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OAG Question

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#1 Domer

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Posted 25 November 2020 - 11:46 AM

I'm planning to purchase an Esprit 100 and ASI2600MC for imaging.  I've been doing more research into the use of an off-axis guider and it's raising more questions than answers...

 

1) My paltry little ASI120MM-S seems like it might struggle with finding guide stars with its tiny little sensor (4.8mm x 3.6mm).  Should I be concerned about that?  Coupled with the Esprit 100, it would have a resolution of 1.41"/pixel, the same as the ASI2600MC imaging camera since the pixels are the same size, but it would have a much smaller FOV.

 

2) Is using an OAG overkill for focal lengths of 550 mm or less with an APS-C sensor (~3 degree FOV)?

 

3) With the SkyWatcher telescopes and a ZWO camera, are there specific OAGs I should be considering?



#2 bobzeq25

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Posted 25 November 2020 - 12:10 PM

Many try a 120 with OAG.  Few are satisfied, just not sensitive enough.  A 290 works well, the mini form factor is important for achieving focus with some setups.  Mono is a must.

 

Some people just like OAGs, period.  I use a guidescope with my 580mm, OAG with 910mm.

 

Backfocus is the key issue.  I have and like a Celestron OAG, but its backfocus requirement is extreme.  I had the good fortune to have a flattener available with 103mm spacing requirement.  I needed almost all of it.

 

Thin OAGs are quite popular, for that reason.


Edited by bobzeq25, 25 November 2020 - 12:13 PM.

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#3 ShortLobster

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Posted 25 November 2020 - 12:11 PM

I have that same setup (Esprit 102, ASI2600MC and ZWO OAG with ASI120mm). It actually works great. Spacing is pretty easy. I've never struggled to find stars. 

 

The one challenge is getting the ZWO OAG into focus. Focus is achieved through a wonky mechanism involving 2 screws, 2 additional set screws and manually moving the camera and mirror stalk in and out. I highly advise doing this during daytime to avoid trying to get it right in the dark. 


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#4 erictheastrojunkie

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Posted 25 November 2020 - 12:11 PM

I moved to an OAG with my WO GT81 and Flat6Aii, focal length of 368mm, using a QHY5l-iim guide camera and an ASI183mm Pro imaging camera, I haven't had any issues finding guide stars in any field of view yet. I switched to an OAG to save a bit of weight (the WO guide scope is a big, heavy beast) and also free up space on top of the scope so I could mount my computer on top, I don't think it's overkill although I can't say it's made much of an improvement in terms of performance. 



#5 drd715

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Posted 25 November 2020 - 12:22 PM

Nice combination of scope and camera  I think you will like the 2600.  


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#6 MikiSJ

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Posted 25 November 2020 - 12:29 PM

I think it comes down to cost. My Celestron OAG retails for around $225, is solid and since it is Celestron mates up well to my C11EdgeHD.

 

A refractor for piggyback guiding will range from ~$75 to ~$150 and will require rings and a scope to scope connection. This connection may introduce flexure into your imaging train that can be a bear to chase down.

 

I would recommend an OAG is in your budget and if it will fit into your imaging kit.


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#7 AZ Maverick

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Posted 25 November 2020 - 12:57 PM

I switched from a separate guide scope and ASI120MM-S to the ZWO OAG with ASI290MM Mini when I started imaging with my longer focal length telescope and now I guide with the OAG on all of my telescopes.

I get better guiding on my ES ED80 with the OAG and ASI290 than I did with my 60mm guide scope and ASI120, I don't know if that's because of the camera or the OAG but it's probably a combination of both.

I don't have any problems whatsoever finding guide stars with the ZWO OAG and ASI290MM Mini on my ED80 or my ED127, sometimes it's problematic on my 9.25" SCT though.

