ZWO OAG with ASI290MM (USB 3.0 version) & Edge HD 8"

[michael8554]

Arcturus is not a suitable guide star, it's so bright it's over exposed.

Of course its jumping around, that's what you're trying to get rid of when you guide !

If you've run out of focus range,  add a couple of mm of spacer into the main imaging path.

When you come to guide for real you'll have to employ your FOV frames to find stars, just having the target in centre of frame is only the start !

Michael

Edited May 14, 2020 by michael8554

[AndyThilo]

I wasn't trying to guide on it, just use it to focus the 290MM. 

[michael8554]

Sorry, I misunderstood when you said the star looked horrible and was not guideable.

A bright star like Arcturus is good to start setting focus with, as even way out of focus it presents a large bright Airy disc, still bright enough to see - a dim star would be invisible if way out of focus.

Fine tune on dimmer stars like you'd expect to guide on.

Michael

[jimjam11]

On 14/05/2020 at 10:39, AndyThilo said:

Well.... Finally got out last night with the OAG. Did a quick polar alignment, slewed to Arcturus, plate solved, focused and tweaked bobs knobs. Synced in Stellarium and used that align Arcturus within the guide scope FOV (theoretically). Slewed to the spot and I saw a star in PHD2! Not far out of focus, but out of focus none the less.

Couldn't get any better focus as I needed to move the camera inwards, but it was already in as far it would go, tried moving the prism down a little as I was only seeing image on 2/3 of the sensor. Tweaked and played, no joy. Star looked horrid and wasn't guidable. Shimmering and moving about. All the little dots are hot pixels as I hadn't done darks in PHD2. 

So I don't know where to go. I think I'll stick with a guide scope. I was getting 2mins no problems with my 50mm, might swap to a 60. The 290MM seems unsuitable for the OAG, too hard to adjust to focus and I'm wasting too much time messing about.

 

 

 

An oag is a pain and causes all manner of issues which need to be resolved. However, if you get it working and your mount can handle the improved sampling you will get measurably smaller, rounder stars. It might be worth waiting for the moon because it is easy to find and you can then nail focus?

Moving the prism in will not alter focus, you need to move the upper part of the stalk to change focus, or physically move the oag backwards/forwards with spacers.

Also be very careful of shadowing the main sensor such as this:

 

[Merlin66]

James,

OK, sounds like you need the change the spacers on the main imaging camera to give more room for the OAG/ Guide camera to work.

I've been using a "on axis" guider - the reflective slit plate on the spectrograph, on the C11 @ f10 successfully for many years. Once you're set up it will work!!

[jimjam11]

Another thought, do you have a different ota which can focus at shorter distances? I use my zs73 to get main and guide cameras parfocal. The zs73 can focus down to a few metres so this is easily done during the day.

 

You can also capture flats to look for shadowing issues, although this varies from ota to ota...

[AndyThilo]

Final update, success! Sold the ZWO OAG and my 290MM and bought a Celestron OAG and a 174 mini. I was careful setting up to get my measurements right and not block the 294's sensor and last night used it for the first time. Focusing the 174 was so easy, I focused the main scope on Arcturus, synced my mount and plate solved in NINA. Then in Stellarium, I moved slewed the mount so the OAG would see Arcturus and focused the 174. 

I started imaging M94 and was having trouble with guide stars but soon realised I needed a brighter star to guide on. The 174 shows a huge FOV in PHD2, very impressed and so easy to focus compared to the 290MM. TBH I originally ordered a 290mini but the supplier sent me a MM as the minis were out of stock. 

 

[michael8554]

On 17/05/2020 at 10:57, jimjam11 said:

So I don't know where to go. I think I'll stick with a guide scope.

Well done, got there in the end, OAG was the right way to go.

Michael

[AndyThilo]

2 hours ago, michael8554 said:

Well done, got there in the end, OAG was the right way to go.

Michael

Yeah the 290MM might be an excellent planetary/guide scope camera, but it sucks for OAG. Can't see any planets from where I live anyway so no loss :D. 

[SimM]

On 14/05/2020 at 10:39, AndyThilo said:

Well.... Finally got out last night with the OAG. Did a quick polar alignment, slewed to Arcturus, plate solved, focused and tweaked bobs knobs. Synced in Stellarium and used that align Arcturus within the guide scope FOV (theoretically). Slewed to the spot and I saw a star in PHD2! Not far out of focus, but out of focus none the less.

