Getting the best performance from my AM5: Page 23

Getting the best performance from my AM5

CHriss

ASIMount@ZWO
Thanks for checking but no, I never got a response. Only the auto-reply after opening the ticket.
Since I'm running my own mail-domain and mail-server, it is very easy to check the logs.

I've replied to the auto-response, maybe that helps.

Jan75

ASIMount@ZWO Thank you for the explanation. I was talking about the guide rate (that I was asked to change to 0.25x), not about the min_move (that I know cannot be changed in Asiair). The guide rate is an option in Asiair for other mounts but not on AM5 (disabled in the current version from what I understand from other posts).

In all cases, on my side, the 2.9s-period oscillation is still present even with the beta so maybe that has nothing to do with the SW parameters you changed - just guessing. Anyways, I sent you the most recent guiding logs as requested, let me know if you need anything else from me, otherwise I will just wait for your response by email.

ASIMount@ZWO

Jan75 The guide rate is an option in Asiair for other mounts but not on AM5

The option is on our plan. We'll release this option later.

Kevin_A

Just adding my 2 cents worth, the new beta version released to help fix inconsistant guiding did not change anything in regards to performance, if anything my mount is running worse.

CHriss

ASIMount@ZWO
I didn't get any response so far, neither by e-mail nor here :-(

ASIMount@ZWO

CHriss Sorry for that. Our support will send you an email again later.

fotosgrafiabymiguel

@w7ay - Thank you Chen for your well-written description "getting the best performance on the AM5" I used your suggestions and got the best guiding on my AM5, AA+, RedCat 51, ASI 2600MC Pro and ASI220mm Guidescope.

Do you have any guiding suggestions for AM5, AA+, Celestron EdgeHD 8, OAG w/ASI174mm and ASI533MC Pro?

Thank you in advance.

fotosgrafiabymiguel

@w7ay StarzLite Thank you Chen for your well-written description "getting the best performance on the AM5" I used your suggestions and got the best guiding on my AM5, AA+, RedCat 51, ASI 2600MC Pro and ASI220mm Guidescope.

Do you have any guiding suggestions for AM5, AA+, Celestron EdgeHD 8, OAG w/ASI174mm and ASI533MC Pro?

Thank you in advance.

w7ay

fotosgrafiabymiguel Do you have any guiding suggestions for AM5, AA+, Celestron EdgeHD 8, OAG w/ASI174mm and ASI533MC Pro?

Sorry, I do not use the ZWO mount to know how it works with an OAG, especially with an ASIAIR (assuming that is what you mean by "AA+").

To get good guiding (0.35" total RMS with an RST-135E), you need the camera to have the ablility to set an extra 20 dB of gain after getting 12 stars. I am not sure an OAG can do that.

Chen

Jan75

w7ay My own observation is that OAG makes guiding performance worse, but at this point I don't know why that is. I suspect it is the number of stars detected: the Asiair interface displays some stars with circles around them, so I'm assuming these are the ones used for guiding. So far I have never seen more than a handful circles (let's say 5-ish at best). I use the ASI ZWO 220 camera with the gain at 450. The telescope is an RC8 (at 1627 mm FL). I have seen small improvement when attaching a counter-weight.
Next time I have a clear night I will try the same setup with a guidescope and confirm that the key problem is with the number of stars used for guiding.

w7ay

Jan75 My own observation is that OAG makes guiding performance worse, but at this point I don't know why that is.

With a strain wave mount, I suspect that it is because

you need guide exposures of 0.5 sec or shorter
this means that you will need multi-star guiding
in turn, this means the need for a good star field with all the stars in the field being as sharp as possible (this is where an OAG falls apart). If one star has coma and the other star does not, or another star has "wings", as the weights of their centroid changes over time due to scintillation, the weighted sum will also change.

Even before strain wave gear mounts, I have tried various OAG over the past 15 or so years, and have always gone back to using a separate guide scope.

Chen

Jan75

w7ay This might be the key to the entire problem of bad guiding performance. Can't wait to re-run all tests with a guidescope. I used to use it before I bought the OAG but I also had the ASI ZWO 120MM camera that is not as sensitive as the 220 so pairing the 220 with the guidescope will be a new experiment. Now that I use a counterweight I am no longer that worried about the weight (or torque) of the main telescope + guidescope group. Just wondering if a 246mm guidescope is a good match for a 1627mm main scope... Will see!

w7ay

Jan75 Can't wait to re-run all tests with a guidescope.

With an RC8 type focal length, you will need to make sure there is no differential flexture, though :-).

That being said, I have been able to get consistent 0.35" type total RMS guiding with ASIAIR, albeit with a different manufacturer's strain wave gear mount, with just a 250mm focal length guide scope with flattener and ASI178MM guide camera (large, clean FOV).

