Off Axis Guiding + Mono/OSC team up

[david_taurus83]

Loads of threads on this already but looking for recommendations. What's good, what's not..

 

Having a bit of a think about my setup. Currently using a side by side, scope and guidescope. My guiding RMS is typically 0.6 to 0.8 but there can be quite a large peak to peak error. At 336mm the WO 71 is fairly forgiving but at 564mm the AA 102, not so much. Noticing centre stars having funny shapes in the same direction. Also, I'm having this eerie voice in my head telling me to mount 2 scopes with ASI1600 mono on the 102 and a colour camera on the 71. Currently have a 178MC for a guide cam. Could I pair this with the GT71 to collect colour? It has a Sony sensor with small pixels and low read noise. It's the uncooled version. The resolution would be a close match to the 1600/102 combo. Or could get a modded DSLR for the bigger sensor but extra cost and weight.

Anyway, I understand there are a few hurdles to overcome first with an OAG, focus, prism position etc

What's everyone's thoughts? Is there a noticeable improvement with guiding after switching from a guidescope?

[david_taurus83]

Well, that escalated quickly!

 

I paired up the Starwave 102 side by side with the WO GT71 last night for some testing. Despite having a couple of hundred quid worth of mounting bar the 2 most definitely did not line up after centering Vega in the FOV of the ASI1600/ED102. I had to use tin foil shims on the side of the GT71 dovetail to nudge it over and a Dunlop Tortex 0.88mm guitar pick under the dovetail to tip the nose down. A 5 pence piece was too thick, I found! Obviously a temporary solution for now. Managed to get the FOV close enough for what I needed. Trained the GT71/ASI178MC at the Pacman last night and ran a plan of 60 x 60s subs at gain 200 and set the ASI1600 to capture 60 x 60s Lum subs, all controlled and dithered by APT. No guiding, obviously. The result below isn't perfect, theres lots wrong with it. The ASI178MC uncooled is definitely not a DSO camera, not without a cooler anyway. Theres lots of amp glow that doesn't calibrate out even though I captured darks at the same temp. So it's still a bit noisy. Not saying I'd stick with this camera for OSC but the idea shows promise I think. Just an hours worth of short exposures and I managed to get a full colour image. With mono only I wouldnt settle for anything less than an hour on each LRGB filter. That would take about 5/6 hours in a single night when you factor in focusing, flipping, dithering, guiding woes etc. We just haven't got the weather and skies here. It's so frustrating having only a couple of hours Lum or Ha on a target and then you have to wait weeks for a clear moonless night to try and add a different filter to it. Might be on the lookout for another OSC camera soon. Need to decide on a modded DSLR or do I get a dedicated astro camera? First thing I need to do is figure out a way to align both scopes a bit more accurately. Not keen on leaving the tin foil in place...

[david_taurus83]

Is this close enough? The 102 will have a full frame Canon 6D so it will overlap the edges of the ASI1600 on the 71. I replaced the tin foil with another guitar pick! 0.5mm spacing makes a huge difference on the pointing of the scope.

OAG now in place. Had me scratching my head as to why it wasnt pointing at the same position of the ASI1600, of course, the prism is outside the FOV of the 1600 sensor.

 

[Hallingskies]

Can’t say I noticed any improvement on guiding when I swapped from a guidescope to an OAG, but then I’m working at a fl of 550mm so guiding accuracy isn’t that critical.  The reason I switched was that at some orientations near the zenith, it was difficult to get the main and guidescope both pointing through the slit in the obbo dome.  The OAG greatly simplifies matters.  There was a big weight saving as well.

[david_taurus83]

Had a first light last night with new astro modded 6D and OAG. Guiding went really well. The ZWO non rotating focuser is a godsend! I had a remarkable improvement in guiding considering I didn't bother to polar align and calibrated fairly high up. My target for last nights testing was M33. Despite a preliminary alignment at the weekend both scopes are still not quite centred. The GT71, OAG and ASI1600 are fixed with no shims in the clamp and is the primary scope and camera. The 102EDR and 6D are the secondary so this is the one to adjust. I literally need 0.5mm adjustment in both axis to get the target centred. After a heart stopping moment when the mount performed a meridian flip, the 102 slipped down the saddle by 3 inches but luckily caught before disaster struck! I'm not convinced now its 100% secure with plastic shims wedged between and under the dovetail. So I need to come up with an alternative. Yes, I know there is the JTD Astro bar but I really dont want to spend that much cash for the sake of less than half a millimetres adjustment!

@Davey-TI have found this image on another thread. It looks very interesting. How did you get on with this? Any more pictures of the parts you've used and how its bolted together?

[Davey-T]

1 minute ago, david_taurus83 said:

I have found this image on another thread. It looks very interesting. How did you get on with this? Any more pictures of the parts you've used and how its bolted together?

Hi David I'll dig some pictures out, I made three of them aiming to have three WO Star 71s but only managed 2 so far, the adjusters work really well and as you say you only need a minimum of adjustment, the JTD adjuster is very nice but has heaps of adjustment that's not needed and weighs more, mine could probably be made lighter.

Dave

 

 

[david_taurus83]

3 minutes ago, Davey-T said:

Hi David I'll dig some pictures out, I made three of them aiming to have three WO Star 71s but only managed 2 so far, the adjusters work really well and as you say you only need a minimum of adjustment, the JTD adjuster is very nice but has heaps of adjustment that's not needed and weighs more, mine could probably be made lighter.

Dave

 

 

Thanks Dave, that would be great. If I centre the GT71, and use no shims the target is still in the FOV of the 102 but top left. 1mm shim is too much! Needs a tiny adjustment but I'm really uncomfortable now unless i use something more secure.

[Davey-T]

3 minutes ago, david_taurus83 said:

Thanks Dave, that would be great. If I centre the GT71, and use no shims the target is still in the FOV of the 102 but top left. 1mm shim is too much! Needs a tiny adjustment but I'm really uncomfortable now unless i use something more secure.

I found it nigh on impossible plus very time consuming using cut up bits of aluminium drinks can as shims, with the adjusters I can make miniscule adjustments and they could probably be made simpler as there's loads of adjustment to spare and once adjusted they're locked securely. 

I'm using the 10Micron unguided and can use the orthogonal error readout from the handset to get the centre scope aligned then centre the other two scopes using crosshairs on screen  and play around with plate solving to get all the cameras at the same angle, obviously helps to have it all permanently setup.

As I said I made 3 for Star 71s also one with a Losmandy dovetail clamp for the WO110FLT but have just ordered an ADM dual dovetail clamp from FLO to save changing stuff around so will have a spare Vixen clamp one, if you really want to have a go at making one I could send you the spare to copy, I made the prototype by hand with only a pillar drill but the other two with CNC machine.

Dave

[Davey-T]

Astro camp setup on tripod

Three vixen dovetails, had to buy longer dovetail to mount them on.

Losmandy in centre