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PE on EQ-G

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#1 Grouptele

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Posted 06 June 2009 - 03:13 PM

Dang it... deleted the original post due to an error on the PecPrep analysis.

OK... After a little over 1 yr of using an EQ-G, I have finally joined the "guiding" bandwagon; thus, proceeded to evaluate the pe. Following analysis were done with a C8 + finder scope + 6.3fr + Phillips webcam + PDH module (the webcam was installed on the C8 for ease of the exercise.

The below screen shot is for the unguided run. Basis my scant experience with guiding, looks like the pk-pk pe is 6 arcsec for a little over 1 complete worm cycle, and it also looks like there are lots of higher frequency events going on, as evidenced by the rippled raw data dnd the FFT survey.

Unguided images (less than 2min typ) at 1200fl show fairly round stars, yet my guided images (3min typ) show some irregular star distortion (but no tails). Thus, the aim of this exercise was to find if the source of star distortion is the mount pe or the guiding set-up. So far, I have used the default PDH set-up, except for agressivness (80%).

Any comments are appreciated.

Victor

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#2 Grouptele

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Posted 06 June 2009 - 04:59 PM

Here's the guided screen shot.

Victor

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#3 Dennis Sakva

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Posted 07 June 2009 - 11:41 AM

Victor, look at your DEC axis. PECPrep allows that to do. It will show you how atmosphere is behaving. If it is close to +/-1" then there's not much you can do. Also look carefully at 10.2 seconds cycle. Is it relatively large? It causes the most problems with guiding. Longer cycles are guided out much more easily.

Regards,
Dennis.

#4 Dennis Sakva

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Posted 07 June 2009 - 11:45 AM

One more thing. How do you guide? Separate guidescope or OAG?
If it's a separate guidescope differential flexure (when main scope and guidescope flex relatively to each other) can be the reason for distorted stars.
If it's OAG then check how securely the guidecam and main cameras are attached.

#5 Grouptele

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Posted 07 June 2009 - 01:33 PM

Dennis,
Freq spectrum shows little amplitude at 10.2sec, but slightly higher at 5sec. After worm cycle, largest amplitude visible is the stepper gear, at 125sec. At 1sec, amplitude is also very small. I do not recall bad seeing during the capture session.

How do you view the DEC axis trend on PECPrep? I have been using the default settings, which show the "Y" axis.

As to your question about how I was guiding, for this particular run I was guiding through the main scope (8" SCT) and was not imaging; thus, no flexure problems. When I image, I use a side-side set-up and guide through a 66mm refractor.

Victor.

#6 Levente

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Posted 07 June 2009 - 05:22 PM

Victor, are you absolutely sure that your evaluation is correct? I think +-2.8" periodic error unguided is too optimistic even for a Tak or Astrophisics mount.

#7 Grouptele

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Posted 07 June 2009 - 07:43 PM

Levente,
That is a good question. The answer is: I am not sure.

The basic input to PECPrep [using PDH] is correct (e.g., 1282mm focal length (C8 at f6.3) and 5.6um pixel size (NC9000 webcam]. Otherwise, I used the default values for PECPrep.

I agree that my raw data looks optimistic when compared to other EQ6 data, but I just do not know what the error might be.

Victor

#8 Dennis Sakva

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Posted 08 June 2009 - 01:29 AM

There's an option like Open DEC data, next to where your load your RA data.

#9 Tapio

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Posted 08 June 2009 - 02:39 AM

I agree, 6" peak-to-peak sounds too good to be true.
However if you get pretty round stars with 1200mm focal length and 2 min exposures it tells that you have a good mount.
I get about 12" peak-to-peak values with my HEQ5 Pro (aka Sirius mount).

#10 Grouptele

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Posted 08 June 2009 - 07:30 PM

With the help of other fellows, just found that I did not compensate for the declination of the guide star (e.g., it was nearby Mizar, at about 55deg DEC). Re-ran PECPrep with this new information, and the pe now came to 12.7" pk-pk, which seems to be more in line with the typical EQ-G - see the attached screen capture.

Thanks for the comments and hints with this issue.

Victor

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