That type of error should work well enough for the RC8. I have started to tune the autoguiding to prepare for when a Mewlon 180 arrives later in the year, although most of the time, it will be used unquided (planetary).

Your 246mm should do fine.

Good luck.

Chen

prastro

I have an RC6 on the AM5 at native 1370mm fl. Guiding is PHD2, not ASIAir.

When it was being used on a CEM, OAG setup of Askar OAG (10mm prism) plus 174mm mini worked very well with guide exposures of 1.5-2 sec.

On the AM5, I have tried 0.5 to 1.5 sec guide exposures and settled at 0.8 sec. Location is in a metro, so high LP (skies around 17 mpsas).

The OAG works fine in star rich areas at 0.8 sec. I get 8-9 stars in multistars. Where star fields are poor (Virgo, etc), or near the light dome, I get only 1-2 stars, maybe 3. You might get more if you have low LP.

I then tried a 60mm F4 guidescope with an old ASI385 I had lying around. With this, there are always 9 guidestars and I feel the guiding in star poor areas is better - lower by about 0.1"-0.2" rms total. Not significant when imaging at 1.6"/px, but desirable at 0.55"/px.

In most areas, the difference in guiding doesn't show up in the images. But in the star-poor areas, it does. So I just use the guidescope now.

I could probably go down to 0.5sec as well. I fiddled around with the minmove - had to reduce it compared to OAG.

The guidescope is in rings which are bolted to the top vixen dovetail. The guide camera and imaging camera are well aligned - both put the same star in the centre.

The 220mm actually has larger pixels than the 385. That would push the guiding pixel scale away from the imaging scale. I am planning to try a smaller pixel camera (a 290mm I have) and see if that improves it further as the guiding pixel scale will come closer to the imaging scale.

w7ay

Roughly speaking, the following is what I find with ASIAIR multi-star guiding with my strain wave geared mounts. ASIAIR uses signal-to-noise ratio (SNR) weighted centroid averaging (https://en.wikipedia.org/wiki/Weighted_arithmetic_mean). Autoguide programs that use uniform weighting works better, but if you are using ASIAIR, you have no choice.

more than 1 second exposure -- limited by the slope of the mount's periodic error -- basically limted to no better than 0.75" total RMS error, even if "seeing" were perfect (hah! only happens on Mauna Kea),

For 0.5 second exposures:

1 star: guiding is limited by atmospheric turbulence ("seeing"). Typically 2" RMS per axis, even for an average night, if you are at sea level.
2 stars: RMS error is a factor of about 1.4 (== sqrt(2)) times smaller than the 1 star case iff the two stars have the same ADU. If one of the stars has an ADU of about double that of the second star, the SNR weighted scheme in ASIAIR will only reduce the RMS guiding over the one star case by about 12%.
11 to 12 stars with the brightest star also picked: again, depends on the distribution of the guide stars. If one star is much brighter than the other stars (say, Vega is picked as a guide star), ASIAIR's SNR weighting when computing the centroid again makes the RMS error no better than a single star case.
11 to 12 stars without an abnormally bright star picked: equivalent to about 3 or 4 stars. RMS error due to "seeing" is about half of using one star. A 0.5 second exposure equivalent to using 1.5 to 2 seconds of exposure using 1 star -- i.e., it starts being usable.
12 stars plus an additional 10 dB camera gain after achieving 12 stars -- the brightest stars should now be rejected by ASIAIR because it has become saturated. If the brightest two or three stars are rejected, this might give the equivalent of using 6 to 9 stars when the weighting is uniform instead of SNR weighted.
12 stars plus an additional 20 dB camera gain after achieving 12 stars finally, you are getting the benefit of using the centroids from 10 to 12 stars. A few of the brightest stars should now be rejected, making the rest of the stars have more equal weights. The limiting factor (under decent "seeing" and no wind) of about 0.15" to 0.25" RMS, so the error is most likely limited not by ASIAIR centroid computation, but limied by the mount itself. Notice that an OAG (or a heavy filter on the guide camera) may not be able to achieve this.
12 stars plus an additional 30 dB camera gain after achieving 12 stars -- you are likely to hit high noise because of the excessive guide camera gain at this point.

Notice that ZWO cameras uses 0.1 dB per gain unit. So, 10 dB of extra gain means adding (adding and not multiplying since the gain units are already in the logarithm scale) a gain of 100. 20 dB of excess gain means adding 200 to the gain.

Given a night with good transparency, my 55mm/250mm focal length guide scope and an ASI178MM guide camera typically gives me 11 to 12 stars with a gain of about 60. I do have a flattener (but not reducer, to keep the focal length at 250mm) on the guide scope to get clean stars all the way to the edge of the guide sensor -- distorted stars will only hurt as you add more (since the SNR weighting will jitter the centroid), so make sure you have a decent guide scope with a flat field. I typically set the gain of the ASI178MM camera to between 250 to 300. When transparency is not so good but "seeing" is still respectable), I add even more gain; but at some point, the guide camera ends up being too noisy.

