OK, so I have got my system almost doing what I want...attached is a crop of a 10 min test run on the Keyhole region. My stars are not round but little triangles (no its not a collimation problem). I use PHD to guide and the corrections were applied on both axes with DEC in one direction only. Exposure was 3 secs and I set the erorr window to .05. The error log shows corrections only being applied in one direction and typically taking 2 small pulses to re-centre the star.
So I think my problem is lack of sensitivity to the errors but I am not sure I can go below .05 (ie guiding to less than on twentieth of a pixel?) so I need to barlow?
My Guide scope is a WO 66ED - 388mm fl with an Orion Starshoot on the back (3.43" per pixel), imaging is the VC200L - 1800mm fl with the 20D on the back (0.74" per pixel).
Using those numbers I thought I would be ok without a barlow???
is PHD correcting both axis at the same time? so it can correct Dec aswell as RA at the same time, might be that PHD corrects for one but misses a command to correct for the other so it catches up by then the star has moved too much in RA or Dec? Only a guess mind you, I know of some software that only being issued with one command at a time. I havent actually used Autoguiding on a mount yet, always used Pulse guiding.
Paul, pretty close to middle - the stars do not vary in shape though no mater where I look on the image. Attached a 50% reduced full frame so you can see the issue...
Ok IC. I had similar triangular stars in a series of images, but it boiled down to a Focal Reducer issue. Using the Meade 6.3 on an Orion ED80, the stars out near the corners all looked like little triangles.
In your secondary, do you have screws to hold the mirror to the secondary mount and screws to adjust the collimation or just collimation screws?
But I am sure this is a guiding problem, here is a short, unguided, exposure (30s) - no hint of triangular stars - just a little tailing due to PEC/poor polar alignment....and yes the VC200L mirror is fixed.
Chasing down auto guiding problems can be real fun eh – not!
I once solved one of my problems by turning down the aggressiveness of the guiding correction. I think this reduced the magnitude of the full correction to say, 50%, as I was finding that the correction was too aggressive and overshooting, so the next one would then overcompensate in the opposite direction and so on, leading to a oscillation effect.
This was in CCDSoft which has an aggressiveness setting option – I’m not sure if PHD does?
Dropped the exposure time to 2s, set aggressiveness up to 100% (was at 70% previously). This is a crop of a single 600s exposure at full scale - calibration and levels only. I think this is close to the limit for my setup, pixels are .7" so seeing would make a star blur out over a 3x3 area correct?
Zooming in on a faiter star seems to show perfomance is close to that...
The brighter stars still look just a little bit blobby (egg shaped) at about the 6:30 position on the stars. Maybe try a deconvolution on it at see how it turns out.
i had this problem with my 8" F/4 that all the stars are in triangular shape. Came to the conclusion that my collimation wasn't correct and that my EQ6 mount tripod legs were not tight enough. great image btw john.
I knew my gear was better than it's operator and the seeing/lp in my back yard - but I am very that I can now extract close to the best out of the conditions I have to work with, well, some of the time anyway.
Autoguiding is a black art indeed, not helped by no PEC on the Sphinx...
What I am seeing is mostly very small errors/corrections. The attached plots show dx/dy in pixels at a scale of about 3.7" per pixel with an exposure time of 3s.
But are these large spikes normal - would it be seeing related? They do not appear to be periodic, also if I review the log files I see the jump is reported in position and then corrected - normally in a single cycle....
I had thought to put my dead band higher - but I cannot set it to 1 pixel and anything less would not alter this behaviour - is it real, is my seeing likely to move a centroid by 4" for 3s or more or is this spurious. Can it be wind loading - I do not have an observatory by my location is pretty sheltered...
If this is not seeing related where to look? It seems odd to me that it is always ~1 pixel and impacts both axes by the same amount - it is almost like a noise issue -perhaps I should see if it is still present if I run all gear off batteries. Perhaps I should consider chokes on the cables going to the mount/cameras?
