Switched to a ZWO OAG - very bloated stars in guide camera 120MM-S - solved!

[g__day]

Folks,

I am thinking of switching from a giant Lumicon OAG to a ZWO one to hold my primary ZWO 1600MM-C and guiding 120MM-S cameras.

I wonder if anyone can tell me:

1. If they have tried this and does it work well? (answer - Yes)
2. Do I need any special adapters to mate the ZWO OAG to a Meade Motofocuser? (Answer yes - plus extract spacers)
3. Will I find the 120MM-S on a SCT gives near pin point stars? (answer yes - once the extra spacers are added - in the correct location)
4. Is the unit solid? (answer yes - once you tighten the Allen bolt on the Prism arm - else the guide camera wobbles badly)!

On the third point I think I have done something rather wrong. My Imaging camera gives stars the size of peas at the moment - so I think I must have the spacing way wrong (very correct) - but it's proving hard to correct what should be a simple thing to adjust.

Many thanks for any thoughts, insights,

Matthew

[Camelopardalis]

Matthew, I don't have one yet but here goes...

1. it should have been designed with their cameras in mind, but it looks pretty versatile being so short in optical path
2. that depends on the camera end of the Meade motofocuser...the OAG ships with M42 and M48 adapters according to their product page https://astronomy-imaging-camera.com/products/accessories/zwo-oag/
3. that's the tricky part...it will depend on how well corrected your scope is. If you get coma/field curvature off-axis (conventional SCT) then I suspect not. Corrected scopes like the Edge HD should. It will depend also on how far the prism can intrude into the optical path...the 1600 isn't huge in the vertical, so you should be able to get close enough that star shapes shouldn't be too bad.

Regarding the size of your stars...are you at the end of your focuser adjustment or are they peas at the minimum?

[g__day]

They stars were peas at what seemed to me optimal placement!

So I got the ZWO today. A few comments:

1. The prism being able to slide in and out is very useful - I have positioned it just outside the light cone of the 1600M's chip

2. The holder for the 120MM was a bit wobbly - just a smidgeon when I had tightened everything and showed the folk in the store. Turns out when I got home there was an Allen key for the prism cell that just hadn't been tightened correctly - everything is snug now!

3. Mating the OAG to a Meade motor focuser gave Bintel some confusion (or just the first server). Luckily I brought in the whole old unit - Lumicon focuser, attachment to Meade Motorfocuser (that I'd originally bought of BinTel) and both ZWO Cameras. It took some struggles but two threaded adapters later and I had exactly the right connectors. It took (quite) a bit of asking it looks like a smooth 50mm wanting to connect to a 48mm threaded socket - what can you sell me to mate to it - until eventually something worked perfectly!

4. The ZWO 120MM body is large enough that it can only be connected to the OAG and not hit the Filter wheel when mounted upside down.

5. The sliding ring tube spacer on the 120MM has a tightening bolt who's head is wider than the tube - so it can't sit in flush to the tube if its almost racked all the way in!

Attached is a shot of all the gear attached - now for a clear night to see if it's all looking good when focused and space correctly! Last night I tried my backup guider - a piggy backed WO 98mm using a QHY5-IIL - stars were pin point in PHD2 - so fingers crossed!

[glend]

Just one comment on your spacing. You still have the black adaptor collar on the front of the camera. The camera should be able to screw directly to the filter wheel (mine does to my QHY wheel). This ensures no vignetting by keeping the filter as close to the sensor as possible. This will also remove spacing at the rear which means you can add spacing in front of the wheel if required.

[Merlin66]

Matthew,
The star images, as mentioned previously will always be larger at longer focal lengths.
I'll be interested to see your results.

[g__day]

Hey Glen,

Thank you for that - it was set up this way by Bintel!

How noticeable do you expect Vignetting will be with the gear set up this way?

I have only done a short series of gear tests so far, nothing that will really push my gear whilst I am still checking it out.

