PHD and autoguiding options....

[Tim]

Soooo, being new to this malarkey.......

I want to be able to expose for a lot longer (as the actress said to the bishop). At present there seems to be a limit on my equipment of about 300 secs exposure before the trailing becomes too noticeable.

I appreciate that I am imaging at F10 with a long focal length tube, and that any errors will be amplified considerably. But I also have the gut feeling that I may be missing something obvious.

Last night I was trying desperately to get something on M51 (wife likes the swirly pix) although it was very very murky until about 2.30am at which point it cleared considerably. Over the course of a couple of hours I tried and tested just about every option and combination in PHD, re-calibrating each time I tried a new setting......

geez, head keeps hitting laptop from 2 nigths no sleep,,,,gonna go to bed, will finish this and post pic of it later, cheers

TJ

[Tim]

Ahem, sorry about that. I crashed into bed and slept for 10 hours last night, something of a record for me.

What I am hoping is that somebody may be able to take a peek at the image attached, and have a bit of a clue as to what, if anything, can or should be altered to achieve more accurate tracking.

So here is the piccy;

So what do you guys reckon? The thing that seems most likely to me is that the guide scope, a 72mm WO Megrez thingy perhaps doesnt pick up the star movement accurately enough or something, in comparison to the c9.25 I was using for imaging, but i'm clueless, and as I say, nothing I could find to alter in PHD seemed to make a difference.

I should point out that polar alignment is only carried out via the polar scope. Does that pic indicate whether the tracking errors are in RA or DEC? Is proper drift aligning necessary when using phd? Is there another, preferably free autoguiding option?

Any ideas gratefully recieved. Cheers guys!

TJ

[Whippy]

Teej, not sure and I'm no means an expert but have you tried...

checking the autoguding rate on the mount? Should be at x0.5

what sort of exposure time is the 'loop' running at? You might want to go longer to 'calm' down the guiding.

Saying that, some nights my guiding (ZS66, QHY5 and PHD) works beautifully, some nights it plays up a bit so I shorten the exposures so I don't waste any time. I don'tknow why but that's how it is for me. I polar align in the same way as I do visually, just get polaris aligned in the polarscope and off I go.

Have you got that EQ6 yet? All your gear on a HEQ5 would make it a bit sensitive to vibrations..

Tony..

[Tim]

Yeh its all plopped on the EQ6 now, the odd thing is, the heq5 worked a treat, in fact I was even considering bringing it out of retirement....

Autoguiding on the mount is at 0.5x. As far as the looping on PHD is concerned, I was under the impression that no matter what the loop time is you have set to "see" the stars, the actual checking is done every 0.05 secs. Is that wrong?

I usually have the loop time set to 2 secs, and usually focus until the star is at its smallest size.

The other thing I meant to ask, how long SHOULD I be able to track for? I would have thought almost indefinetly, aside from the fact of the cables and ota catching the mount etc???

I just noticed although its murky that there are some stars visible, I'll have a go at drift aligning, and play with the settings again. All suggestions gratefully recieved!!

Cheers

TJ

[Ant]

I've not ventured into this yet, but there are lots of settings built into the hand controller of these PRO mounts.

It is possible (maybe?) that there is a setting in there that the PHD is fighting against. How about trying the HEQ5 hand controller on the EQ6 mount (they are the same I believe).

Good luck mate - I will be following you down this road in a few months hopefully

Ant

[Tim]

I've not ventured into this yet, but there are lots of settings built into the hand controller of these PRO mounts.

It is possible (maybe?) that there is a setting in there that the PHD is fighting against. How about trying the HEQ5 hand controller on the EQ6 mount (they are the same I believe).

Good luck mate - I will be following you down this road in a few months hopefully

Ant

Thanks Ant. I am using the heq5 controller anyway, they are exactly the same, and that one was updated.

I did even try turning tracking off all together to see if PHD could handle the job on its own, ie, completely tracking the stars via movements sent by PHD, but this seemed impossible, the stars were whizzing across the screen. So as I understand it, the mount does its usual tracking routine, at sidereal rate, and PHD just adds a little nudge here and there to speed up or slow down as required.

Cheers

[Beyond_Vision]

I have been getting exposure length of 15 to 20 mins with no hint of trailing with a scope of 600mm focal length for imaging and 1000mm for autoguiding using PHD and a Sirius EQG mount. With your longer focal length imaging scope guided with a shorter focal length scope the set up will be more critical. Any fluctuation in brightness caused by atmospheric turbulence will affect the guiding as you are using the centroid of the guide star to track at a sub pixel level. You say the skies had cleared but looking at the images they look a bit murky. The seeing has been terrible for the last few days. A combination of turbulance and too aggressive settings on PHD may cause it to track badly.

PHD worked straight out of the box for me and has rarely misbehaved.

Regards

Kevin

[Tim]

Thats interesting Kev, so the seeing will affect it so much? It was still murky at the time of this pic, but at least to the naked eye, the stars weren't jumping around. I would have thought that with seeing causing problems, the star images would have been more blobby and rounded????? They always seem to be errors in RA, but I have no idea whether the mount is going too fast, or too slow, not sure how to check that really.

The FL of the imaging tube is at least 2350mm (not sure if the extension caused by the camera adapter lengthens that??), Im not sure what the FL is of the Megrez jobby. One thing I do know, theres no way this mount will take TWO C9.25's next to each other!!

I did try adjusting the RA aggressiveness right up to 120% and right down to 70%, with little change, if any, in the images.

