Guiding - setup and improving

[Spookyer]

Hi all, I am new to the whole guiding thing. I was just after a bit of advice on how I could improve the results I get from my setup.

To guide my 925mm focal length refractor I have a ST80 guidescope in Losmandy rings clamped to a top plate fixed to the rings of the OTA. It all seems pretty solid. I have threaded spacer tubes extending from the focuser of the ST80 to allow focus with the QHY5ii guide camera. The focus tube on the ST80 itself is extended less than 10mm. I got longer spacers than I needed to achieve focus with the guide cam so I wouldn't have to rack the focus tube of the ST80 out much as I was concerned it might flex when extended. The QHY5ii camera on the end is very light and overall the system seems very rigid to me.

I have been using PHD2, I don't know if it guides any better than PHD but I do know that the interface is easier for me to read on my laptop.

The QHY5ii guide camera seems very sensitive and gives a few stars to choose from each time I have used it. It seems to calibrate okay an hold the target star no problems.

I ensure I have balanced the scope in both axis when setting up on my AP Mach 1 mount.

Last weekend I left the PHD2 settings at default and didn't mess with them. For the most part the guiding graph while going up and down constantly was within +/- 1 pixel and most of that time is was within .75 of a pixel. My parameters on the left of the graph were typically between .25 and .5 of a pixel in RA and Dec.

I did have a few times where there were departures from the above and had multi pixel deviations over a period of 5 seconds or so which subsequently ruined the subs as there was noticeable smearing of the stars. I do not know what caused these departures and they corrected within about 5 seconds without me changing the settings.

I guess I am wanting to know how I would go about improving my guiding using my current setup to get more consistency so I can do longer subs?

Is there any advice about settings I should be using in PHD2 that would suit my setup?

Brett

[Lee]

I'm far from an expert - but I would estimate with a Mach1 mount and a 400mm FL guidescope, you can expect much better values than you've quoted.... with PHD1 I improved guiding with decreasing RA aggression, increasing hysteresis, incr exposures from 1s to 2s, and improving polar alignment....

Sometimes in poor seeing, the graph with bump everywhere regardless....

the multipixel deviations (I call them b@stards) may be from cable dragging, wind, crud in the mount... or just guiding gremlins....

[John K]

Hi Brett,

Over the last few months I have gone through a learning process with PHD and an 80mm short focus refractor - I have now switched to an OAG because I shoot at 1600mm f.l.

The key that I found with using a short refractor is to get the min motion as low as you can - had mine to 0.05. The default on PHD is 0.25 I think. However, as I was shooting at 1600mm, this still was not good enough. It will depend on the quality of your mount and how well you are polar aligned I would say.

You should be getting an RMS below 0.2 at least or way better.

The bottom line is if you are getting nice round stars. Is this happening for you? How long are your exposures?

I have also found that sometimes PHD goes "off it's rocker" so to speak - I think this can depend on the Dec guide parameters you set (e.g. resist switching; North, or South) and also sometimes the quality of the guide star.

I have not switched to PHD2 yet.

Hope this helps for you and good luck.

John K.

[Spookyer]

Thanks Lee and John for the tips. I realise this is a difficult area and there are many factors potentially at play.

To answer your question John I have been doing 5 min subs and generally getting round stars. I have probably done 5 nights of imaging with this setup and in that time I have probably kept 90% of the subs I have taken with the remainder smeared to varying degrees. The images I have produced have been pretty reasonable for a beginner I think and have been reasonably sharp.

However like you said I think there is a fair bit of room for improvement to get smoother guiding and more consistent results leading to the ability to do longer subs where appropriate. I will put your suggestions into place.

If there are others out there that have experience with PHD/2 and are using guide scopes I would welcome hearing about your experiences and what you found improved your guiding.

thanks
Brett

[White Rabbit]

I'm no expert either but I've spent years pulling my hair out with this whole guiding thing. One night you get flawless guiding the next you get utter rubbish.
I agree with John though on the min motion thing. It sounds as though phd is over correcting if it is bobbing up and down. I'd dial the aggression down a bit and then adjust your min motion down a bit as well.

Can you post a picture of your graph?

Cheers
Sandy