To beginners and YouTube watchers: do not blindly use the gain that I use. Your gain will be different from mine. What you need to do is to slowly increase the gain of the guide camera until you get between 10 and 12 stars. Then add an additional gain of 200 to work around the ASIAIR SNR averaging problem. You may need a gain of 200 just to get 12 stars (in which case adding an extra 200 would make the guide images too noisy -- you would need a better guide scope). Ditto OAGs, if you can't get 12 stars at a gain of 200, you will only be able to add 10 dB of additional gain before the camera noise becomes intolerable. And in some cases, you won't even be able to get 12 stars with an OAG to start with.

Chen

Kevin_A

w7ay what about when using a low end camera like the asi120mm mini where unity gain is at 28. I use one of those plus a 290mm mini where unity gain is at 110. I have no problem cranking up the 290mm mini gain to 210 but the 120mm mini maybe useless and a stretch at gain 128 and currently I find gain 70 is about max. Both cameras are on 240mm scopes. I may try a bit higher as I do notice that asiair does use a few overly saturated stars still.

Kevin_A

w7ay what guide rate did you use to get these guide results for all the exposure lengths and different star saturation/number results? 0.5x or less?

w7ay

Kevin_A when using a low end camera like the asi120mm mini

Nah, I stay away from bargain basement stuff when it becomes the limiting factor of a multi-thousand dollar (or Loonie) imaging set up.

Cost wise, my current primary mount costs the most, then comes the OTA, then comes the imaging camera, then comes a set of filters. My guide system is already pocket change, comparatively.

Favorite guide system right now: Borg 55FL objective, Teleskop TSMPT60 flattener, Astronomik L1 filter, ASI178MM camera, Borg helical focuser belt driven by ZWO EAF motor. When experimenting on Indigo, the QHY678M is swapped in, in place of the ASI178MM.

Even the above (with the ASI178MM) is often star starved away from the Milky Way when using SNR-weighted centroid averaging.

Chen

Kevin_A

w7ay i was looking at the 220 mini but the pixels are just too big. My scopes produce fairly good round stars but the choice of zwo guide cameras all have pixels 4uM and up now and the 178 wont fit my scopes. I am not sure why the ditched the 290mm mini with its small pixels.
I still think that its not a camera issue but a asiair guiding issue that is too finicky with its devices.

w7ay

Kevin_A I am not sure why the[y] ditched the 290mm mini with its small pixels.

Probably because the SC2210 (Chinese) sensor in the ASI220 is cheaper than the IMX290 (Japanese) sensor. It always comes down to nickel and dime with ZWO.

This is why I chose the QHY678M (mono) for my Indigo guide experiments. I have learned long ago never to use a Bayer camera for guiding, since a star at the boundary of a sub-color pixel causes sudden centroid changes, even if you were to mono-bin it. Worst for 1-star guiding of course, but the SNR-weighted stuff in ASIAIR does not help.

220 mini but the pixels are just too big

Can you imagine that camera with a short guide scope and a MinMo of 0.2 pixels? Yeehaa!

Chen

w7ay

Kevin_A what guide rate did you use to get these guide results for all the exposure lengths and different star saturation/number results?

Stock ASIAIR v2.1.1 guiding. 55mm Japanese objective (Borg uses fluorite glass from Canon), 250mm focal length. ASI178MM.

0.5 second exposure (2 FPS), 0.25x sidereal guide rate, Astronomic L1 filter (which cuts a little more UV than a typical UV-IR cut filter, but probably no more than 5% to 10%; but helps with any UV star bloat).

Max declination pulse duration of 30 ms (not typo -- thirty milliseconds -- since polar alignment of the order of 15", so there is really negligible declination drift), and max RA pulse duration of 20 ms (this is with the RST-135 'e', not the barebones RST-135, which would require more than 80 ms). Declination and RA feedback aggressiveness of 10%.

Focused to achieve HFD of about 2.3 pixels (measured with ASIAIR detect star utility, at 3 second exposure).

In addition to the Astronomik L1, the flattener glass also cuts some light, but not enough to make me go without a flattener. I aim for wide FOV with good star shape to the corners -- with multi-star centroids, you do yourself no favor with a wide FOV if some of the stars has coma or look like seagulls. You want the average centroid to remain constant as the stars scintillates and the centroids of different stars carry more weight. This "moving centroid" problem is also present if the polar alignment is not perfect and the guide stars field rotate after an hour or two -- PHD2 does not handle field rotation -- if only handles ∆x and ∆y image shifts -- i.e., Galilean Transformation.

Chen

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