED on order.  I'll be pairing these with a Star Adventurer 2i to begin with, which has an autoguider port.  Although it only guides in RA I'd like to make use of it in the near future.

 

While I'm looking into guiding solutions for this setup I also want something that will compliment my larger 8" f/8 1600mm FL reflector down the line.

 

This has brought me to two options:

 

Option 1:

  • Order a small mini guidescope and mini guide cam to use with the AT60ED on the Star Adventurer
  • When using the 1600FL newtonian attach the AT60ED with mini guide cam and use that for guiding

Option 2:

  • Go with an OAG and mini guide cam and use it for both scenarios

 

I like the OAG option (it's the simplest after all) but I don't know anything about OAGs.  What are the pros and cons vs a dedicated guidescope?  I like that it operates at the system FL, so will give me the accuracy I require when shooting at 1600mm or longer.  What about obstruction of the sensor frame?  I assume the small sensor on the 183 won't have a problem with it, but what about a full frame sensor down the line?  Other considerations?

 

Thanks



#2 jonnybravo0311

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Posted 08 April 2021 - 12:57 PM

Well, that poor little SA won't be able to handle the 8" reflector. Might want to consider upgrading laugh.gif.

 

Jokes aside, the benefits of OAG are that it uses your main imaging scope. No differential flexure, so you get more accurate guiding, which is definitely a necessity at the longer focal lengths. Cons are it's a bit fiddly to setup. The prism can intrude into the light path. Typically requires a "better" guide camera... typically, the 290MM is recommended for OAG, whereas the 120MM is recommended for a guide scope.

 

My current setup is the William Optics GT81 with the William Optics UniGuide 50mm. If/when I ever get a big old light bucket, I'll be upgrading to OAG.


Edited by jonnybravo0311, 08 April 2021 - 12:58 PM.


#3 matt_astro_tx

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Posted 08 April 2021 - 01:12 PM

Yeah I have a feeling mounting the 8" to the SA would void the warranty.  I have a tracking mount in development and build right now as a hobby project.  Might be worthless, might be awesome.  Might purchase an EQ6R in the end.  Time will tell.  



#4 TelescopeGreg

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Posted 08 April 2021 - 01:53 PM

The mini guider will not be strong enough to guide a longer focal length scope.  The "rule of thumb" is to shoot for something under 5:1 ratio of the pixel scale between the guider and imager.  Pixel scale is 206 times the camera pixel size (in microns), divided by the scope's focal length (in mm).  Compute those for the two optical systems, and look at the ratio.

 

Second rule of thumb is to not try to use a guide scope for an imaging system with a focal length longer than about 1,000 mm, or if the imaging scope has a moveable mirror that can't be locked (many SCT scopes are in this camp).

 

I think you break both of these "rules" with the longer scope.  An OAG will work on the smaller scope; a mini guider will not work on the longer one.



#5 Stelios

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Posted 08 April 2021 - 02:11 PM

Pros of OAG:

1) Better guiding under pretty much all circumstances.

2) Totally eliminates differential flexure, aka the dog that didn't bark: Good RMS but bad stars.

3) Portable between any scopes. I have the same OAG on a 336mm F/L scope and a 2030mm F/L scope. 

4) Easier to balance.

 

Cons of OAG:

1) Cost. An OAG requires a better quality (more expensive) camera than a guidescope. Figure at least $200 more.

2) Initial setup: You need to bring the OAG camera and main camera to simultaneous focus. May require spacers and/or extenders depending on setup.

3) More problematic/fewer options with DSLR cameras when paired with reducers/flatteners with 55mm backfocus.

4) Needs some advance planning to make sure that selected OAG, camera and/or FW (filter wheel) and backfocus of reducers will work together.

 

I must also add that I agree with the others that your decision to use the SA will hurt your imaging far more than an OAG would benefit it. 



#6 matt_astro_tx

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Posted 08 April 2021 - 06:13 PM

Ok, so what I think I’m hearing is that an OAG is the proper equipment to guide both scopes. Especially in the case of the larger one, and won’t hurt anything in the case of the smaller one.

Admittedly the SA isn’t a particularly precise tracker, but it’s what I have for the time being. I acquired it mainly for wide field use with a DSLR in the long run. I’ll be shooting through the AT60 and the newt with the ASI183MC.

As for equipment, what are your thoughts on the ZWO 1.25” OAG paired with an ASI290MM? Other similarly priced options I should consider?

#7 DaveB

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Posted 08 April 2021 - 08:58 PM

 

Cons of OAG:

1) Cost. An OAG requires a better quality (more expensive) camera than a guidescope. Figure at least $200 more.

2) Initial setup: You need to bring the OAG camera and main camera to simultaneous focus. May require spacers and/or extenders depending on setup.

3) More problematic/fewer options with DSLR cameras when paired with reducers/flatteners with 55mm backfocus.

4) Needs some advance planning to make sure that selected OAG, camera and/or FW (filter wheel) and backfocus of reducers will work together.

While I agree with the last three items, I've never had a problem with finding a guidestar with cheap cameras. I was using an old Meade DSI-II for a long time, and I just started using a QHY 5L-II Monochrome camera ($169) as my guider. I have yet to have any problems finding a guide star. I used to image in Bortle 4 skies and am currently imaging in Bortle 5 skies between Baltimore and DC.




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