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Guiding w/PHD CG-5 with short focal lengths

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#1 Gregk

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Posted 30 April 2010 - 05:11 PM


Hi,

It's time to drag out the 6 inch SCT for some galaxy work
My Cg-5 has been down for a month finally got it fixed.
I use a .8x or 6.3 reducer flatner also

Last time I tried guiding with a 135mm camera lens and moog adapter for the DSI the results were not that great. Oval stars.

My results are always spot on imaging at 600mm and guiding at 388mm. Now my imaging scale will be much smaller < 2. I use the Kochab Method of polar alignment

My calibration Steps , for the shorter focal length setup, are set to 1500 to 2000 I assume in milliseconds. Which if I got the math correct is 1.5 seconds and 2 seconds.

Is it safe to assume that the DSI exposure needs to be at least 2 seconds to match the maximum calibration step?
Or is it unrelated

Also what should the Max Dec Duration be the same as the Calibration Pulse Duration.

Any other comments suggestions welcome. Please feel free
settings..Especially those using lenses or finder scopes
for guiding

Clear Skies

Greg

#2 Jason B

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Posted 30 April 2010 - 09:15 PM

I use a AS-GT to image with a AT111EDT (777mm) and guide with the Starshoot Autoguider, PHD and a 300mm guidescope. I got very good results with this combo. I also tried the same set up with a C6 at F10 but had mirror flop issues though the stars were round on the good exposures (5 min.)

I just upgraded to a CGEM for my AT111 and a new RC8. I plan to keep the AS-GT as a portable imaging rig with my AT65Q or a Vixen F:L80S (80mm F8) and the 300mm Guidescope.

#3 Bootlegger

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Posted 01 May 2010 - 03:11 AM

Hi.

I guided using a 350FL 67mm camera lens.
Imaging at 480mm i got the best images my mount has ever produced in my 2 years of imaging.
I only tryied this combo once before changing guidescope and still regret that decision.

Your math ads up i think. 1000ms is 1 second. Guiding at such short FL needs a long guide pulse.

Your exposure time does not need to mattch the guide pulse. After exposing the program evaluates the movement of the guide star and sends correction to the mount. Next correction will not occur until next exposure is finished.

Max dec pulse will only limit the DEC pulse the program is permittet to send per guide corrections to limit jumps in the Dec axis.

I try to have my guide calibration finish in about 16 steps and change the guide pulse duration accordingly.



Nonni.

Image using setup

Setup

#4 Gregk

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Posted 01 May 2010 - 05:00 PM

After some more testing I think I need to get one of those smaller pixel auto guiders to more closely match the imaging scale.

The Meade DSI Pixel size is 2x that of the newer auto guiders....

Greg

#5 Bootlegger

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Posted 01 May 2010 - 05:05 PM

I´m not so sure of that, I was guiding very well with 350mm FL and DSI 2 PRO at 8.6x8.3 pixel size.

#6 Gregk

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Posted 02 May 2010 - 05:18 PM

Here's What I meant

Scaling Formula = (Pixel Size * 206) / Focal Length

Imaging at 960mm FL and Pixels of 8um = 1.72 arcsec/pixel

Using a 50mm guide scope and comparing 8.6um pixels to 5um

Guider Imaging scale (50mm Guide Lens) = 20.6 arcsec/pixel
DSI Imaging scale (50mm Guide Lens) = 37.08 arcsec/pixel

So the Guide camera is significant closer to the imaging scale of the imaging camera Much better scenario for guiding


In your case I'm not sure of your imaging scale I too can guide well with the dsi when my imaging scale is close to 3/arcsec/pixel and I'm guiding with a 388mm guide scope
4.51 arcsec/pixel


Greg

#7 Bootlegger

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Posted 03 May 2010 - 01:58 AM

yes.

One thing that PHD does is that it calculates Minimum motion in fractions of pixels and that number can be adjusted under the brain.

Default value is 0.25

So if guidescale is 4.64 as is for my camera lens and i was imaging at 2.45 i should have had larger stars if it wasn´t for the Min motion.

Try and see what happens if you lower Min motion value.

#8 Craig

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Posted 03 May 2010 - 10:48 AM

Give Bootlegger's suggestion a try. In addition, if your guide scope is very short so that your star isn't spread across a number of pixels, PHD won't be able to accurately locate its center. The easy solution is to just defocus a bit. But, the star must be spread out over a few pixels to have any chance at sub-pixel localization.

Craig






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