If I do see it should be easy to move the spacer to the other side of the filter wheel - thus preserving the parafocal distance between the two imaging cameras.

/edit/ - Just checked - the black protusion on the 1600MM-C attached to the filter wheel is the actual chip, not a spacer! - ed -totally wrong sorry - it is a spacer, where it's not needed, attached to the camera way too tightly!

Of course a flat frame will help correct images post processing - but make sense to minimse the need for correction post capture!

Hi Ken,

True - but seeing the two cameras are at the same focal length with the same pixel size - I would expect the stars in both cameras to appear as sharp as each other. I think I did something wrong that I couldn't figure out in my last setup. I am hoping a simpler, more compact design should eliminate this issue!

[Merlin66]

Matthew,
Just a thought...
When I used the Giant Easy Guider on the 12" Meade SCT I used to make use of the built-in reducer. Is this the set up you were using?

[g__day]

Hi Ken,

I did originally plan to do that - but my shots had significant coma and the guys from Lumicon had no idea why. I did have a Meade motofocuser in the light train - making it about 3" - 4" longer than standard - but they said this shouldn't have caused what I was seeing!

In the end I removed and stored the unit in its original boxing. If this all works well I may sell all the original gear (modded Canon 40Ds and 400Ds and mains power adapters, Lumicon OAG and focal reducer, Hutech IDAS light pollution suppression filters, Shoestring astoronmy DSUB etc).

[glend]

Ken, i suggest you check the camera drawings on the ZWO website product page. That is not the chip, the chip is inside the red top cover of the camera. It is recessed 6.5mm from the front of the red end cap. That black collar is a male to female thread adaptor which is supplied with the camera. It may not affect your setup but for some buyers having access to that spacing could make alot of difference.
Good luck with it.

[Camelopardalis]

Yeah the scope-side ring on the 1600 unscrews to reduce the chip to filter distance to ~10mm.

Regardless, I'd doubt you'd see any vignetting at f/10, but like Glen says it gives you a little more flexibility if you need it.

[g__day]

Guys,

From the pictures on the web it looks like you are right - will double check again tomorrow when it stops raining - puzzled how this tricked my eyes!

[AstroApprentice]

Matthew, the 1600 manual (p 16-18) shows pics describing how to screw off t ring and attach efw directly to camera :thumbsup:

[g__day]

Just read that - but blowed if I can unscrew it by hand! Not keen to use any tools on this, as when I tried hard (extreme grip strength) it felt like the red unit housing might start buckling.

Did anyone else find the ring felt like it was really put on hard (like by machine)?

[g__day]

So spacing is a bit closer to correct - but issue persists with a new OAG!

I need to move the focuser about 3-5mm away from the OAG - simple extension tube should sort that.

However, using the OAG - even shifting focus until stars were the smallest I could get them - stars are still HUGE - even when the imaging time is 1/100 of a second.

What is going on here? What amd I missing?

I took shot of some random stars at first using TSX - nice and tight and small stars on the 1600MM-c (which has almost the same size pixels as the 120MM-s) - at full 2.3 metre focal length of the C9.25. Same stars on the 120MM-s - see below where montrously huge - even at best focus (not using a Bhatinov mask - just visual inspection moving the mirror in / out with a 10:1 fine focuser and looking at 2 second light frames).

Then I tried PHD and PHD2 - same behaviour between the two cameras.

Finally I tired my QHY 5II-L on a WO 98mm refractor - lovely tight stars again.

Be grateful for any advice as to what is happening here and how to fix it!

[peter_4059]

Is the camera gain set too high?

[g__day]

Looked like it - but reducing it from 95% to 0% - yes 0 - still produced that result.

OAG pick off prism was about 3-4 mm outside of the main imaging chip's light cone by visual inspection. Tried binned and unbinned shots - no material difference.

Seeing was very average - with very poor gain accidentally set on the 1600mm-c here is what its stars looked like - same focal length, almost identical pixel size 4 scond shot.

