How to force PHD2 to loop and find new guide star

[Dick C]

I am running a Planewave CDK14 on an AP1100 mount.  Due to issues with my OAG, I recently switched to the ASI 2600MM Duo camera. The guide chip didn't like being behind the 6.5nm Ha filter, so I bought a 12nm filter one. I still have issues. Understandably, when NINA switches to the Ha filter, most (importantly, not all) guide stars disappear.

The problem is when NINA switches to the Ha filter, the guide star it was using is lost but PHD2 keeps trying to reacquire it (apparently the same one), which is now gone--invisible. PHD2 keeps saying "guide star lost-low SNR", then it switches to "guiding" (but there's nothing there to guide on, and a bad zig zag in the star profile window), then back to "guide star lost". The guide graph is off the chart, as it is not actually guiding on a star.  After 3 or 4 of these "cycles", it says "no star found" and gives up.

When the above happens, there are in fact 2 or 3 perfectly good stars in the guide star field of view. I can manually make it loop and manually choose one. When I do this, the guiding is fine. If I make it loop I can let it choose one of these available stars, and it will do that; but PHD2 won't loop on its own and choose one of them without my forcing it. When I manually choose one of these available stars, their SNR is 150, HFD is 3 or so, and they guide perfectly. But PHD2 won't choose them unattended.

Upon expert advice, to try to get PHD2 to choose one of these stars, I changed SNR to 30, minimum star HFD to 2.8, max 5, noise reduction 2x2. I use 6 sec exposure and guide camera gain of 85, to make dim stars more visible. PHD2 is set to retry every 15 seconds.

The problem is it doesn't "loop" so it doesn't "find" one of the perfectly good stars available. Instead, it keeps retrying an invisible guide star that is long gone and the guide graph goes off the chart.

How do I "force" PHD2 to loop, so that it will acquire one of the available stars?

[Brian Valente]

Hi Dick

Fancy seeing you here ;)

I've been thinking about this for a bit. PHD doesn't control the automation for something like this. I think what you need to do is (take a deep breath) move to the advanced sequencer in NINA, and when you switch to the HA filter in NINA, add a stop guiding, switch filter, start guiding set of instructions, something like this (this is an unconfigured NINA, so you would choose your HA filter)

Brian

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Brian Valente

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[Dale Ghent]

To be honest, I would have tackled the root problem differently. I don't know what issues you were running into with your OAG, and which OAG you were using, but I really think it's worth revisiting that in light of the sacrifices, extra expense, and hacks that your are now experiencing or needing. You've now traded one set of problems for another set of problems - and some notable sacrifices such as filter bandwidth - which really isn't progress at all.

Many folks have CDK14s and many of them successfully use OAGs on them with similar, non-Duo cameras. This is a reasonably well-traveled path and is nothing exotic. I personally run a practically fuss-free CDK14+Optec Gemini+Optec Sagitta setup that feeds a QHY600M and its filter wheel. It would work just as well if you have an IRF90 instead of a Gemini.

The key aspect to success here is using the right OAG; one that not only has good operational properties such as a large pick-off, but is also well-designed so that adjusting it and adapting it into your imaging train is an easy task. The OAGs from ZWO, QHY, and other such camera vendors are mediocre at best. They have small pick-offs and require frustrating disassembly of the imaging train to adjust, which often leads to knock-on issues around camera tilt and alignment. Their threaded or bolt-on adapters can force you into suboptimal OAG and guide camera orientations relative to the filter wheel and main camera behind it. They are ill-suited for something such as a CDK which, I presume, is situated in an observatory.

Looking at the ASI2600 Duo specifically, its integrated guide chip is the SmartSens SC2210 which is a Type 1/8 format sensor. That is *significantly* smaller than the Type 1/1.2 size IMX174 and IMX585, which are often used in large-format guide cameras. So your guiding FOV on a ~2560mm CDK14 is going to be so stifling small. Not only are you attenuating all guide star light by guiding from behind the filter, you are also severely limited in the number of guide stars you can even have the possibility of seeing. That small FOV might be mostly ok for the current season, with the star-festooned areas of the inner milky way being popular. But that can change later in the year with the popular targets often existing in relatively star-sparse regions. It's entirely possible that you may be forced to limit your imaging to targets that specifically have a bright-enough star that's also close enough.

