Guiding failure after meridian flip [Deep Sky] Acquisition techniques · montygill · ... · 10 · 328 · 0

Dear experts,

There could be many reasons why guiding would fail after a meridian flip - I am just wondering what are the common causes

Edge HD 11 at 2800mm, ZWO 2600MC Pro Duo (ie guidecam within main camera) with ample image circle. ZWO Asi Air. Filter to remove ambient wavelengths. Imaging a galaxy (Whirlpool)

The session starts off wonderfully with pin point guide stars (3-4) . Albeit I need some aggressive configuration ie 3 sec exposure, higher than usual gain of 450 and Bin2 but it works.... until meridian flip at which point stars are gone or very faint

- Non availability of stars after flip could be a reason I suppose. I am in Bortle 6
- Can mirror flop on the EdgeHD be a reason? I use an EAF so the mirror focus lock knobs are kept loose (so that EAF can work). Do I need to tighten it just enough?

I have not had any problems with other telescopes I have though they are considerably less in focal length and I use a 30mm F/4 guidescope with them

What else could possibly be the issue?

Thanks!

what mount are you using? I know the paramount's have this weird setting that is required in phd2 post-meridian flip

I image at 2350 and it’s not uncommon to have reliable guide stars on only one side of the meridian. If guide stars are already marginal, then whether I find one or not can depend on the quality of the seeing, transparency, and/or moon phase. If conditions are good and still not finding one, sometimes increasing the exposure length can help.

That said, mirror flop and lost focus can also be the issue, and mirror flop is most likely to occur during a meridian flip. Tightening the knobs “just enough” isn’t going to work. You can loosen them and tighten them (fully) before and after every focus run, but if it were me, I would definitely forget to loosen them at some point……and it would also prevent you from being able to do automated focusing while imaging (temp or fwhm change). Anyhow, if you focus after every meridian flip and whenever you switch targets, mirror flop shouldn’t be an issue. 

Cheers,
Scott

Thanks for the the very useful tips!

@Nick Grundy  I am using the AM5N - hmm its not the setting of 'flipping the calibration on meridian flip,' if that's what you are thinking..ZWO ASI AIR does that automatically and I have seen that to work

@Michael Gehrisch I see. I need to leave the EAF connected since I want it to refocus after meridian flip, for new target, every two hours so can't really disconnect it after focussing initially. I think my guide sensor is focused (manually though) since pre-meridian flip, stars are quite pin point, less than 255 Peak and decent magnitude. I haven't tried the method you suggested though. Will try it. The filter is close to sensor. Just telescope -> filter -> Askar backspace adjuster -> sensor. In fact adding the filter, makes the stars look more clear and focused

@Scott Badger I see.. yeah, makes sense

@Himanshu Pandey  yeah seeing conditions are certainly a factor, I choose to do the imaging on good nights but the problem still persists. I do think as you say, I am at the limits of what the guide sensor can do. 

My plan is to try other targets. Surely not all targets won't find good guide stars in FOV after the flip!

Will leave focus knobs loose since I am doing automated focusing and it focuses after every meridian flip anyway

Worst case I might have to add a rotator to keep the guide stars the same before and after flip as @Alex Nicholas mentioned

Great responses friends. Appreciate it. Will update when I've found a solution

Not a lot of good guide stars around M51at a narrow FOV and OAG. This is just something what will happen with certain objects, especially in galaxy season. It's a bit of mucking about but you might mount that small guide scope and use it for solving and guiding if there's no other way.

Alex is correct. You need different guide stars pre and post flip. Or rotate the Camera/OAG/duo 180 degrees. If you have a good guide star after flip and the mount misbehaved, then toggle the guiding software (reverse dec commands after flip). 

CS Tim