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

[AndyThilo]

Hi

First time trying to use an OAG. Last night was only a test night, get the scope focused, find stars etc. Well I succeeded with the 8", eventually got it aligned with Vega and focused up, tweaked collimation to get a central donut with Vega out of focus. Then slewed to M51, plate solved and no issues. NINA got M51 slap bang in the middle (excuse the red light, it was my headtorch). This was just a 10sec exposure :-

Then I load up PHD2 and nothing, just noise. The 290MM is mounted directly to the OAG so minimal focus adjustment via the stalk. So this morning I decide to put my limited AutoCAD knowledge to use and measure and draw it up but I'm not 100% sure my distance in Stellarium is right. I'd assume the prism/ccd distance is the distance from the centre of the image circle to the bottom of the visible image circle on the 290? Image below from CAD :-

I've drawn it for both vertical and horizonal orientation of the 290 sensor. The large centre rectangle is my 294MC Pro sensor, the small purple rectangle the 290MM sensor. The round circle around the 290 sensor is the 5.5mm hole from the OAG prism to the camera.

In Stellarium, my photographed image shows a rotation of 90degs and the stars in the image line up with what Stellarium shows. 

So clearly I've no guide stars in the FOV. So, some questions:

1. Is the 290MM a suitable guide camera and can focus be achieved? Anyone out there use a 290MM (NOT MINI) with the ZWO OAG?

2. Is my process correct? Distance assumptions correct?

3. What gain should I be using on the 290MM? Unity?

4. Is the 290MM sensor best vertical or horizontal in relation to the 294 sensor?

I considered a 174 Mini but it would be a waste as the image circle of the OAG is only 5.5 mm. The 290 covers this almost 100% and I believe sensitive enough for this purpose?

 

Appreciate in advance any help/advice. 

Thanks

Andy

[knobby]

Been through the same process bit with an Altair gpcam, had to use a pylon about 2 miles away to focus. However I worked it out on paper it didn't focus 😒 I guess it may be the variables of filters adding 1mm here and there ?

[michael8554]

Hi Andy

1) I thought from your description that you had focus, that finding stars is the problem ? Check daytime on a distant object as knobby suggests.

2) Dunno, but doesn't look implausible.

3) Dunno 

4) Hardly makes any difference.

290MM is a modern sensitive camera, ought to be good enough.

Armed with the excellent frame you've generated, couldn't you rotate the OAG next time to this position ?

Michael

[AndyThilo]

Hi

I can't get focus on the OAG as the farthest object I can see is only a few hundred metres away. I did some tests yesterday and left the Edge at the same focus required for stars. I compared the image of both main and guide camera looking at the out of focus tree and think they are similar. Both show out of focus moving leaves so it must be close. 

 I think I'm going to struggle with the 290MM though and would be better with a mini as that allows for easier focus adjustment. I'm not sure how adding the nosepiece to the 290MM would affect focus compared to being hard mounted to the OAG, I'll have to take some measurements. 

On a slightly different note, I stuck my 50mm guide scope on the edge and did a test early this morning. Only managed 5 reasonable frames before clouds rolled in. Guiding was around 0.7 total on my HEQ5 but I think i have either star trails or back focus is out. Stars aren't pin point for sure but not too bad. Again this was only 5 frames of 120secs each.

[michael8554]

Hi Andy

Why is the Mini easier to focus? 

Focus the imaging camera on Altair, then use your frame to put Altair on the guidecam, and focus. 

Sorry, I don't understand why that's hard to do. 

Michael 

[AndyThilo]

My 290MM (USB3.0 version) screws directly onto the OAG. A Mini can use it's nosepiece to move in and out for focus. Figured the latter would be easier?

[AndyThilo]

Or do I just put the nosepiece on my 290MM?

[michael8554]

From what I can see of the OAG construction on the ZWO site:

You set and lock how far the prism intrudes into the FOV with one set screw.

Then move the camera carrier up or down the locked periscope with the other set screw, to achieve focus.

Tricky if you don't have a distant daytime object  :-<

Temporarily move it into the front garden ?

If there's not enough range, add a spacer.

You don't want to throw away the advantages of OAG by introducing a wobbly nosepiece connection.

Michael

[AndyThilo]

Yeah it’s all set and I’ve measured everything. The only thing that’s an unknown is the guide camera focus. There is about 5mm of movement in the prism post, its nothing. Most people use a mini style nosepiece camera but I need to check the sensor depth difference. I’m sure I’ll figure it out, just hoped someone else had been in this situation.

[michael8554]

Are you unable to get the guidecam close enough to the prism to focus ?

Or not far enough away with all of the 5mm post adjustment?

The mini has 4mm less back focus, so would get closer, if the eyepiece holder doesn't add too much depth.

