Celestron Edge HD 8" with OAG-L - any experiences?

I own both, though I haven't tried the ZWO OAG on the Edge myself yet. Many use it though, the prism isn't that much smaller I reckon. The ZWO OAG is a lot smaller so it leaves a gap to fill in terms of backfocus, but it should work well once that's settled. The advantage with this setup is that you can move the entire imaging system between scopes, at least if you bolt the OAG to the EFW like I do.

Rich Sornborger:
The only way you're ever going to achieve the proper back focus is by using the Orion Thin OAG.  I'm assuming that you're going to use the Celestron .7 reducer, correct?

The ZWO OAG L that OP has is only 17,5mm. That's 7mm or so thicker than the Orion one (if I'm thinking of the right one). There shouldn't be any issues getting the right backfocus with the ZWO OAG L - but it will be more fiddly than with the Celestron as you may have to source some adapters or make life easier and get some custom made ones.

Backfocus is a good point I forgot to mention. I have an adapter from the SC-thread on the reducer (yes, it's the .7x) to an M48 thread, which has only 2mm optical length. This way, I have 103mm backfocus left for all my gear. Adapters are no problem, I own enough to get the spacing right. The only concern I have is if the 30mm opening of the reducer is wide enough for the 294 sensor and the OAG-prism together.

But thanks for your replies, so far.

Hi, Christian.
I have the EdeHD 8 as well.  I posted the following elsewhere recently which addresses your key question.  You'll see I have a rotator in the image train too.  
Note that the ZWO OAG-L has the same size prism as the Celestron - I never have any trouble finding guide stars.

Here goes...

I had the Celestron OAG, which was great but too thick;  I changed to the ZWO OAG-L for use on my refractor which has a much shorter back-focus.  It stays attached to my filter wheel (36mm) and camera (ASI2600MM) assembly.  The whole lot just moves from one scope to another.  See photo below.

I've incoporated a Prima Luce Lab Esatto focuser into my image train so that I don't have to fiddle with the mirror once it's in focus.  Because it's behind the EdgeHD internal lens elements, technically I am moving the length of the image train forward and aft of ideal backfocus, but my calculations suggest that it is less than 0.1mm focus range (although I may be wrong here) which is likely immaterial.  Once I have initial focus as close as I can get with the mirror knob (using a Bahtinov mask), I'll then start using the EAF for autofocus; an initial run and then each time I change filters. 

Here are my image trains for EdgeHD 8 at native FL and with 0.7 Reducer.  The 'half excursion focus' (language my supplier used, which is confusing but I kept it) is simply how far back I set the Esatto focuser as my starting point to achieve nominal backfocus.  Also, the Difference in actual vs nominal (0.7mm) accommodates approx 1/3 the thickness of the filters (2mm) which is the generally recommended backfocus adjustment for filters. 



And the OAG-L with 36mm EFW and ASI2600MM (no tilt plate).  The black tape on the EFW/Camera joint and on the base of the EFW is to prevent light ingress during the taking of flats in the mornings.  The guide camera, once focused, never needs to be touched again.

I hope that helps a bit.
Cheers.
Paul

I use a very similar arrangement to what Paul describes.  I use the ASI 174mm as my guide camera.  To get guide camera focus the sensor on the guide camera needs to be the same distance from the OAG pick off mirror as the sensor on the main camera.  My guide camera is almost inserted all the way to the helical focuser.  If you are less than 55mm from the pick off mirror to your imaging sensor, you might need to add spacers between the OAG and you imaging camera to get the correct spacing.

Hi Paul and Keith,

sadly that is not what I experienced. I was trying to find some info about the position of the sensor in different ZWO guide cameras but can't find anything with a quick search. We are talking of about 4mm difference here, which I find huge. Maybe I will try another cam as well. As previously mentioned, this is no issue when using the QHY 294M, because the backfocus distance is larger. Yesterday, I already put all the mechanical parts for the new scope together and installed everything. So I will try another guide cam later on.

Today I will install all the electronic parts and cables (I'm a set and forget guy) and then I am ready to go for first light as soon as the clouds disappear.

Thanks for your help.

Christian

Paul Larkin:
Of course, one needs to have the imaging camera precicely focused before setting focus for the OAG camera. Then once set, it is set and forget

Something must be wrong with what I've done back then. Of course, I did focus VERY precisely but I will check this with the next opportunity. I guess this might be the reason.

Thanks
CS

I have a C8 EdgeHD that I am using with a 0.7x reducer and a ZWO OAG-L (with a ASI174MM Mini for guiding). I have mostly achieved the correct back focus (~105mm). I was able to achieve this despite also using the Baader Diamond Steeltrack SCT focuser. Currently away from home. But will post pictures of the assembly asap.  

I was using the Celestron OAG. As an OAG, it is fantastic. However it is so thick and leaves no room for any other devices/mods. I had to use the ZWO OAG-L because I was fed up of the mirror flop and focussing issues with my EdgeHD and moved to the Baader focuser. Couldn’t have done this with the Celestron OAG

tldr; It is doable and ZWO OAG-L with ASI174MM has worked for me very well.

If you have 6.5mm+20mm÷OAG, then you will be too short to get your guide camera in focus.  This needs to be about 55mm total for the guide camera to come to focus.

I show you an image of my configuration with the ASI 2600MC.
The guidance camera is very important. Initially I had an ASI120mm but it has a very small chip size (1/3") and I could barely find stars to guide.
I changed it for the ASI 174 mm (1/1.2") with a larger chip and I have no problems.

Hello to all of you,

I am back from vacation and had the chance to set up the scope and all its acessories. I put together all the things I had in mind and tried some first images. After some tweaks, all seems to work and for the first time, I did some off axis guiding. This worked quite well and even the 20 minute Ha exposure was okay. The seeing wasn't good enough to judge the star shapes, but I am confident, their quality might increase a bit. Sadly, with only 3 hours of darkness, I was not able to collect enough subs to stack. Now the weather is bad again, as usual.

So this is the time to say thank you for all your comments. It helped a lot.

There is only one thing left I worry about. My star shapes in the guiding camera are quite poor. I had the mirror quite far from the image center and the stars were really elongated and unusable on the first try. So I tried to place the mirror nearer to the tubes center which helped a lot. But it is still not perfect. Going more to the center may cause vignetting on the sensor of the main camera, I'm afraid. I also collimated the scope, which lets me assume, that the stars in the guide camera might not be perfect anyway. After the guiding tests, this seems to be ok. But what are your observations here? Is there a way to improve? How does the star shape influence the guiding? I guess, PHD2 calculates the star center anyway and a bit of elongation might not be that problematic. Sadly, I did not save a screenshot of the guiding image to show.

Christian

Yes, you want the mirror as close to the sensor as you can get without vignetting.  You are picking guide stars from the very edge of the Edge 8 field-of-view, so your stars may not be perfectly round.