[glend]

Have a look at the "OAG and FW setuo for ASI1600" thread on Cloudy Nights forum. There is a good diagram which shows the required spacing to the guide camera sensor. The tread is in the Beginining and Intermeaduate imaging sub-forum, started on 5 Nov by anismo. You will need to know the exact setback required for your guide camera.

[peter_4059]

Can you get good stars with the zwo120mm through the wo98mm refractor?

[g__day]

Started to try that but the weather closed in. Will also try the old Meade DSI II Pro mono in the ZWO OAG to see what it sees. More data should help isolate the root of the problem.

BTW - Merry Christmas to all!

[Merlin66]

What's the measured FWHM of the star images??

[g__day]

Didn't try that - but at their very smallest it was probably spanning 50 pixels instead of 3 or 4! So large that PHD and PHD2 couldn't even recognise them as stars - try to centre on them and it would just say star lost, star mass 0!

[g__day]

Fingers crossed things go a bit better tonight - this thread was useful

http://www.cloudynights.com/topic/555636-oag-and-fw-setup-for-asi1600/

Doesn't seem to match the way Bintel set things up for me!

[Camelopardalis]

I think the moral of the story is, don't use spacers if you don't have to...but at the same time make sure you pad out the correct back focus.

[g__day]

Actually Dunk et al - spacers turned out to be the solution!

After a morning finishing at 4am - pleased to say everything is beautiful now - possibly just a tad more tweaking to do but that should be easy now.

Turns out there where three issues:

1. the prism needed to be moved in about 6-8 mm closer to the main imaging chip (which is still way outside its light cone) and most importantly

2. mated the filter wheel directly to the 1600mm-c and checked all the filters turn easily - they do!

3. moved the spacer in front of the filter - between the OAG. This helped but still no focus - I did get doughnuts rather than huge, wierd bloated stars on 1 second guide camera shots - meaning it was working now so I went to add another spacer - smaller doughnuts and finally a third spacer did the trick with no adjustment - perfect tiny stars - FWHM under 10. Still some tweaking to do on these tiny pixels - will try binning next. But easily good enough to guide off!

Some pictures of the correct set up follow.

Many thanks all!

PS

The last thing I have to sort now is why stars are so bright after I messed up the gain / offset or some other setting in TSX. I reset gain to 0 and Offset to 10 - but its like the gamma on the main imaging camera is set way too high.

Any ideas how I fix this issue if it already says gain and offset are right down low in the driver. A bit too tired to work out what dum thing I did when I thought I was playing around with the guide cameras gamma in PHD or TSX I believe.

Cheers all , Matthew

[Camelopardalis]

Good to hear it's all working nicely now :thumbsup:

Another moral is that you need to find the right imaging back focus :lol: looks like you've found it by trial and error. Celestron publishes theirs for the Edge HD scopes with a tolerance of +/- 0.5mm to ensure tight stars across the field. Many smaller imaging scopes are setup for DSLRs and their 55mm distance, and the 1600 was somewhat setup to mimic this. Sounds like your scope has quite a lot of back focus!

Ultimately, the optical path needs to be right for both cameras...

[Camelopardalis]

Have you tried opening the FITS file with another viewer? Considering it's linear data, the viewer part will be stretching the image to make it visible. Maybe TSX has settings to control the preview?

[g__day]

I could easily believe its a TSX setting I have applied wrong in the FITS viewer itself. I am not at all skilled working with FITS or TSX - so I need to figure what setting I tweaked incorrectly. With any luck the RAW data is still fine - its the viewer I have amped things up to far!

Oh and I also discover TSX has some problems with JMI PCFC -> Meade Motorfocuser. Native PCFC controlled the focuser fine - TSX spat the dummy!

[Camelopardalis]

Doubtful that TSX applied a display function to the saved file, but you could check in something like FITS Liberator or PixInsight or similar.