Edge HD 8" and OAG

[Grierson]

Apologies in advance as I know a lot has been written on this subject. Prompted by the views of Olly and other members of SGL I have been trying to get a ZWO OAG set up correctly on my Edge HD.

I am waiting for FLO to come up with a reducer but in the mean time I have the back focus set correctly at 133mm. The imaging camera is a ZWO ASI224 and the guide camera an ZWO ASI120mm.  As far as I can judge the distance to the guide camera sensor is the same as that to the imaging camera but despite trying a variety of spacers in the light path to the guide camera I can not get it to focus. I see that it has been suggested on another forum that the ZWO OAG and the 120mm is not a good combination given the size of the prism and sensor. True or not I am keen to get it set up and at least in focus.

I wonder if any other members use the combination above successfully and who would care to share their experience. 

Many thanks. - John 

[tomato]

Hi John,

I have a ZWO OAG with a helical focuser fitted on an Esprit 150 refractor (back focus 96 mm). I could not get the older wide bodied ASI 120 guide camera to focus, but this is a known problem. The solution was to use the 120 mini, which will move further down the helical focuser.

Do you know which way your camera has to move to get to focus, further in or out?

[michael8554]

Do focus during the day on a distant object, if both your cameras will work with SharpCap in Video mode on Auto, my 120MM does.

Focus the imaging camera first.

Then fire up the 120MM and focus on the same distant object.  

Now both cameras are focused on the same object, now whatever you focus the imaging camera on, the guidecam will be in focus too.  

So next try the moon.

First focus the imaging camera.

Then fine-tune the 120MM focus.

And finally check both on a star.

Michael

[Grierson]

Thanks for the comments chaps. The camera appears to want to be further in to achieve focus as far as I can tell. Having watched one of The Elf's videos where he apparently took a saw to the prism post it seems others have had similar problems. I am not going down this extreme route!! I am indeed doing all my experimenting during the day although from my small garden the only distant objects are tops of trees wafting in the wind. Not the ideal subject to look at focusing issues. The moon is just a blur in the guide 'scope.  The picture shows the back end of my Edge and although I have tried various positions for the OAG the one shown appears to be the best to get anything approaching an image on the guide camera. I wonder if anyone has an idea of what the optimum distance from the imaging sensor the OAG needs to be? 

[michael8554]

The spacers between the ASI 224 and the OAG are to get the 133mm sweet spot I assume ?

Depending on how many more mm of inward you need, instead put that thickness of spacers in FRONT of the OAG, then you'll have that amount more in the ASI 120MM light path to play with.

I've realised that's wrong,  makes it worse - Michael

Or dispense with the sexy helical focuser and adjust focus by hand, trickey but you only have to do it once 😆

22 hours ago, Grierson said:

I wonder if anyone has an idea of what the optimum distance from the imaging sensor the OAG needs to be? 

You mean the ASI 224 ?

Can be anything you like - so long as it's 133mm from the Reducer, and isn't vignetting.

Michael

Edited June 4, 2020 by michael8554

[Grierson]

Yep, before I started down the OAG route and having read that focusing distances were critical on the Edge I thought the helical focuser would be a good investment and divorce beckons anyway given the rest of the cost! 

I don’t have a reducer as yet, but yes, the Heath Robinson selection of spacers is to hit the 133mm. (The spec. is 133.35mm)

John

[michael8554]

The Baader Helical Focuser looks shorter and is only £31 :-<  you should be able to sell the ZWO one.

https://www.rothervalleyoptics.co.uk/baader-125-helical-focusing-eyepiece-holder-t2.html

Michael

Edited June 4, 2020 by michael8554

[michael8554]

I've realised that moving the spacers is wrong, it will make it worse !

Yes, you need to get the OAG CLOSER to the Edge HD, is there such a thing as a shorter Edge HD to T2 adapter ?.

As you've shown me yours, I'll show you mine....... 😆

Old-style ZWO 120MM, with a Baader MPCC Coma Corrector between the OAG and the Canon T2 connector.

The large diameter metal shim near the ZWO is just to get the ZWO rotated parallel to the Canon sensor when it's screwed on.

Michael

[Grierson]

Thanks for the link Michael. I will bear it in mind but I am going to continue to play with the distances and positioning as I've just received a set of Baader shims from FLO which may or may not provide me with the answer. 😬 

[michael8554]

Time to investigate how increasing the 133.5mm distance affects the imaging quality.

Adjust the helical focuser to shortest position, then back a little for wiggle room.

Focus the guidecam with the HD8 focus knob.

Add spacers between OAG and 224 to get focus on the 224, fine tune with HD focuser, refocus 120 with that wiggle room.

Take image of star field and check quality - may be acceptable ?

Michael