Dither on a Dual Rig

[tomato]

On the dual Esprit 150/ASI 178 rig, I have been running  SGP on the OAG guided scope which takes RGB and the slave scope takes Lum using SharpCap. I do it this way round as there is a single fixed filter on the slave scope and I find it easier to control the filter wheel with SGP. SGP dithers between subs but because the slave scope is not synced to it the dither can occur at any time during the 120 second Lum exposures. I have been fortunate in that Ivo Jager (creator of the StarTools processing software) offered to process the data, and he has noted that the Lum background noise appears correlated, (i.e. not random), and this has made itself apparent on the final image.

I should also note that all subs were binned 2x2. Is it possible the random dither part way through the Lum exposures is causing this effect? I introduced the dither to eliminate some walking noise that was visible on this set up with a single scope, so at the moment (if my identification of the cause is correct) I cannot see how to fix it. I suppose it would be better to capture the Lum data on the guided scope rather than the slave scope, but then I would have to control the filter wheel with SharpCap, which could be done.

 

[Laurin Dave]

Did you know that you can run two instances of SGPro with one licence ? no need to run Sharpcap..  I run my dual rig that way, and just junk the RGB subs that are affected by dither..  also have you checked the dither pattern and that it’s big enough .. You can do this by stacking without registering/ rejecting..  on occasions I’ve noticed fixed pattern noise and found that the dithering was in a diagonal line rather than being random ..  

nice image by the way..  6” refractors don’t you just love them 

Dave 

[ollypenrice]

I would certainly shoot the lum with the guided scope and the RGB with the unguided so as to drive any flexure artefacts onto the RGB where they will have little significance.

And surely dithering during an exposure, especially an L exposure, will do far more harm than good. I solve the problem by not dithering on the dual rig and I can't say I miss it. You're combining two datasets with different residual noise which has a similar effect. 

Alternatively, with two filterwheels you could shoot through all filters on both scopes. This, though more expensive, would bring several benefits (provided flexure isn't an issue.) You'd be combining two sets of residual noise per channel and you could shoot L and blue at the highest elevation which can be a major benefit. (The best way to preprocess under this régime is slightly more laborious in that you should calibrate all files without stacking them and then sigma-stack all the calibrated files in a single pass.)

Olly

Edited March 12, 2020 by ollypenrice

[tomato]

Thanks for the replies, the simplest thing to try will be to stop the dither. Putting the Lum camera on the guided scope will be trickier, as I don’t think I have the right spacers to put the OAG on a camera without a filter wheel. However, with the ASI 178s I am not imaging out to the edges so spot on spacing is not as critical.

I know an OAG is not considered to be a good idea with a dual rig, but I definitely get superior guiding performance with this arrangement.

I love this hobby, so many things to get right.😊

[Laurin Dave]

If I remember correctly Ian King told me that he didn't use a flattener with his Esprit150/QSI6120 setup, so you should be ok without them as your sensors are smaller .. worth giving it a go I'd have thought..

Dave

[tomato]

Cameras now reconfigured as best as I can to take Lum on the guided scope and RGB on the slave scope. The latter optical train is a bit too long but shouldn’t be an issue with SW’s claimed 44 mm imaging circle and the 178 sensor diagonal at 8.9 mm.

Bring on a clear night for M101 or M51...