PHD Guiding

[Roy@Aldermaston]

I've been guiding with PHD and a QHY5 on an ST80 for a few months now, and generally calibration and guiding run smoothly. The one minor glitch that I haven't been able to solve is how to avoid star saturation. This puzzles me because my choice of guide star seems fine for calibration and PHD will start to guide without any problems, but within a few minutes it reports that the "star is saturated" and the guiding graph becomes jagged. This happens every time no matter how careful I am with the guide star selection, making sure to avoid excessively bright stars.

Does anyone know what causes this problem and how to avoid it?

Thanks,

Roy

[gfletcher]

What exposure are you using ?

[Roy@Aldermaston]

It's usually 2.0 or 2.5 secs depending on conditions

[OzDave]

Hmmm, I've noticed this too. It seems to happen when PHD detects too many bright pixels within your star. Or it could be because the star profile isn't a good shape, possibly due to focus or seeing. Things you could try are decreasing the exposure time so that pixels do not saturate (i.e. reach the maximum level of brightness), or reduce the camera gain (use the Brain icon when PHD is not guiding to reduce this a bit).

Also, try asking PHD to show the star profile when this happens. It should look "normal" and not concave or anything else odd looking. If you do get an odd looking profile, post a screen capture here for analysis.

Anyone got any other solutions?

[Peter_A]

Roy,

I am very interested in this thread because I am having the same problems with my QHY5/ST80/PHD setup. This is a new setup for me (EQ5 purchased in the spring) and I have been trying to get it going for the new season. Last night I managed to get it to guide for a couple of minutes before I got the dreaded star saturation warning. I was also seeing the PHD icon bar flash red, I don't know why it does that??

I will post my settings if I manage any improvement. I think poor seeing could be a factor, it was much clearer last night in between the clouds.

Peter.

[chrisshillito]

Just defocus a little A defocused star image will also help the accuracy of centroid calculations as it has more area of non saturated pixels over which to calculate its weighted average.

Chris.

[Peter_A]

Just defocus a little A defocused star image will also help the accuracy of centroid calculations as it has more area of non saturated pixels over which to calculate its weighted average.

Chris.

Thanks Chris, I'll give that a try. Pete.

[RikM]

I've never seen that error with my QHY5 on a finderguider. I do know my focus is out a little bit (on purpose) so it could well be that. I remember reading a thread that said the default gain setting for the QHY5 in PHD was 50%. I don't know what mine is set to but I haven't altered it from the defaults.

[Peter_A]

I've never seen that error with my QHY5 on a finderguider. I do know my focus is out a little bit (on purpose) so it could well be that. I remember reading a thread that said the default gain setting for the QHY5 in PHD was 50%. I don't know what mine is set to but I haven't altered it from the defaults.

hat's also a factor, there are quite a few settings to experiment with which will take time, although less time hopefully with kind help available here on SGL. I have tried various gain settings from 25% up to 90%, I imagine lower gain is better to avoid amplifying noise?

Pete.

[Davey-T]

I've found it worth using the option in PHD to take a dark frame as it shows more dimmer stars and gives more choice if it complains the star is to bright.

Dave

[Peter_A]

I just noticed that the objective on my 80mm was in dire need of a clean, that was almost certainly a significant part of the problem. I havn't had a chance to do another test yet.

Pete.

[Roy@Aldermaston]

Some interesting thoughts: I read on another forum that increasing the camera gain to between 90 and 95% might be helpful, and also defocusing. Peter's last post prompted me to think about condensation on the ST80 objective as a possible cause for star saturation - could it be that simple? The fitted dew shield on the ST 80 seems too short to be of much use so I've decided to add a DIY version and see if that helps - clear skies permitting of course.

Roy

[Peter_A]

Looking at the weather forecast it doesn't look good for the next few days. If I manage any success later I'll post my settings.

[ollypenrice]

I don't see dirty or misted optics causing saturation, though they might cause other problems.

