PHD and CGEM 2000mm guiding Solution

[MarsG76]

Hello Astrophotographers,

Now that I have my CGEM on a permanent pier, I figured that I should get much better accuracy in guiding quality, especially at 2032mm (F10) focal length on my 8" SCT.

I spent a bit of time getting guiding results that are at the very least acceptable and thought I’d share my experience with anyone who is looking for info on PHD autoguiding.

In the pasten setting up for each astro session and using PHD I was generally getting RMS about 2.2-2.6ish and after stacking and processing, the soft effects were able to be negated to a great point, revealing detail and with results that I was quite happy with (on a good night) imaging at 2032mm using a modded Canon 40D.

The stars were round and generally I was able to use most (if not ALL) of the subs generated, even when I was exposing through Halpha or SII for 30-40 minutes per sub. That in it self, I thought, was pretty good for the setup that I'm using... or... I'm just easily pleased.
   
I use a OAG for my exposures so guiding on the same FL as imaging.
That said I did spend a bit of time playing with PHD settings, as well as the backlash setting on the CGEM, along with autoguide rates to try to get better guide graph.
 
After a spending a bit of time on both polar alignment as well as tweaking the autoguiding parameters in PHD, I was still getting a graph that showed large jumps, see pics… Nowhere near the near flat line that a few imagers were getting. Although the RMS level at 2032mm did improve, now I’m getting numbers of between 0.83-1.3, so it is definitely an improvement but still didn’t look flat.

The test exposures I done at those RMS levels using the 40D at ISO100 on a 40 minute exposure showed round stars and the frame exposure looked good.

I decided to investigate to try to improve the graph, and when turning off the guide commands the graph showed large bumps generated by star movement caused by the atmosphere, the graph was very similar, although slightly higher RMS, due to the star moving around obviously due to seeing.
At this stage I put my larger graph RMS in comparison to other very flat graphs to perhaps me guiding at 2032mm on a 1/4” CCD and others guiding using a much smaller/shorter FL guidescope where such large seeing related star movements are not picked up at a shorter FL, I base this on the fact that when imaging and guiding while using my 80mm/500mmFL frac where I generally got a RMS of 0.3ish.

Using the size of the pixels and CCD on the focal length the results are 0.548 arcsecs per pixel so multiplying that by the RMS I get the guiding is 0.45 – 0.71 arc seconds accuracy (?) which if I’m right, sub arc sec accuracy is OK for AP. I used http://www.celestialwonders.com/tools/imageScaleCalc.html for the calculation.

NOTE That until I had decent backlash set on the handset I was still getting intermittent saw tooth like spikes in DEC and RA, and the guide star did spontanously jump large distances periodically.
The way I adjusted the backlash on both RA and DEC was by centering a star on the laptop screen and at 1X guide speed moving forward, forward, back, forward, back, back, adjust the backlash and repeat until the star responded instantly.
Also my autoguide rate is set at 40% for both DEC and RA.

My PHD parameters that seem to give the best guiding at 2032mm are below:
RA Aggressive: 50
RA Hyster: 10
Max RA Dur: 350
Search Reg Pix: 15
Min motion: 0.15
Calib Step: 500
Time Lapse: 0

Dec Guide Mode: Auto
Dec Alg: Resist Switching
Dec Slope: 4.5
Max Dec Dur: 350
Star Mass Tolerance: 1.00
Dither Scale: 0.05 

Hopefully some of this helps someone.

Also if I'm missing something and anyone has advice to improve that graph, please feel free to share.
Clear skies.
 

[Freddie]

Couple of issues with that graph you have posted. You are guiding on a saturated star which is not ideal and using 0.5 second exposures will mean you are "chasing the seeing". 2-3 seconds would be better.

[newbie alert]

Great to hear on someone using a sct for imaging..30-40 min exposures!! Crikey.. .I'd of thought 10 mins was exceptional 

Not tried my oag on the 8sct as yet as mounts messing about... 

[MarsG76]

9 hours ago, Freddie said:

Couple of issues with that graph you have posted. You are guiding on a saturated star which is not ideal and using 0.5 second exposures will mean you are "chasing the seeing". 2-3 seconds would be better.

I see your point, and in theory that is a fact but in practice the results were different.

I tried 1, 2, 3, 5 & 10 second exposures.. thinking that a longer exposure will be better as in actually "draw" a round star from teh seeing fluctuations on a longer exposed but in reality the 0.5sec gave the best graph, 1 sec was not too much worse.

Simply the CGEM is not fast enough to respond at 0.5 sec exposures so in reality the delay the CGEM had makes it act as if the exposures were longer. Software might be displaying the seeing but the mount reacts much slower.

 

[MarsG76]

2 hours ago, newbie alert said:

Great to hear on someone using a sct for imaging..30-40 min exposures!! Crikey.. .I'd of thought 10 mins was exceptional 

Not tried my oag on the 8sct as yet as mounts messing about... 

SCTs are quite capable of producing some great images. I never managed more than 7 or 8 minutes consistently on a SCT before using the OAG... when guiding with a OAG on the imaging FL, I dare say that the exposure time is only limited by your polar alignment accuracy, ie field rotation... so even 40 minutes is not the limit.... seeing, sky glow and clouds will limit it too, and for actual imaging 30-40 minute exposures are, in my experience, only useful for NB, Halpha or particularly very useful for SII NB imaging.

 

[newbie alert]

Sounds great..ive got a soft spot for HA images..

What do you use as a guide cam,something like a lodestar?

[MarsG76]

1 hour ago, newbie alert said:

Sounds great..ive got a soft spot for HA images..

What do you use as a guide cam,something like a lodestar?

I use a IS DMK21au618 on PHD and also the NexGuide... NexGuide does a good job too, but I use it mainly for wide angle inaging on the 80/500mm frac.