integration times

[sarg314]

Lately I've been chasing my tail trying to find the best integration time.  It seems to have started when I bought a new guide camera; I replaced a very old QHY 5-II-M with a ZWO ASI 220 mini.  I didn't expect to see any difference with the new camera (and I may be imaging the whole thing).  I use an Off-Axis Guider on a Celestron 8" with an AM5 mount. I use a counterweight on the AM5.  I'm used to using integration times of 2.0 - 2.5 sec., with the old QHY, anyway.  G.A. always seems to tell me to use between 1.5 and 3.5 sec. I always run G.A. and always accept its recommendations.

Last night I was surprised to find that while 3.0  or 3.5 sec gave me rms > 0.9 arcsec, 1.5 sec gave me about 0.7 and sometimes even less.  The plot and scatter diagrams  definitely looked nicer at 1.5 sec. than 3.5 sec.

Some questions:

1- What can I expect from this set up? The best I ever seem to get, when seeing is good, is 0.50 or 0.55 rms.  Is this good, bad or indifferent?

2- Trying to find the best sweet spot in integration time is difficult and time consuming.  Your basically trying to match a sampling frequency to the seeing frequency without knowing the power spectrum of the seeing or how the mount is wiggling.  Is there any rule of thumb to use to select integ. time?

3- After I change integration time, how many samples (or minutes) should I wait for before deciding if an integration time is better or worse than the last one I tried?  Seeing can change a lot in a 10 minute period sometimes.

4- Is there any way to guess if the integ. time should be increased or decreased?  Or should I just always step through the whole range?

[Bruce Waddington]

Many of the strain-wave mounts don't track very well at the sidereal rate, at least not by imaging standards.  So a common approach is to try using 0.5 or 1.0 second camera exposures with multi-star guiding enabled assuming you can find reasonable guide stars with those exposure times.  The GA recommendations for exposure time are tuned to non-strainwave mounts that do a better job of sidereal tracking.  Once you get some guiding time under your belt using the reduced exposure times, use the PHDLogViewer to analyze the periodic error in RA.  For some of the strain-wave mounts, the periodic error is fairly well-defined, so you can then start using the PPEC algorithm for RA and setting the period-length to the largest periodic error component.  When you set it, disable the "auto-adjust period" feature in the algorithm and increase the predictive weight parameter up to 80 or 90.  Many users have found this combination of adjustments will produce good results.

Regards,

Bruce

[Brian Valente]

Adding additional commentary to Bruce's reply, strainwaves seem to exhibit a lot of variability by manufacturer and even by individual unit. Some have periodic error that can be well characterized, others are all over the place with no clear period. primary PE ranges from 5-60". So take any input from others with a grain of salt: You will need to test settings with your mount and see how it reacts. 

Generally strainwaves seem to top out in the 0.50" RMS in consistent total RMS (there are of course exceptions), and they do better with faster guiding cadence 1sec or faster.

Brian

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Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[sarg314]

I always leave have multi-star guiding enabled, but it's very rare to have more than 1 star in the field.  With an OAG and 2032 F.L. the field of view is only 8'.  Also, I observe galaxies almost exclusively so I'm usually at high galactic latitude so there usually aren't a lot of field stars.  (I preselect guide stars using software I wrote that searches the NOMAD catalog to find candidates. It gives me the position angle of the guide star so I nearly always have 1 reasonably good guide star.)

I use a counterweight, but the mount is still out of balance.  So there is always a small preload.  Do harmonic mounts have a preferred preload direction - in the easterly or westerly direction? (I have a vague suspicion that  it guides a little better on objects in the west than the east.)

You have given me a lot of new information here. Thanks very much for those suggestions.  

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Tom Sargent

[Brian Valente]

>>>Do harmonic mounts have a preferred preload direction - in the easterly or westerly direction? (I have a vague suspicion that  it guides a little better on objects in the west than the east.)

That's probably a question better answered in a user forum for specific to your mount. There hasn't been any comments or discussion here that i recall regarding preload or biasing the balance. 

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