Guiding failure after meridian flip - Forums

Topic starter

May 1, 2025

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!

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Alex Nicholas

May 1, 2025

When you do a meridian flip, unless you're using a rotator and rotate your camera 180⁰, you're guiding on a completely different set of stars.. (fov is diametrically opposite).

The best plan is to frame your original image, paying attention to bright stars both above and below the main image to ensure you will have guide stars both before and after meridian.

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Nick Grundy

May 1, 2025

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

Michael Gehrisch

May 1, 2025

I am using the ASI 2600 MC Air, ( integrated camera, guide camera and asi air) + ZWO EAF + Optolong Pro filter mounted on a Rainbow Astro 135E, also in Bortle 6 (ONTC 8" f4).
I double check my focus before the meridian flip, lock down the focuser screw and disconnect the EAF from the asi air, just to avoid any potential movement, (after the session stopped once when the asi air went into the Autofocus dialogue and just waited forever).
My setup works perfectly after the meridian flip, averaging around .5 arcsec before and after.
I fine tuned the focus of the guide camera which improved guiding. (make the guide camera the main camera and view the preview while you adjust the guide focus)

Where in the optical train do you have the filter? As I understand, it's better closer to the sensor.

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Scott Badger

May 1, 2025

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

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Himanshu Pandey

May 1, 2025 · Edited: May 1, 2025

I think its the combination of focal length and your guide camera being behind a filter that makes your setup very sensitive to mirror flop as well just provides a tiny number of stars to work with even when conditions are good.

When I image at 2032mm using an asi174mm on an OAG, phd2 gets 4-5 stars to work with. I have been imaging with the reducer at 1422mm but with a smaller sensor guide camera (462 mono) and here I sometimes see only 1 guide star. My guide cams are not behind filters and I'm using less focal length than you and it still causes problems occasionally.

You are working at the edge of your camera's capability just to get it to work under normal conditions "…Albeit I need some aggressive configuration ie 3 sec exposure, higher than usual gain of 450 and Bin2."

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montygill

Topic starter

May 1, 2025

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

Michael Gehrisch

May 1, 2025 · Edited: May 1, 2025

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.

If you didn't check and adjust the focus of the guide camera, it may very well be off. It seems to be a common complaint on the Duo and AIR. Mine was on the asi 2600MC Air. So my guiding improved from over 1 arcsec to under .5 arcsecs after correcting the focus on the guide camera and that was on a brand new camera.
On the ASI 2600MC Air the process to correct the focus on the guide camera is:
first focus the main camera
then turn off the main camera and make the guide camera the main camera
now you should be able preview and zoom in on the stars that the guide camera sees
Turn the silver focus knob to make the stars as sharp as possible.
Thereafter switch back to the main camera.

Butr maybe the duo is different? You might be able to see the guide camera view all the time?

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Tony Gondola

Frequent contributor

May 1, 2025

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.

Jeremy Wright

May 7, 2025

I had this exact problem on M51 with a C11 and oag with ASI 174mm, which is a large guide sensor. You should be able to look at the guide log and find the problem but almost certainly failure to find a guide star. Very frustrating, it was a rare night of good seeing.

Tim Ray

May 7, 2025

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

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