Hi fellow astro photographrers, last week I bought a used Celestron Edge HD 800 used with nearly all accessories I need for astro photography. The offer was very good and I can't resist. Sometimes, you just have to go for it. Today it arrived and I am really happy about the condition of the scope and the parts. The only thing that's missing is the Celestron OAG. I can't buy it now (once again I'm broke :cool  , but I will do later. Until then, I have to use what I already own. Actually I am planning the backfocus distance and take into account all the equipment I will use with the scope. Basically, I plan for a QHY294M Pro, a EFW 7x2" filter wheel, a Wanderer Rotator Mini V2 and hopefully the ZWO OAG-L. Everything should work fine except maybe the OAG-L. That's where my question comes in. When I measure the opening of the reducer, I get about 33-34mm. Reading the specs on the TS website,it is specified with 30mm, which is even less. This means, I will get a hard time using the OAG and in the end, it may not make sense to even use it. So are there any of you out there that use this combination (or tried to use it). What's your experience? I own several SVBony 60mm guide scopes with a focal length of 240mm. These may work at least with the 1422mm focal length. But the OAG (paired with a ZWO ASI220M) would be the more precise and preferred option. Thank you for your replies. CS Christian
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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.
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I can attest to the difficulty using this setup. 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?
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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.
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Here is a picture of my setup. As I look at it the ZWO OAG should fit fine as you say.  |
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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.
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It should be as people both use larger cameras and larger OAG's (I believe the Celestron has a bit bigger prism).
I use a IMX571 camera and didn't notice any intrusion with the Celestron OAG at least.
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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
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Hi @Paul Larkinthanks for your post. This is exactly what I was hoping for. It tells me, that I should give it a go and the opening is wide enough to use the OAG-L. Seeing your image of the camera/filter wheel/OAG combination, there is one more question I'd like to ask. I used the ZWO 294MM with the exact same combination. But it seems, that the correct focus setting for the guide camera was with the helical focuser of the OAG screwed all in. I used a ZWO 220M Mini camera. How about your setup? is there some room left to focus the guide cam properly? In case of the Edge setup, there should be no issue, because the QHY294M has 17.5mm backfocus distance to the sensor, other than the 6.5mm of the ZWO cameras. So I have 11 more millimeters left to find the right focus distance for the guide cam. CS Christian
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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.
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Hi, Christian.
Like Keith, I have the ASI174MM mini as my guide camera. It is inserted all the way in and without the need for a spacer (as per photo above). The helical focuser is at position 4.2 (out of 6) on mine; plenty of room to adjust in either direction.
Cheers.
Paul
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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
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Hi, Christian. I found the following on the ZWO ASI website, here: https://www.zwoastro.com/product/mini-cameras/It seems they all have the same distance to the sensor. The drawing below indicates 8.5mm. Maybe QHY is different, but as Keith and I suggested above, we have no problem focusing the ASI174MM (which by the way is great as a guide camera). Naturally, this will change if you have the extender screwed on. I use it with just the threaded cover, as per the drawing and photo at the bottom of this post. 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  Cheers. Paul   With just the threaded cover - no extender:  |
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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
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If I understand your image train correctly I believe you have:
Camera 17.5mm
EFW 20mm
OAG-L 17.5mm
Rotator 16mm
Focuser/telescope
If this is indeed the case, you have the same spacing between the OAG and your sensor as Paul and I do, so you should be able to focus the Guide cam. Can you describe your image train and spacing - is it as above? Is there a tilt adapter between the camera and the filter wheel?
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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.
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Keith Mombourquette:
If I understand your image train correctly I believe you have:
Camera 17.5mm
EFW 20mm
OAG-L 17.5mm
Rotator 16mm
Focuser/telescope
If this is indeed the case, you have the same spacing between the OAG and your sensor as Paul and I do, so you should be able to focus the Guide cam. Can you describe your image train and spacing - is it as above? Is there a tilt adapter between the camera and the filter wheel? Not exactly. I use the Wanderer Rotator Mini wich has only 10mm optical length. I had to fill up the rest of the space with about 38mm of spacers. I can't tell exactly now, but I calculated everything. There are some adpaters to get from one thread size to the next. There is also a 2mm adapter between the camera and the filter wheel. As mentioned, I don't see any problems here. But the setup I had problems with is the ASI 294MM mounted on the filter wheel and then the OAG-L. This is 6.5mm + 20mm + OAG. That's why ZWO mentioned in the manual of the OAG, that the setups need a ZWO guide cam which in general have shorter backfocus distances than i.e. GPCams or else. Yesterday I finished the scope setup. It is mounted on my EQ6 now, but I haven't tried all the added stuff, yet. The next 3 weeks I am very busy and I can't see room for working on the scope, sadly. But after this it's summer break and hopefully the weather will be good. CS Christian
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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.
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Hello I use OAG-L with C8 EDGEHD, focal reducer f7, ZWO Filterwheel 7x36mm and ASI533MM Pro
it works very well with an ASI174MM.
no annoying tilt has been reported so far.
final real focal length should be around 1480mm when you respect backfocus.
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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.  |
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For my oag, either zwo oag or askar oag just work with 174mm-mini, with no vignetting on 2600 sensor. For me, it looks like guide camera is more important. Previously my 462mc failed although it's more sensitive, due to it's small pixel size.
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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.
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Rich Sornborger:
I can attest to the difficulty using this setup. 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? I have an EdgeHD8 with the celestron 0.7x reducer and 174mm. I don't use anything from Orion in my image train. All the parts/connectors are stock from either Celestron or ZWO. This setup has been working for me for a few years.
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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.
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No problem for me with Edge HD 8'', reducer with OAG-L in 36mm filter wheel with ASI174mm and ASI2600mm.
Only a "dark" curve in the 20/25% of the guide frame in the long side.
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