An Optec Sagitta OAG paired with a guide camera that has an IMX174 or IMX585 mono sensor would be a very good base to start from. Its pick-off can illuminate the whole field of these large guide sensors, which is important at this focal length in order to ensure that you are getting as many stars as possible. Its adapter system is flexible and based on a circular dovetail, making the orientation of the OAG simple. Adjusting the pick-off stalk of the Sagitta up and down can be accomplished using a small hex key to turn an external adjustment screw. There is no need to remove the camera in order to directly access the stalk to adjust it like on so many other OAGs. It has a manual and motorized option for focus. I have the motorized version on my CDK14, however I've not really found the need to use that after zeroing its focus with the general location of focus for the main camera.

I highly suggest that you pause and reset on the current path you're on and reconsider the OAG route only with a better idea with how to implement and use one. I guarantee that you will have a better overall experience in the long term.

/dale

[Bruce Waddington]

I agree with the advice you've been given and I think you are at risk of having to spend time and frustration trying to work around these limitations.  IMO, you are essentially starting out standing in a hole with this setup.  The historical context on this "duo" camera is a little amusing to me, speaking as an old-timer.  Back when amateur CCD imaging was starting to take off, the leading camera manufacturer was the Santa Barbara Imaging Group and they developed and patented dual-chip cameras.  At the time, this was attractive because the guide sensor was co-focused with the imaging sensor, it came in one tidy package, available OAGs were scarce and small, amateurs were doing broadband imaging, and the guide sensor was roughly competitive with other then-available guide cameras.  Fast-forward a few years and the picture became very different.  Free-standing guide cameras had larger, more sensitive chips, people were wanting to at-least shoot in H-alpha,  and OAGs became substantially larger and more capable.  Presumably in response, SBIG then introduced the "remote guide head" machinery that would at least move the guide camera sensor in front of the filters, but the competitive writing was on the wall.  They intentionally allowed the patent to expire which allowed ZWO to produce a dual-chip camera of their own.  And of course, the original problems remain. Just my perception of course...

Regards,

Bruce

[Dick C]

Thank you Dale, I can't disagree with anything you have said. In fact, just before I saw your post, I had removed the Duo camera and was getting the OAG-L/2600MM Pro ready to go into the image train. The issue I had in the past was what you described:  the Zwo OAG-L (their "big" one) was nothing but trouble in terms of disassembling it 20 times to adjust the prism trying to get rid of vignetting, etc. (I was using it on a C-14 Edge).  Which is why I gave up and went with the Duo when I got the CDK14. I had looked online for "better" OAG's with bigger prisms but didn't find any. 

I will look into the Optec Sagitta. I think I'm done with the Duo. Nice idea, but just doesn't work for all around imaging, which must include narrow band. 

Having said the above, I still think it is ridiculous that PHD2, set to "restart" on a failure, won't pick out a perfectly good new guide star that is obvious in the field of view and fits all the parameters set in the software. This would seem necessary and fundamental for unattended imaging, which we all do regularly.

[Dale Ghent]

If you're interested in the Sagitta, call up Jeff @ Optec. He can walk
you through the front and rear adapters you'll need with it, as well as
any spacers needed to get the sensor at the CDK14's back focal length.

As for PHD2's methodology, consider the history. The original integrated
dual-chip SBIGs were not a design that was long for this world for
already-stated reasons. That was nigh 20-25 years ago and predates PHD2.
PHD2 (and other guiding apps) come along and the dominate guiding
configuration settles into a combination of coaxial guide scopes, OAGs,
and the niche ONAG. All of these configurations place the guide camera
in front of any main camera filters.