The impact of the spikes is worse on shorter subframes as the excursions form a larger %age of the exposure time thus my 10 min subs have rounder stars than my 2min subs....attached is a 300% zoom of one of my Canon images at 300s - the stars are distorted in x and y...
Still digging into this guiding issue...needing help/suggestions/advice
Thought I would share the journey with you. My latest experiments with guide parameters - faster slews, shorter slews, backlash adjustments etc etc have not yielded any significant improvement in guiding accuracy. I still get egg shaped stars....I did a run last night with the guiding outputs turned off.
The result is attached. As you will readily see I did not bother with accurate polar alignment for this run - so there is significant DEC drift.
I ran the mount off a battery and did not use TEC on the guidecam, I only had the guideport on the mount connected to the PC and only one USB connection - all to eliminate PC load and electical noise.
There are two things worrying me here. The DEC motor was off and yet there are step changes in the DEC trace and the RA trace shows jumps also. Exposure was 2s.
Is this slippage somewhere (clutches) or is it backlash. DO I need to unbalance in DEC as you do in RA to keep the gears loaded?
Your advice/input would be appreciated - I am runnig short of things to tweak...
Hi John,
I owned a VC200L for over ten years, 4-5 of those years I used it for imaging with a webcam and HX516 the latter working at around .85 arcsec/pix. with a fOV of 7x9.3.. like looking down a drain pipe you might say! Super critical. The HX916 when I got it was a little more forgiving.
First of all, what sort of mount are you using?? Good gearing and worm with little PE is essential.
Next you have to be pretty well spot with your polar alignment. Drift alignment is the only way.... guiding at these fL's is super critical for success. Many people think because their auto guiding, rough enough will be good enough... wrong!
Software is also an issue.... I've used Astroart and Maxim because their guide sub exposures are at a sub pixel level and it is what's needed at these critical levels imo..... not like the freebees available on the net, you don't get all the program flexibility where compensating for your mount type and drive speeds are available, heaps of other stuff as well etc. I tried many availably at the time when I was working through these issues.
Unfortunately, this hobby's demands are pretty heavy for good achievement with long focal lengths, unfortunately spending money is a big part of it.
If your mount can do consistent unguided exposures out to 2min, then 2 secs on your guided exposures, if only 1min consistent unguided exposures, then 1 sec... etc. With a Canon DSLR, things will be a little less critical than what I was experiencing with my stand alone astro cameras.
Secondly, how are you fixing your guide scope to the mount.
DON'T fix it to the VC200L as it is!!
The tube is quite flimsy. Even if you have a rail from the top of the primary and secondary housing, this will still make the tube flex.... maybe not when you are imaging with the scope perfectly upright (90*), but when it's starting to get on its side, (90*+/-) it will be pulled out of shape and warp.
Also the stock (native) dovetail rail that is used to fix the scope to the mount is very inadequate for such critical imaging purposes. Just look at it and think about it.... grab your scope and put some pressure on the tube, watch it flex... only has to be a little, that's enough to spoil any guiding at this FL.
The solution here is to spend some money on heavy duty tube rings. Forget the tube rings from the Vixen RSS200, they are still too flimsy!
"Parallax" are the best around and will do the job that you need your setup to do!! Because the tube dia on the VC is a non standard size, you'll find Parallax will be able to custom a set for you.... but it will cost you!
Now you'll be able to fix a guide scope to the top of the Parallax tube rings without any fear of differential flexure or flexure/ warping from the VC200.
When I had my GPDX (long gone) all tuned in using PEC, I was able to get out to 1.5- 2min exposures @ F9, I had no dramas with any sort of flexure, triangular stars. But when I went down setting it up for guiding, this is when my hair started to get pulled. I persisted for a couple of years
So all of the above was my findings with this scope.
Good results can be achieved however, you'll just need to remove all flexure!!
Here's a link to one of the best VC200L users around, check out how he attached his guide scope.... that's why he has great success, rigid tube rings. Look at his images.. http://members.inode.at/wasshuber/eq.../equipment.htm
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
that I can now extract close to the best out of the conditions I have to work with, well, some of the time anyway.