I need to nail this problem, not just for getting better piccies, but to help me to decide whether to build an obs in the garden, so all the advice is especially welcome.

Cheers

TJ

[Kaptain Klevtsov]

So the guiding doesn't seem to be working because:-

a) The cable wasn't plugged in. (Sorry, but I have the T shirt)

PHD was talking to the wrong port. (This is a guess as I don't know PHD at all)

c) The guide star was a double one and PHD swapped between the two.

d) The seeing was pants causing the guiding to go erratic.

e) Some setting was off or wrong in PHD.

All the error seems to be in RA, so I'm guessing that it didn't work at all, your polar alignment was good, but what you are seeing is the mount's PE.

To check that the guiding works, I slew the 'scope away from the target and watch to see if it comes back. That's with K3CCD, I don't know if you can do this with PHD as the handset has to be in serial port mode IIRC.

Kaptain Klevtsov

[anthony]

Imaging at 2350mm FL is very demanding stuff - no two ways about it and all the varigacies (poor spelling porbably) of tracking and guiding will take delight in revealing themselves. To get anywhere near 300sec at that length is a considerable achievement and needs applauding.

Anthony

[MartinB]

Some people are lucky enough to plug everything in and have guiding work first time. It wasn't like that for me!! One consolation is that when things don't work you get to know an awful lot about how guiding works and how to tweak it!!

It's best to be systematic and adjust one thing at a time.

First off, polar alignment needs to be reasonable but not exact. Some people even do a tiny offset to make sure that dec adjustments are all in the same direction.

The other important thing when setting up is to offset the balance so that the set up is slightly heavier to the east. This means that the RA motor is always pushing against the gears and gives smoother tracking.

Your small scope should guide the 9.25 perfectly.

Guidestar - very important that it isn't saturated. If it is saturated the software wont be able to work out the centroid position of the star and guiding will be all over the place and quite likely oscillating too and fro. PHD calculates the centroid to sub pixel accuracy which allows use of smaller guidescopes such as yours.

However the guidestar needs to be reasonably bright for good s/n ratio. This is also important for guiding accuracy.

When calibrating, if there are 2 bright stars in the frame the software can confuse the 2 totally wrecking your calibration. Ensure there is just one dominant star in the frame when calibrating

Backlash - not an issue in RA because the tracking is always in the same direction, just speeding up and slowing down. this isn't the case with the dec axis. Backlash can wreak havoc and mess up calibration. 3 ways around it - use backlash compensation -either software or using the mounts control system (quite tricky), offset the balance in dec so that gravity keeps the gears meshed (worked a treat on my old GPDX where backlash was a big issue) or, as I said, offset the polar alignment by a tiny amount (not a good idea with very long exposures because of field rotation).

False tracking errors - your system doesn't know the difference between errors induced by poor seeing, wind and genuine errors of tracking. Seeing has been pretty grim across the UK this week and this will make the guidestar wobble and the mount tries to keep pace. In doing this it starts to chase its tail and can start oscillating wildly. I don't know if PHD provides you with tracking info but it is useful to have a look to see what your guiding is doing. If you are seeing oscillations there are several things to look at but one of the most useful is to turn down the aggression. In poor seeing I sometimes use an aggression setting of 30% - in other words a mount correcti of 30% of the tracking error is made. This can really calm things down. The other way to deal with the problem is to increase the lenght of the star exposure (making sure the guidestar isn't saturated). This averages out the changes in seeing. The downside of this is that all the time you are exposing the star tracking errors are going uncorrected. It is a matter of getting to know what works best for your set up. Aim for guide exposures of around 1-5 secs depending on conditions and ability of your mount.

If you are still having problems there are other tweaks but I'm not familiar with PHD. Maxim allows you to set a maximum and a minimum move which can refine things a little.

The main thing is to have a methodical approach to diagnosing the problem, get the set up right and then try adjusting one thing at a time.

[cygnusx1]

You could also try Guidemaster, very intuitive and much nicer graphical output than PHD to assist in analysing the tracking.

[Tim]

ok, thank you all very much for those points.

If we get some clear spells tonight as they say then i'll start working through them methodically. I might even try k3ccd as a guide option, and I also want to try the polar alignment app I just downloaded.

I'm pretty sure that PHD is communicating correctly with the mount due to the calibrating process. I tend to use the nearest bright star to the DSO I am trying to image as the guidestar. I dunno if its just the camera that I use, but there is rarely more than one star for guiding even visible on the screen, and I usually get hold of that one by popping an eyepiece in the diagonal, adjust the scope rings to centre the nearest star, and then drop the camera back in.

I think PHD automatically compensates for dec backlash, it does a couple of checks for it.

Lets see what the weather brings, clear skies hopefully!!

TJ

[Paxo]

TJ, in the tools section of phd there is a graph function which plots the signals to the mount, it may give some clues, it runs alongside the main window. I have it reasonably ok most of the time, but for me phd tends to occasionaly just go off the rails for no apparent reason which is usually only rectified by a computer restart. I run phd via a gpusb connection to the autoguider port on an eq6 pro, this little box of tricks has a bi-colour led on it which is red while guiding but no signal and green while a signal is being sent to the mount. Last night, the whole system was flawless for two hours, hardly a flicker from the led to green, then, alarms going everywhere and phd was driving the mount crazy, why?

I can confirm what Martin said about the Megrez 72, I use one very successfully in conjunction with a Meade DSI for guiding.

I hope you sort your problems out soon, there's nothing more frustrating.

Steve..