Strugling with PHD?????

[Nico13 (Ken)]

I must be the Dummy that PHD was written for but it's still giving problems.

The guide Camera setup is an un-moded Phillips Toucam fitted to a 35mm camera lens adapter I made so I could fit any focal length lens I liked.
The lens in the picture is a 135mm f/2.8 and I have also tried a 77mm f/1.8 and a 50mm f/1.7

The main problem is RA calibration keeps failing due to the star not moving enough error.
I did get it to cal once after making some Azimuth corrections to the mount to help with DEC drift but that was short lived as when I moved to a new area of sky and tried to recalibrate, same old problem.

How far does the star have to move to get the calibration to pass and can you force this, also the numbers displayed at the bottom of the PHD window during calibration, what significance have they to the cal.

The Laptop connection is via a GPUSB and is functioning ok as I can select manual drive and that works ok.

Any help grateful.

Nico.

[Tandum (Robin)]

Under the brain button there is a Calibration Step size which defaults to 750 I think. Increase that step size to make it move the star more when calibrating. Try tripling it to something like 2000.
Once you got it going you can adjust that to suit the guide scope/lens you are using. Shorter focal length = bigger step size.

[tilbrook@rbe.ne (Justin Tilbrook)]

Hi, Ken.

Can't really help you with the software you are using, but I think your focal length is too short.
I find with my homemade setup guiding manually, that 600mm f/l was needed to have any chance of making corrections.

At 135mm, what you are using, there is a 40 arc second tolerance.
At 600mm it's about 8 sec.

My guess is thats the problem.
There should be something in your help file that will tell you what you need.

Of coures it's just my guess.

Cheers,
Justin.

[cventer]

I set my calibration anywhere from 2000 - 3000 to get it to work with short focal length lenese.

This all all you will need to do.

[BlackWidow (Mardy)]

Make sure your scope is well balanced. On my rig I eas way out of balance and had the problem you speak of. Got my balance better and the system would calibrate

[mithrandir (Andrew)]

Nico, the trick is to keep fiddling with the calibration step until PHD calibrates in about 12-15 steps. Shorter focal length -> bigger step. I use about 1500 on a 400mm guide scope. I haven't had a chance to work out the values for a mini-guider.

The balance should be good but slightly east heavy, no matter which of the scope or counterweights are on the east side, and adjust it after crossing the meridian.

[leon]

Yes Ken I agree, I too feel that you need to increase the focal length to at least 400mm or so.

Leon

[Tandum (Robin)]

Originally Posted by leon

Yes Ken I agree, I too feel that you need to increase the focal length to at least 400mm or so.

Leon

Not to sure about this Leon, I guess it depends on the main scope.

Have you seen the new SBIG guider kit? 100mm FL @ F2.8.

[Nico13 (Ken)]

Thanks everyone for the great responses.

I reckon that step size is gunna be the key to this little problem as mentioned by Robin.
The default of 750 and the popup message that tells you to reduce it for longer focal lengths is a bit misleading I guess so I've bumped that up right off.

The focal length of my setup is a bit of a mystery at the moment will have to do some calcs on that but was guessing it's a lot longer effective length due to the small sensor size in the toucam.

As an example on my SLR with a APS "C" size sensor the 135mm is roughly 200mm effective F/L or just over so with the very small sensor in the web cam I was guessing it had to be out around the 300-400mm as I was getting so few stars to guide on.

Anyway will look at that as well.

Just had a quick look at Sensor size versus crop factor and effective F/L here and going on a 1/3 sensor which is about the toucam size and the 135mm lens the calculation comes out at just under 1000mm and that would match up with what I'm seeing on screen with the amount of visible stars and star size.

So now I have the info I need to more than likely fix this guess what clouds. Oh well maybe tomorrow night.

Thanks again guys for that info will keep you informed as to my progress.

Ken.

[cventer]

A 135 mm lense at F2.8 will give FL of 378mm

toucam has 5.6um pixels at 640 x 480 you get an image scale of 3.05 arc sec per pixel with FOV of 24.4 x 32.6 arc min


My guider setup is 210mm fl at f3.5. In order to callibrate this in PHD my calibration setting is 2500 assuming a guide rate of .5x sidereal

You dont need longer focal legth to guide your setup. see this article here:
http://www.wilmslowastro.com/tips/autoguiding.htm

with 5.6um pixels assuming you were aiming for 1 arc sec per pixel resolution anything greater that 190mm FL will do the job. (Thats assuming a mediam level of centroid accuracy. Ie between best and worst)

You may find you have to defous your guide camera very slightly if your guide star is only a few pixels wide as to calculate the centroid accurately the guide star shoudl be spread nicely across 4 - 6 pixels.