Celestron OAG vs ZWO M68 OAG - L?

[oymd]

I’d appreciate some advice regarding which OAG to buy for my setup, and pros and cons of each?

Imaging will be an Edge HD 11 + 0.7 reducer. 

Camera is the 2600MM Pro + 7x2” EFW. 

Guide camera will be the 290MM Mini, but I’m planning on buying the 174MM Mini. 

Both OAGs have 12mm prisms, and I believe both have bigger openings in their stems, so allow much more light through. 

The Celestron is considerably wider and heavier, and will take up much more space in the imaging train. 

I think the ZWO OAG has a better attachment mechanism to the EFW, where it screws on directly. I do not think the Celestron one can be screwed to the EFW. 

Which OAG do you suggest to buy?

Many thabks

Ossi

 

 

[ONIKKINEN]

I dont have experience on either of the ones you had in mind, but i have an Askar OAG which i think is great value for money (and good quality otherwise). It is natively M54 and comes with threaded adapters to M48 and M42 for both the scope and camera side so you can fit it to almost any scope without extra kit. This one has a 10x10 prism, so a little bit smaller than the 12x12 on the 2 bigger ones. But on the topic of that, for your 290MM you definitely will not need to have a bigger prism and the 10x10 is plenty enough. Not so sure about the 174MM though as the sensor is a lot larger, but you might be surprised how many stars even the small sensor will pick up.

While i dont image at 2m focal length so its not an exact comparison, i think its somewhat close. I am imaging with 1025mm focal length and 200mm aperture with a 120MM using just 3s exposures. So far i have never come across a situation where there aren't at least 5 suitable guide stars in the tiny little sensor with this short exposure. The 290MM is more sensitive, but has smaller pixels so i think it evens out in terms of usable sensitivity. I would be very surprised if you have trouble finding a guide star with the 290MM with sensible exposure times.

The bigger ones are probably a safer bet if you do intend to buy the 174MM but thats a fair chunk of cash so you may want to think about that and try with the 290 first.

[newbie alert]

I use a Celestron oag with a 8inch XLT and .63 reducer, 290m... Never had a situation where I can't find a guidestar, or even rotated the gudecam as recommended by that American u tuber.. I find the 290 sensor and prism just fine on a 8inch tube, if it makes much difference with a bigger tube I wouldn't know.. 

Biggest issue was finding an adapter to fit the SCT and m48 and not encroach into the backspacing distance as Im also using a efw

[MikeBY]

You have two issues to deal with.  1st is that the back focus requirement for the EdgeHD.  It's 146 mm. That's the same with or without the 0.7x reducer!

The 2nd is image circle, especially if you contemplate going to a full size sensor. The 2600mm is APS-C, so you probably haven't run in to this issue full force yet

Here's the problem with EdgeHD scopes and full frame sensors:

The image circle is only 42mm. With the 0.7x reducer for 9.25 and larger the image circle is maintained .

EdgeHD 800 it shrinks to under 30mm with the 0.7x reducer!  

Although you might be tempted to use a larger diameter OAG because you've got a 2" EFW. You will run into trouble picking off light with the prism and getting focused. 

Any OAG larger than 48 mm will likely NOT work.  It's  a problem no-The EdgeHD scopes cannot fill a full size sensor. Buy the over priced and smallish M42/M48  diameter Celestron. OAG? It might be too small

I'd go with ZWO OAG-L  (NOT the OAG-L 68 Version). Why?  Because you have the large filter wheel.  And it went cause as much vignetting. It's a true 48mm OAG.

It's kind of a toss up though under these circumstances getting the back focus to 146 mm will require extra  spacers with either.

 

Look at the guide port. The helical focuser is  about 37 to 45 mm high.  Move it to mid travel.   Depending on which camera and how you attach it (i suggest threading the guide  camera on) calculate how much the camera. adds or subtracts from the focuser length.   For example a ASI178 adds 12.5 mm using the 1 1/4 nosepiece.  It adds only 5 mm threaded on to the top of  a helical focuser. .An Asi174 mini might slip  down Into the top of the focuser a bit so it's a negative number (sensor is closer then the top of the focuser.)

Attach the main camera to the OAG as  a visual aid and adjust prism position to get prism taper in  the image circle as far as possible while not interfering with the main camera sensor light path. 

Measure optical path from the middle of the OAG (1/2 of the thickness of the OAG is used) to the main camera sensor. Compare to length  from guide sensor to the point on the prism where the 45° cut starts.  The distances need to match. The focuser gives you +/= 4 mm. Adjust so they are equal.

Next measure from main camera sensor to the  edge of rear glass (is either 0.7x reducer or at back of edgeHD scope). . This Includes full thickness of OAG.  Add spacers as needed between OAG and 0.7x reducer. To get to 146 mm.

 

 

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[newbie alert]

5 hours ago, MikeBY said:

Here's the problem with EdgeHD scopes and full frame sensors:

The op is using a 2600, which is APS-C 

[MikeBY]

Which I noted in my comment.

The 2600mm is APS-C, so you probably haven't run in to this issue full force yet.

Going to an OAG the size of the image circle and drop- off of light intensity outside the area of the APS-C sensor becomes important. 

Mentioning the issue in terms of full frame support might be considered a bit off topic, but it's relevant tovmention because the OP is asking sbout the larger OAG. One consideration is supporting full frame devices.