 

170922613.LcdFSeHJ.jpg

 

170954483.UQWX1LEo.jpg


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#8 Domer

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Posted 25 November 2020 - 01:22 PM

I have that same setup (Esprit 102, ASI2600MC and ZWO OAG with ASI120mm). It actually works great. Spacing is pretty easy. I've never struggled to find stars. 

 

The one challenge is getting the ZWO OAG into focus. Focus is achieved through a wonky mechanism involving 2 screws, 2 additional set screws and manually moving the camera and mirror stalk in and out. I highly advise doing this during daytime to avoid trying to get it right in the dark. 

Yeah, I saw that when I was reviewing the ZWO OAG - the focusing mechanism looks like it would be a pain-in-the-a**.  That's THE reason I asked for suggestions on OAGs - it seems like maintaining a tight focus over the entire night of shooting would be difficult if you have to keep adjusting set screws.



#9 dswtan

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Posted 25 November 2020 - 01:30 PM

Re: focusing the ZWO OAG - just get the low-cost helical focuser add-on though, problem solved. AZ Maverick's post #7 photos show this fitted (red ring). 

 

Also, you don't have to refocus the OAG *separately* all night. Once set to be equidistant with the imaging camera (in daylight at first), it's set-and-forget. 


Edited by dswtan, 25 November 2020 - 01:32 PM.

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#10 Domer

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Posted 25 November 2020 - 01:32 PM

I switched from a separate guide scope and ASI120MM-S to the ZWO OAG with ASI290MM Mini when I started imaging with my longer focal length telescope and now I guide with the OAG on all of my telescopes.

I get better guiding on my ES ED80 with the OAG and ASI290 than I did with my 60mm guide scope and ASI120, I don't know if that's because of the camera or the OAG but it's probably a combination of both.

I don't have any problems whatsoever finding guide stars with the ZWO OAG and ASI290MM Mini on my ED80 or my ED127, sometimes it's problematic on my 9.25" SCT though.

Question - which one is the "mini"?  Looking at the two versions on the HPS web site, the one they call a "mini" is actually larger than the one that isn't:

 

ZWO ASI290MM USB 3.0 Monochrome Imaging Camera - ASI290MM

ZWO ASI290MM Mini Monochrome Astronomy Camera - ASI290MINI



#11 dswtan

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Posted 25 November 2020 - 01:43 PM

This is the 120, but same difference. Mini on left. 

ZWO-ASI120MINI-asi120mm-mini-usb2.0-mono


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#12 AZ Maverick

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Posted 25 November 2020 - 01:53 PM

Question - which one is the "mini"?  Looking at the two versions on the HPS web site, the one they call a "mini" is actually larger than the one that isn't:

 

ZWO ASI290MM USB 3.0 Monochrome Imaging Camera - ASI290MM

ZWO ASI290MM Mini Monochrome Astronomy Camera - ASI290MINI

 

ASI290MM Mini is in the OAG.

Attached Thumbnails

  • AsiMini.jpg

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#13 Domer

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Posted 25 November 2020 - 01:59 PM

Ah, OK, so the "mini" has a smaller diameter but a longer tube?  And you recommend the mini for the OAG, correct?



#14 AZ Maverick

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Posted 25 November 2020 - 02:05 PM

Ah, OK, so the "mini" has a smaller diameter but a longer tube?  And you recommend the mini for the OAG, correct?

I do - especially if you want to use the OAG helical focuser option.

The sensor on the mini is located near the end of the nosepiece while the sensor on the larger camera is located in the body of the camera.


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#15 bobzeq25

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Posted 25 November 2020 - 02:07 PM

Ah, OK, so the "mini" has a smaller diameter but a longer tube?  And you recommend the mini for the OAG, correct?

Yep.  It can prevent focus issues that the t-shaped cameras can have.


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#16 Domer

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Posted 25 November 2020 - 02:09 PM

I do - especially if you want to use the OAG helical focuser option.

The sensor on the mini is located near the end of the nosepiece while the sensor on the larger camera is located in the body of the camera.

Thanks, guys.  This has been a great help.


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