Couldn't get any better focus as I needed to move the camera inwards, but it was already in as far it would go, tried moving the prism down a little as I was only seeing image on 2/3 of the sensor. Tweaked and played, no joy. Star looked horrid and wasn't guidable. Shimmering and moving about. All the little dots are hot pixels as I hadn't done darks in PHD2. 

So I don't know where to go. I think I'll stick with a guide scope. I was getting 2mins no problems with my 50mm, might swap to a 60. The 290MM seems unsuitable for the OAG, too hard to adjust to focus and I'm wasting too much time messing about.

 

 

 

Hi Andy (and Knobby too)

How are you finding the Celestron EdgeHD 800 + Celestron 0.7x Reducer and Celestron OAG "combination" for AP?

I read through more of the topic - you switched from ZWO to COAG and small camera to 174 too. With it working, what height did you set the prism?

I bought an EdgeHD 800 because it has a flat and crisp image out to the edges (according to the published "blurb") so ideal for AP as well as visual use.

I also wanted a Reducer because the accepted wisdom here and on Cloudy Nights (CN) is that imaging with a longer focal length and slower F/10 makes the task particularly difficult.

I also bought (but returned) two Celestron OAG whilst waiting for a Celestron Reducer to become available.

Simon

Edited June 6, 2020 by SimM

[AndyThilo]

My limited use so far has shown the Celestron OAG to be excellent with the 174. I also have problems with rotation but usually I have a guide star or 2 to chose from so it’s no issue. My rotation issue is caused by the auto focuser though and certainly isn’t an issue with my feathertouch so not sure why you have issues? Photo below showing my setup with the reducer.  Ignore the focuser, I’ve been collimating mine and needed manual focus 
 

[SimM]

6 hours ago, AndyThilo said:

I also have problems with rotation but usually I have a guide star or 2 to chose from so it’s no issue. My rotation issue is caused by the auto focuser though and certainly isn’t an issue with my feathertouch so not sure why you have issues?

Glad to know that you have a working solution.

The problem was when not using the Reducer (which I didn't have when I tried it).

Without the Reducer the gap between the OAG and the back of the OTA is minimal, so with the thumbscrews pointing towards the scope, they caught on the  focuser and focus locks.

How close can you get the prism to the 294 e.g. centre of the prism to the centre line of the OAG?

Your OAG is perhaps a little different e.g. there isn't a ring under the helical focuser - I could rotate the lock and this would limit the rotation of the focuser - so the thumbscrews were out of the way.

However, if it fixes the thumbscrew problem, then it may have introduced another one.

The minimum height I could lower the prism was to a centre line of 17mm and the view of the Reducer (when it arrived) was not very good.

If you remove the guide camera and look in, do you see the image circle or only a small portion of it e.g. is your prism perhaps further in than I could achieve?

One effect I noticed is that the 294 gives a very nice image but the guide camera is all washed out e.g. it struggles to see anything clearly. When not used with the OAG it's absolutely fine, so it must be how it's positioned? 

Simon

[AndyThilo]

Not sure why without the reducer you have this issue, the OAG should be on the end of the T adapter using the 42mm adapter that comes with the OAG. It certainly shouldn’t be directly on the back of the OTA as that would mean the guide camera would have to be spaced out a long way to match the main camera.

Edited June 6, 2020 by AndyThilo

[SimM]

4 hours ago, AndyThilo said:

Not sure why without the reducer you have this issue, the OAG should be on the end of the T adapter using the 42mm adapter that comes with the OAG. It certainly shouldn’t be directly on the back of the OTA as that would mean the guide camera would have to be spaced out a long way to match the main camera.

I only have a problem using the Reducer with the COAG.

The setup is the "normal" COAG one as shown in the manual (COAG instructions don't have a Reducer example) or with the Reducer (neither instructions references the other product), as you would expect:

  a. EdgeHD 800  - COAG - T adapters - ASI294MC, with QHY5III-178M in helical focuser (this is OK for both cameras);

  b. EdgeHD 800 - Reducer - COAG - T adapters - ASI294MC, with QHY5III-178M in helical focuser (this is NOT OK for the guide camera but is OK for the imaging camera);

 

There is also without the COAG e.g. for "normal" use with/without the Reducer and one camera:

  c. EdgeHD - visual back - camera (QHY5III-178M or ASI294MC) (this is OK for both camera);

  d. EdgeHD - Reducer - T adapters - camera (QHY5III-178M or ASI294MC) (this is OK for both camera);.

