RASA 8 image problems with ZWO ASI 1600mm-pro OSC

[Petrol]

I have recently bought a second hand RASA 8. I've fitted my ZWO ASI 1600mm-pro OSC cam and managed to achieve back focus using a couple of adapters I had.
Decided to give it a first light with a temporary guide cam/no filter, very poor conditions etc etc

This is 180s sub of M81 & M82

[img]https://i.imgur.com/ctrXkLJ.jpg[/img]

I was a bit surpised at the outer edges of the image

[img]https://i.imgur.com/v5fl3sG.jpg[/img]
 

Is this a collimation problem or something else?
Any help would be appreciated.
Many thanks for looking

Pete

 

[Petrol]

[michael8554]

Focus in the centre is slightly off (or is it slight dew ?)

The corners have coma.

Michael

 

[tomato]

The RASA8 at F2 is super sensitive to focus and tilt. I agree the focus is off a bit but the bottom LH corner stars look a better shape than the others so that would indicate to me that the backfocus is not far off and there is some tilt on the sensor. Other RASA8 owners on SGL have made a simple jig to allow tilt adjustments off the scope, see attached thread. I have a QHY 268c which has a coarse tilt adjuster built in which allows me to make adjustments fairly easily when the camera is on the scope but either way you need to have some form of tilt adjustment with your ASI 1600.  The Celestron RASA camera adapter has some squidgy material on the mating face which I wasn't  keen on, as it introduces a variable into the imaging train at a critical point, but other RASA imagers use it OK.

Don't be put off, the RASA is an amazing imaging scope.

 

[ollypenrice]

It's a fickle beast in my view. I had camera from the one based here on the tilt jig again because our stars have gone off along one side. You really have to get everything right on the RASA, notably collimation and tilt. This is not a scope which is going to like lots of random spacers, etc...  The Artsy adapter dispenses with the pads which troubled Tomato, above.

Have you collimated it? The only sane way to do it is with a small cylindrical camera (we used our guide camera) which lets you get at the collimation screws while still in situ.

When it's going well the RASA is formidable, though.

Olly

[Fegato]

As above, back focus distance has to be spot on for all four corners to avoid this. I had a particularly badly matched 8 Edge HD and Hyperstar lens where I had terrible coma in all corners at the correct back focus distance (at least badly matched is my conclusion, although you might say a bad workman blames his tools!). So when I got a RASA 11, I also invested in an Octopi tilter for my ASI2400MC Pro. This allows 4 point adjustment (so easy to work on the 4 corners of the image) for tilt as well as providing a good few millimetres adjustment for back focus. Using this and the new facilities in the Hocus Focus plug-in for NINA makes sorting this out almost enjoyable. Sadly though, the Octopi devices are bespoke and very expensive.

[Petrol]

Thank you for your valubale feedback. I very much appreciate it.

Dew was a problem when I took the image, the laptop was wet through. I joined 2 heat bands together to try to combat it but it could have been out of focus as well.

I've fitted a dew heater ring and a Starizona filter holder today. I live in a Class 5 bortle area so have also fitted a ZWO duo band filter which enabled me to remove the "clear glass"

The Starizona is fited with a 5mm spacer, (I thought ZWO cameras needed a 7.5mm spacer) The flange is thicker though. Not sure if this is to compensate for removing the white plastic seals or not. Either way I've removed them.

It's probably best to do things one step at a time and see if theres any improvements. Again thanks for your help, I really appreciate it. I can see this scope has real potential if I can get it sorted.

Some pics and btw, I've also upgraded the guide scope to a WO Zenithstar


Clear skies
Pete
 

[Fegato]

Looking good!

I think fitting the Starizona filter holder leaves 17.5mm, which is right for the ASI1600?  Also, you might want to check with Starizona, as I think they say to leave the Celestron optical window in place. The glass all makes a difference to the back focus, so this would be important (each mm of glass in the filters adds 1/3mm to the back focus distance required - albeit the Celestron window is already built in to the RASA back focus stated requirement).  To be clear for yourself, you might want to measure your image train carefully, including the ZWO filter glass addition, and compare to the RASA 8 requirements. 

Do you have a dew shield? It's essential really with the RASA. Apart from the dew, it helps keep out stray light and reduces the chance of reflections from bright objects outside the field of view. I have a flexible Kendrick one, and it works fine even on the larger 11. 

[ollypenrice]

I should have thrown in another snippet of information which we've found out: the edge and corner stars need focus to be bang on. When it isn't, the first thing we notice is not a loss of sharpness in the stars but elongation along one edge and/or in the corners. The stars can look pretty sharp in the middle and bad at the edge but further refinement of focus can fix it. I've never seen this before.

Olly

[Fegato]

1 hour ago, ollypenrice said:

I should have thrown in another snippet of information which we've found out: the edge and corner stars need focus to be bang on. When it isn't, the first thing we notice is not a loss of sharpness in the stars but elongation along one edge and/or in the corners. The stars can look pretty sharp in the middle and bad at the edge but further refinement of focus can fix it. I've never seen this before.