The 290 Mini nosepiece is M28, not M42,

If not enough height, then add an appropriate M42 spacer.

Get your ruler out and measure how far the 294MC sensor is from the prism, the 294MC has 6.5mm backfocus.

Then measure where that distance is up from the prism centre to where the guidecam should be, the 290 backfocus is 12.5mm.

That will give you a starting point.

Michael

 

[wimvb]

I assume that you've seen the diagram on the ZWO site about how to connect oag, main camera and guide camera. ZWO generally provide all the spacers you need with their cameras and oag.

You can focus your guide camera this way:

Target a bright star with your main camera and focus with a Bahtinov mask. Leave the mask in place and move the mount so that the same star is in the guide camera fov. Then adjust focus for the guide camera.

[Merlin66]

Not my images, but taken from a previous thread on OAG.

If I remember correctly there's an optimum back focus spacing on the Edge to the focus. If you've set the imaging camera to this spacing then the only variable you have left is the guide camera positioning........

 

[AndyThilo]

10 hours ago, wimvb said:

I assume that you've seen the diagram on the ZWO site about how to connect oag, main camera and guide camera. ZWO generally provide all the spacers you need with their cameras and oag.

You can focus your guide camera this way:

Target a bright star with your main camera and focus with a Bahtinov mask. Leave the mask in place and move the mount so that the same star is in the guide camera fov. Then adjust focus for the guide camera.

Sounds like a plan, I'll try it on the next clear night.

[Merlin66]

Andy,

In my experience it’s 100% easier to set up the OAG during the day.

once the imaging camera and the guide camera are both in focus, they will also be in focus for all other objects.

I ‘ve been using the equivalent of an “on axis guider” - a reflective slit plate for spectroscopy for the past ten years - no major issues.

 

[michael8554]

Hi Merlin

O to Ic = O to Ig.

Agreed, but hard to measure! 

P to Ic = centre of prism to Ig 

That you can measure, close enough to start with. 

How do you get your prism on the corner of the Canon FOV, is that your Spectroscopy setup? 

Michael 

[Merlin66]

Micheal,

with the reflective slit plates we use in spectroscopy we are “on axis” so we don’t have the same restrictions.

The point is to to focus the imaging camera then find the guide camera focus....

I would make the point that the guide prism needs to be rotated to give the best focus...

 

[wimvb]

56 minutes ago, Merlin66 said:

I would make the point that the guide prism needs to be rotated to give the best focus...

Geometrically, an oag configuration is similar to the secondary in a Newtonian. Just that the prism is small section of that virtual "secondary", as in the drawing. But shouldn't the prism be at 45 degrees then, and the guide sensor shifted? 

Or am I completely off here? 

Edited May 11, 2020 by wimvb

[AndyThilo]

2 hours ago, Merlin66 said:

Andy,

In my experience it’s 100% easier to set up the OAG during the day.

once the imaging camera and the guide camera are both in focus, they will also be in focus for all other objects.

I ‘ve been using the equivalent of an “on axis guider” - a reflective slit plate for spectroscopy for the past ten years - no major issues.

 

Agreed, but it just means I have to take it all out to somewhere. I'm rather lazy and like to do everything from home :D. But in this instant, it would seem I might have to venture out. Now I have it all setup in Stellarium I'll try it again on M27, there's loads of guide star all around it. If I have no success then I'll go out with it during the day. 

[Merlin66]

Yeah but......

The prism only represents a small part of a mirror diagonal - it's at the outer edege.

In my diagram I try to show that the prism needs to be re-aligned (tilted) to bring the image "on-axis" to the guide camera - hence the poor images in an OAG.

 

[AndyThilo]

Can't tilt the prism in a ZWO OAG though? Anyway tilt is the least of my worries, I just want to see stars

[michael8554]

8 hours ago, AndyThilo said:

Now I have it all setup in Stellarium I'll try it again on M27

I would start with Arcturus.

Get imaging camera focus spot-on with Bahtinov mask.

Then use Stellarium to put Arcturus on the prism, and fine-tune focus on the Guidecam to get lowest HFD reading on the PHD2 Star Profile window.

The star shape may well be poor at the edge of FOV, but PHD2 apparently copes.

Merlin's suggestion of prism tilt would improve that, but would be hard to incorporate in the design I'd say.

Then try your luck on some favourite targets.

Michael

[AndyThilo]

Yeah I could do that, once synced I guess it should be easy to manually slew the mount to position the star in the guide camera. 

[michael8554]

Can you not get your scope to goto the centre of the prism once you've Synched on Arcturus in the imaging camera ?

Michael

[Merlin66]

The original Celestron OAG had a tapered pin which allowed the prism to be tilted....

[AndyThilo]

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.