Using shorter exposures has a down side. The shorter the exposure the more the seeing will disturb the position of the centroid so you'll 'chase the seeing.' If you can reduce saturation via the gain then that would be better. Another obvious thing to try would be an aperture mask but remember that this will change the FL and need a fresh calibration.

Olly

[martin_h]

Some interesting thoughts: I read on another forum that increasing the camera gain to between 90 and 95% might be helpful, and also defocusing. Peter's last post prompted me to think about condensation on the ST80 objective as a possible cause for star saturation - could it be that simple? The fitted dew shield on the ST 80 seems too short to be of much use so I've decided to add a DIY version and see if that helps - clear skies permitting of course.

Roy

I run my QHY5/ST80 combo at 95% gain - star slightly defocused. Never had a saturation problem, I run at 3 second loops for guiding.

[RikM]

I checked and mine's set at 95% as well.

[Roy_E]

Thanks for that, I'll try increasing the gain and make sure that the star is slightly out of focus.

[Peter_A]

Another test last night with my QHY5/ST80, better after cleaning the ST80 objective but I am still getting the PHD "Star Saturated" warning after a few minutes of guiding, despite always selecting one of the fainter guide stars. The guide star profile was always a nice bell shape with the peak about a third of the way up the little preview window. When it does guide the graph looks good so if I can get rid of the saturation problem I am hopeful that the system will work well.

I set 95% gain, 2sec exposures. There was the occasional small patch of low cloud and at the end of my 90minute session there was a little bit of dew so these could still have been contributing factors.

If I can't solve this problem I can see myself taking up a different hobby ;-)

[martin_h]

Thats a strange one, I cant see how the saturation could alter over time....what setting do you have for the "star mass tolerance"? mine is set to 0.5

[Peter_A]

Thats a strange one, I cant see how the saturation could alter over time....what setting do you have for the "star mass tolerance"? mine is set to 0.5

Yes, it does seem odd, guiding seems to work fine for a few minutes and then I get the saturation message. Star mass tolerance was one setting that I thought I would try, it was set to 0.5 but then I read somewhere that entering 1.0 disables it, so I tried that but it didn't seem to make any difference.

I am going to have to be a little more methodical to work out which part of my system is at fault. Starting at the beginning and working through each element:

0. Seeing, clouds etc.

1. ST80 scope (dew?)

2. QHY5 guide camera

3. USB cable (I use a long extension)

4. Laptop Computer

5. ASCOM

6. ASCOM QHY5 Driver

7. QHY5 Windows XP driver

8. PHD (all the various settings, I am assuming it isn't a bug considering that other folk have success with similar setups).

9. Anything else I can think of???

[Uranium235]

I've never seen that error with my QHY5 on a finderguider. I do know my focus is out a little bit (on purpose) so it could well be that. I remember reading a thread that said the default gain setting for the QHY5 in PHD was 50%. I don't know what mine is set to but I haven't altered it from the defaults.

Exactly the same here, never had this problem with a finderguider or ST80 (which was reduced by 0.5x). Since getting the NEQ6, ive just been setting the exposure time to 4s and then picking a star that gives a nice spike in the star profile, four seconds should give you plenty to choose from (and average out the seeing).

[Gina]

I generally find a 1sec exposure works well enough with this combination but sometimes go up to 2 if the stars are faint.

[Cath]

I've just reported a source coding bug in one of their star routines to them, although it won't affect the guiding.

Could it be possible that when you start guiding the star is lower down, and if it moves higher in the sky (less atmosphere) it then becomes brighter as time goes by? .. no idea, just guessing.

[Roy@Aldermaston]

It's possible Cath, but I would have expected the problem to arise more widely if that were the cause.

Gina, I'm beginning to wonder if there is a fault with my PHD installation. If I reduce the exposure setting to 1 sec, as you are able to do, the screen becomes very bright even with the gamma slider set to the minimum. That puzzles me as I would have expected the brightness to increase with the higher exposure settings, rather than the other way around!

[Roy@Aldermaston]

I forgot to add, the problem appears whether the guide star is high or low in the sky, and the symptoms seem to be exactly the same as those experienced by Peter.