Olly

Yes I've noticed this sometimes. I wonder if it's because the back focus is slightly out, so you have to find a sort of compromise focus that evens it out a bit.

The Hocus Focus plug-in for NINA I mention above allows you to plot focus curves for the centre and all four corners of the image. This makes working out what is going on much easier - you can see immediately which corners are inside or outside focus versus the centre.

[gorann]

As Robin pointed out you really need a dew shield for the RASA. The good thing with that is that the camera then produces enough heat to keep dew away so you have no use for a dew heater.

I would first try to set focus carefully as Olly points out. If that does not work for the corner stars try to fiddle with the sensor distance (the sensor lies 6.5 mm or 17.5 mm into the ASI1600 without or with the 11mm extension ring that came with the camera). One thing to check is that a defocused bright star really looks symmetrical. It has to be spot on, and if not you need to collimate (using a small camera and the CS adapter that came with the RASA so you can reach the screws.

When you get the RASA8 to work, a very different and very deep imaging world will open up!

Edited April 15, 2022 by gorann

[Petrol]

Thanks for the fantasic feedback. You guys are awesome.

I've made a temporary dew shield. I'll buy a proper one when I decide whats best to go for. I'll definitely need one for flats too. I'll run the dew heater on minimum for starters
I've been away from astro photos for many years but now I have more time to get back into it. Most of my gear low quality and antiquated but I'm hoping to bring things up to the times. I don't want to rush in and buy the wrong equipment though. Got the T shirt and cap on that one!
Might get a chance to image tonight so I'll leave it as it is and see what I get
I've recentely installed NINA and am getting my head around the software. It's all very complicated but I'm sure I can get to grips with it in time.
I now understand that focus is mega critical with these scopes. I'll use NINA to focus and hope I can put the stats up. I'll move on to Hocus Focus plugin later, it looks mega! I'm wondering if an autofocus upgrade is the way to go.
I'll check that a defocused star is symmetrical as soon as skies permit.

Have to say I'm quite excited about this scope, I could see the dso after a 10s preview. Never been able to do that before! I'm hoping the narroband filter improves things as well. Mono imaging with filters seemed all a bit daunting to me along with not enough clear skies so I'll stick with OSC for now.

Couple more pics of where I'm up to

Again many thanks for your help.

Pete

 

[tomato]

Another snippet about the dewshield which might be peculiar  to me as I operate the RASA inside a dome, but on warm nights I have found that the warm air expelled by the camera gets recirculated inside the confines of the dewshield which results in a drop off in performance of the camera cooler. It’s not a big deal as with CMOS cameras as I don’t run them well below zero, and if you are outside it might not even show up as a problem.

The RASA8 with the dual band filter or in broadband imaging is outstanding. I obtained a decent image of the Dark Shark Nebula in a few hours, that my dual  Esprit 150 rig, using the same cameras, could not match.

[Petrol]

Looks like progress thanks to your help and advice

 

Out of focus star
To focus, I set the HFR to the lowest I could achieve
5 Minute sub just stretched. Looks like this

Full moon but the stars look much better. I'll measure the back focus on the camera next I think

I can't thank you guys enough for helping me out with this

Pete

 

[gorann]

That looks very promising! Now you can just go for it and any slightly oval corner stars can allways be fixed in processing. One thing you could think about is organizing your cables in a semi circle so you do not have to deal with odd star spikes. There are plenty of threads about it. This is how I did it (curved steel wire inside and electric tape to secure the cables to it):

[michael8554]

Hi Pete

You should tidy up those cables catching on the mount and dragging on the ground.

Michael

 

[ollypenrice]

16 hours ago, gorann said:

That looks very promising! Now you can just go for it and any slightly oval corner stars can allways be fixed in processing. One thing you could think about is organizing your cables in a semi circle so you do not have to deal with odd star spikes. There are plenty of threads about it. This is how I did it (curved steel wire inside and electric tape to secure the cables to it):

I switched from a small circle to something like this, Goran, but found I had to tape over the camera's little red light which was lighting up the bottom of the cable and giving a huge gradient.

Olly

[AKB]

30 minutes ago, ollypenrice said:

found I had to tape over the camera's little red light which was lighting up the bottom of the cable and giving a huge gradient.

Exactly so.  Found out the hard way when trying to do darks (which I normally do with the camera detached, of course, but I had spent ages collimating and arranging the cables, which I didn't want to redo.)

There should be a 'dark' mode for these status lights.

Tony

[ollypenrice]

1 minute ago, AKB said:

Exactly so.  Found out the hard way when trying to do darks (which I normally do with the camera detached, of course, but I had spent ages collimating and arranging the cables, which I didn't want to redo.)

There should be a 'dark' mode for these status lights.

Tony

Glad to have this confirmed, Tony. The evidence pointed to the light but I found it slightly hard to believe!

Olly

[gorann]

5 hours ago, ollypenrice said:

I switched from a small circle to something like this, Goran, but found I had to tape over the camera's little red light which was lighting up the bottom of the cable and giving a huge gradient.

Olly

Interesting! Never thought about it but from here on tape it will be in case the light gets in the wrong position when I rotate the camera.