plate solving issue

[neilbarber]

Not sure if i'm in the right place to ask this but here goes.
I have an HEQ5 Pro mount and z61ii and ed80. With the z61ii I do my polar alignment and can get it to .02 and .04. When I slew to a target in Stellarium and plate solve it is normally within 2(can't remember the rest. Age thing). Tonight I swapped over to the ed80. Polar alignment was just as good, .03 and .05. When I slewed to the first bright star and got 120 stars it would not plate solve. Said it couldn't find anything within 15 degrees. Moved it across to the moon and centered on that (Was just a little off center), synced in stellarium and then picked another bright star and slewed to it. Once again it said it could not plate solve as it couldn't find anything within 15 degrees.
The only thing that has changed since I last used it a few nights ago, with the Z61ii and had no problems, is I updated to the latest version yesterday. Is there a problem with the update? Have rain forcast for the next few days so no rush ..... thanks. Neil

[admin]

Hi,

I think the most likely cause is that the Z61ii has a focal length of about 360mm, whereas the ED80 is 600mm, so the field of view is considerably smaller with the ED80.

A smaller field of view can make plate solving trickier and make it more critical to get the right settings. It's also possible that your plate solving program does not enough data installed to allow it to work at a narrower field of view. Which plate solving program are you using? Astap? Astrotortilla?

cheers,

Robin

[neilbarber]

Thanks for getting back to me Robin. Bed time here so will check in the morning to see which it is. I thought with having such good polar alignment it would manage quite well. Will post tomorrow. Thanks

[neilbarber]

Morning Robin,
It is Astrotortilla that I'm using and haven't had this trouble before with the ed80 so I'm assuming I have the right data fields downloaded for it. Any ideas.

[admin]

Hi,

the best bet to check plate solving is to save a frame and then upload it to http://nova.astrometry.net/ - that will almost certainly solve the image if it is possible and gives you the co-ordinates (and scale). You can then check that the mount RA/Dec readouts are close enough to the real values and that you have the right index files installed to solve at that scale.

You may want to check out a couple of options in the SharpCap plate solving settings - that is the sigma parameter which controls how sensitive astrotortilla is when finding stars (lower = more sensitive) and the 'send 8 bit images to plate solve). It's usually good to have between 100 and 300 stars found when using AstroTortilla - less than 100 or much more than 300 can cause issues.

cheers,

Robin

[neilbarber]

Thank you for the information Robin and will check it all out once this weather system has gone through next week. Much appreciated.

[neilbarber]

Hi Robin,
Managed to get out for an hour last night to try a few things which you suggested. This is what i found

I've had the Focal length of telescope set to 600mm (ed80) and this didn't work so changed it to Do Not Use
Lowered the sigma setting down to 20 but had to have my camera set to 5sec with very high gain but it would only find around 30 stars but would plate solve.

Could you tell me please if this is the norm or should i have the sigma setting higher.
Thanks

PS ... nice to see a lifetime license but in 10 years I'll be lucky if i'm still here .... How about a pensioners discount on the yearly one

[admin]

Hi,

there are no hard-and-fast rules on the sigma - 20 is on the low side, but if it works for you then stick with it, although as you have noticed the value you need may change if you make big changes to the exposure/gain of the images that you are sending to the plate solving engine.

cheers,

Robin

PS. Sorry, no discounts on the yearly price - the price is already set to be highly affordable.

[neilbarber]

Thanks Robin ... will just have to adjust things as needed. Could you tell me why it doesn't work when I set my telescope focal length. I would have thought that would helped more.

Yearly price is very good and well worth it considering all you put into it.

[admin]

Hi,

well, firstly you need to be sure that the focal length is correct and accounts for any focal reducers or barlow lenses etc in the imaging system. I've also seen cases where it turns out that the focal length of telescopes actually isn't what is stamped on the side (although this seems to be mostly at the cheaper end of the market).

If you turn the focal length back on and try a plate solve, you will find a line like this ends up in the SharpCap log

Code: Select all

Info    13:43:44.486217 #1  Field of view calculated as 0.16x0.12 degrees based on FL of 1700.0mm and sensor size of 4.80x3.60mm         

You can compare the values shown there with the actualy size of your camera sensor and the measured field of view that you get when you are running a succesful plate solve (ie without the focal length set). If you find a significant error, let me know all the details and I will try to work out if it is a bug.

cheers,

Robin