 

Where:

• COAG is SCT Adapter - Adapter 4.5mm - OAG - Adapter 12.5mm (as supplied);
• T adapters or visual back are to make up the BF of 105mm or 133.5mm - a combination of 11.55mm + 21mm + 16.5mm (as required);
• Cameras are ASI294MC (imaging) and QHY5III-178M (guide camera).

So from the above, "without the Reducer": means (a), (c) or (d) and "with Reducer" means only (b).

In the above, Celestron Support say that the COAG is designed for (a) but not (b). I think they mean originally for C8 but now (coincidently) also for EdgeHD 800.

Of course, the scenario I really want to get working is (b) e.g. using EdgeHD 800 with Celestron 0.7x Reducer and COAG, with both cameras active.

"OK" means the image is OK e.g. look at branches on a tree etc. and recognise them. "Not OK" means it's all washed out and very poor.

It's not hard to see why the image is so poor when seen through the prism with the reducer - the prism is so high up that it's out of the field of anything decent to see.

Simon

[AndyThilo]

I don't know why you're having problems, my setup is similar and you can see my photo there are no issues rotating. Also stars are fine through the OAG. Can you post a photo of your setup?

[SimM]

Andy, I know you moved from ZWO OAG and 290 to Celestron OAG and 174. I'm really glad it works for you.

The 174 gives extra height so that you may get a better view of the back of the Reducer. Not sure if it still works when you have the guide camera horizontal and not vertical - that's how most users want to align their cameras.

I have only tried Celestron OAG and 178. The first COAG was faulty and was returned. The second COAG has a ring below the helical focuser (NB it's not on the COAG you have). It may fix the rotation problem but it also puts the prism too far off centre. That's why in the scenarios (a) - (d) the results vary so much. When Celestron Support say it's not compatible, I simply returned it for a refund. I have kept the Reducer and also the 178 and obviously the EdgeHD 800.

There are a couple of other scenarios I also need:

1. Using a Baader ClickLock to mount a diagonal or an imaging setup for ease of switchover (needs a "thin" OAG); 
2. Using Reducer + OAG + DSLR - where COAG only gives BF of 121mm minimum (and I want to use 105mm);
3. Using a Corrector/Reducer with a Refractor - the minimum BF is often 60-65mm and requires a "thin" OAG.
4. Using Reducer (and without the Reducer) + OAG, keeping the parfocus of the guide/imaging camera unchanged;
5. Using the guide camera horizontal and also vertical relative to the main camera (needs to be close to main camera);
6. I already have a 178 guide camera and not the larger 174.

(1) doesn't work with the thick COAG main body. (2) this doesn't work so using a bigger sensor on DSLR is not an option. (3) doesn't work with Refractors that have Corrector/Reducers and a minimal BF. Also not with filter wheels e.g. mono. (4) varying the spacing in front of the OAG allows removal of the Reducer , keeping the focus of the two cameras relatively unchanged. (5) want to be able to use the guide camera vertically and also horizontally and for the spacing to the main camera to be less than the minimum that's available. (6) 178 is what I already have to use as a guide camera. The 290 is probably the most sensitive guide camera available but also has a small sensor so searching for guide stars isn't easy with these, 178 is bigger and 174 is the biggest available.

The Celestron Support "solution" is to vary the position of the prism using additional parts. The effect is to give more options to vary the BF in front of the OAG when using a Reducer. It's also the "solution" if you want to use a DSLR with the Reducer.

Most things can be "fixed" e.g. one "simple solution" is to use a 1/2" SCT extender between the Reducer and the COAG - the space can be taken away behind the COAG to maintain the BF. This allows the two sensors to be placed closer. For a DSLR, another "simple solution" is to use a very thin (4mm)  SCT adapter e..g a Baader one, so that the BF for a DSLR with the Reducer can be  set at 105mm. There isn't an easy solution for using a Refractor apart from using a guide scope, but this increases the demands on the mount e.g. a bigger mount may be required.

Having a FL of 2032mm, 1490mm, 700mm, 400mm etc. is the same as varying the EP for visual. To frame different targets requires a combination of SCT, SCT with Reducer, Refractor, smaller Refractor etc. Most Refractors provide a faster/flatter field with a Reducer/Corrector but the BF for these is often quite minimal - that's why other "thin" OAG are often used. The ideal is one rig that doesn't put too much strain on the mount and can be used interchangeably with every scope/reducer combination.

Simon