PEC and guiding with PHD2

[joecoyle]

Hi

The longest frames i seem to be able to capture using my Atik 414ex and PHD2 are about 2 minutes. Any longer than that and i start to get egg shaped stars. PHD is making corrections, but i can't understand why i can't get to subs of at least 5 minutes. I'm not sure if its a limitation of the scope (6SE, with f/6.3 FR), limitation of the guide camera (orion magnificent mini, 50mm scope), poor polar alignment or poor worm gear. 

Would PEC training help improve my sub length? Or is it settings in PHD to experiment with? I look at the logs, but it's all just numbers to me. I can provide logs if that would help?

Any suggestions appreciated.

Thanks

Joe

[Uranium235]

Its probably the mount as its an alt/az (you will be getting field rotation after a few min), so PEC training wont help. On an decent EQ mount you can quite easily crack off a 20min guided sub without ever going near PEC training.

[joecoyle]

I should have said my OTA is on an AVX mount  Is that a decent mount for a 20 minute sub? 

[libraryman]

Post a screen capture of your PhD guiding screen, you can also down load PhD log analyser.load into it your last guide log and post a screen shot of that.

Ray

[Qwiati]

Hi,

I was on exact the same boat as you are now. 

PHD - should only correct when it sees it need to correct so if the mount is tracing correctly there should not be an issue with subs length.

My issue was simply resolved by:

polar alignment - as best as possible

PEC file  and make sure use set gain to anything other than 0 - I used PEMPro v2 and it produced excellent PEC file

Your guide scope should be ok for telescope up to 1500mm fl

If you eliminate poor polar alignment AND poor worm gear ( good PEC) your subs should be close to perfect even with long exposure times.

 

Kris

 

[Uranium235]

Ahhh so its the AVX!  If im not mistaken, thats a jazzed up version of the old CG5-GT (my first mount).

Well, in theory you should be able to do 10min subs on a regular basis. But with mine, I settled for 7.5min as a compromise in order to get a better hit rate. So there are a number of things you can look at:

1) Polar alignment

2) Any flex between the OTA and guidescope

3) Velcro up all your cables (to remove strain from the cameras)

4) PHD settings - minimum motion should be quite small (ie: the amount a star should move before a guide command is triggered)

5) Check the mount for slop in either axis (most likely in DEC), a small tweak of the worm may help there.... but be careful with it, its easy to overdo it.

6) Try to guide in one direction only (in DEC), either north or south - depending on what way it likes to drift. That will help take out a lot of the backlash.

 

Its all little things that can add up, but check everything!

[joecoyle]

So in terms of Polar alignment then... What is the best way to get that as good as possible? Currently, this is my routine:

1. Point roughly north.
2. Use polar align scope... rotate RA until Cassiopeia is correct orientation. Then adjust bolts to get Polaris in the little circle.
3. Do a quick one star align to get the Handset to generate a model.
4. Use SGPro to plate solve the target then sync the mount to those coordinates.

I used to run the All Star Polar Align routine on the handset but it didn't improve things massively (still 2 minute subs, the Polar Align error said 000 but i don't believe it). And now with plate solving, i supposedly don't need to?

 

Thanks

Joe

[Thalestris24]

Hi Joe

You could use the PHD2 drift align tool. You are imaging at quite a long focal length which makes things more challenging. Make sure you are well balanced too. Also run PHD2 with guiding OFF (can use the PHD2 guiding tool). That will indicate how much you might be drifting. Plus what Rob said

Louise

[joecoyle]

Here is a screen shot of my PHD log. What do people think?

Thanks
Joe

 

[Davey-T]

Doesn't look like it's making many RA corrections, do you know which way it's drifting ?

May be over / under correcting in Dec

Dave

[joecoyle]

Im not sure how I determine which way it drifts?

Joe

[Thalestris24]

Hi again

Just one thing - are you settling down after dithering and before exposing?

Louise

[Davey-T]

Take a pic 'of something that has an obvious way up / round so you can see if your stars are going eggy in RA or Dec.

I think PHD2 has some sort of assistant that runs with guiding off and indicates drift that needs correcting.

Dave

[joecoyle]

Here are my PHD settings.... Ive only changed the calibration step... used the calculate button for that.

Louise - My imaging program (SGPRO) says "waiting for Autoguider to settle" before beginning the next exposure. So I'm guessing it is waiting. It has (distance 0.5, 0.4, 0.3) next to it. It starts the exposure when it gets to 0.3. 

Cheers

Joe

 

[libraryman]

Your guide log shows a very slow reaction to corrections, I would suggest looking at you guiding rate and if possible choose a rate between 0.5x & 1.0!

Ray 

[joecoyle]

And where do I change that Ray?

Thanks

Joe

[libraryman]

Actually, just re-looking at the guide log....it appears to say that your rates differ slightly.0.8x & 1.0x so the rates should be fine...but...the log shows all correction on one side for a time then on the other side for a time when ideally it should be a little more balanced, in addition ..you appear to have some backlash in Dec, and that's taking time to correct. Which led me to suggest upping the guide rate

turn the aggressiveness down a bit ..perhaps 70. But do make sure your alignment is good, use PhD your polar scope alignment is unlikely to be close enough really and that would require higher rates and more aggressiveness to correct the lager excursions.

Ray

[joecoyle]

So, can I use PHD to align ? I'll reduce the aggressiveness to 70 and see what it changes. Can i fix the backlash in dec?

[libraryman]

Sorry joe, I use Eqmod it's easy in there but I don't know what you use, perhaps someone familiar with you set up can assist but at a 1st guess, look in the hand paddle set up.

run the guiding assistant and enable backlash correction it may help with the Dec issue.

Ray

[joecoyle]

Yeah I found the guide rate in the hand controller. Thanks

[libraryman]

What's it set too?

[Thalestris24]

13 minutes ago, joecoyle said:

So, can I use PHD to align ? I'll reduce the aggressiveness to 70 and see what it changes. Can i fix the backlash in dec?

Yes, you can use the PHD2 drift align tool providing you can point south to adjust az and east or west to adjust allt.

[joecoyle]

Both the RA rate and DEC rate are set to 50% of sidereal.

[libraryman]

O.k I would suggest raising them a little ...perhaps 0.7 x (70%). or perhaps 0.1x and test....lower aggressiveness can be used then.

Ray

ps..plate solving has nothing to do with polar alignment and does not guarentee it.

[joecoyle]

Thanks I'll give it a go. I'll try out the drift align too.