Fast forward to more recent times and ZWO reboots the original SBIG
concept with modern CMOS sensor tech. I'd like to point out that ZWO
just kinda launched these cameras onto the scene with no coordination
with guiding app developers or vendors. I don't use an ASIAir, so I
don't know if they put any special handling into their own private fork
of PHD2 that they put into that system. How ZWO has envisioned their Duo
products being used operationally, filter considerations and all, with
the popular guiding apps remains unknown. I have to imagine they
considered or even encountered these issues when developing the cameras.
I guess they figured that they'd just push the Duo cameras onto the
market and leave figuring that out to the scant few guiding app developers.

So, there you go. The existing functionality has been quite sufficient
for the predominant guiding configurations for well over a decade. ZWO
doesn't appear to be interested in helping or advising guide app
developers in optimizing anything for their new cameras. Developers,
many of whom self-fund their projects, aren't likely going to run out
and buy one of these cameras because, no matter how sensitive the
guiding CMOS chip is, trying to guide from behind things like 3nm
narrowband filters is a non-starter. Combine this with mount that
requires a rapid guide cadence, such as the popular SWG mounts, and the
efficacy of this idea seems even dimmer.

/dale

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[Brian Valente]

to be fair, the (roughly) original idea of an OSC camera with integrated guider was a great idea for something like a RASA, hyperstar, or even just a straight scope. Nice idea.

However, and as you point out, once you introduce monochrome and narrowband filters, it gets challenging pretty quickly

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[Dale Ghent]

Even the popular dual and tri-band narrowband filters for OSCs will pose
a problem for the ASIxx00MC Duo's out there. Those filters first
appeared with fairly wide bandpasses, 10nm or more. The current trend in
that space sees those bandpasses tightening up. The Optolong L-eXtreme
claims 7nm bandpasses for H-alpha and OII. So that means yet more light
attenuation. But yes, cheap narrowband filters with non-premium
transmission rates are going to be a struggle with the mono versions of
the Duos.

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> Brian Valente
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[Brian Valente]

>>> Even the popular dual and tri-band narrowband filters for OSCs will pose
a problem for the ASIxx00MC Duo's out there

yes as soon as you get narrowband involved, it's challenging. Dick mentioned to me he tried 12nm HA filter and still it was an issue. 

Being a hyperstar user myself, I can appreciate the "pure original idea". 

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[Dick C]

Thanks, guys. Good advice. I backed into this problem because a club member who got a 2600MC Duo thought it was great and suggested because I shoot mainly galaxies, an OSC camera would simplify my life. Since in processing galaxies one doesn't have to mess with lots of complicated color variations (like SHO, etc) as one does for emission nebulae. So I got the 2600MC Duo, enjoyed the simplicity, and got good results, except there was no Ha showing in the galaxy images--the one color highlight that makes galaxy images more interesting. So I naively decided to go back to mono, but with the Duo. 

I talked to Jeff, and have now ordered the Sagitta and expect it should avoid all the trouble I had with the Zwo OAG-L.  And with my venerable 2600MM Pro, this will get me back to "normal" mono imaging.

Parenthetically, as I told Brian, one of his suggestions caused a light bulb to go off in my head and I figured out the interim "solution" to the Duo/Ha guiding problem where PHD2 won't choose a new available guide star upon filter change. Using the legacy sequencer,  one simply makes 2 sequences for the same target, one for LRGB, and a following sequence for Ha only. This "forces" PHD2 to start guiding from scratch, as if on a new target, and loop, and in doing so it will find one of the (few) suitable guide stars in the field of view. I didn't realize that "restart" won't do that, so I had to stop guiding and make it loop and find manually. Using the "2 sequence" method for the same target, when the Ha sequence starts, PHD2 will loop and find, just as it always does when starting a new target. 

The above interim solution will keep me occupied while I wait for the new OAG and go back to the proper "guiding ahead of the filters" method I should have used all along.

Many thanks again for the advice.

[Thomas Mastrocinque]

ingenious and yet simple solution!  I'm going to do the same.

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[Thomas Mastrocinque]

Lol, I agree with all of this as well.  I also suggest the Celestron OAG, a much cheaper alternative to the Sagitta.  But in any case, definitely move your OAG in front of the filters. 

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