Guiding focal length

[D4N]

I was having some issues yesterday and found removing my OAG was a quick way of getting around them.

However I still wanted to be able to guide so scrambled around for a lens to put in front of my guide camera and found the longest focal length I had available was 50mm which gives 15"/pixel.

Now my main scope is working at 294mm and 2.5"/pixel, so it seems to me this probably isn't going to work so well.  I know PHD can work to sub pixel accuracy but does anyone have any experience of guiding with such a big mismatch in focal lengths.

I'm thinking that I will need to buy a cheap lens to be able to guide effectively.

[riklaunim]

Just try it. I had no problems guiding C11 with a finder-guidescope.

[D4N]

Well it worked, calibrated ok (once I remember per where to enter the focal length into PHD2) and it stopped the trailing it is just hard to tell the difference between round stars from perfect guiding and round stars from random guiding errors.

I'll give it a proper test if we get a night without clouds.

 

On the plus side it is really easy to find a guide star with an f1.8 lens on the guide camera

[ollypenrice]

You can either get PHD to give you its guide trace in arcseconds by telling it about your guide cam and guide optics or you can leave it in pixels. Left in pixels your error is really 6x as bad as it will look in pixels in PHD (15/2.5) so multiply the error by 6 for a useful comparison with your imaging rig.

Olly

[D4N]

Yep I have it in arc seconds, it didn't look great last night going out to 2-3" but I need to sort out my PA and there were no low stars in the East or West last night to drift with, the only cloud clear area was in the South.

Hopefully I can get it down with good PA.

[uhb1966]

Dear Dan, you have a factor of 1:6 between guide scope and primary. My set up has 162:800 or 1:5. I finally came out with an average error of 1,5-2 arcsec (but with 3.5 arcsec peak). So we have a similar setup, and similar results However, in my case, the seeing was mediocre (high temp), i doubt that the seeing was better then 3.5 arcsec, so i have a hard time guessing whether i was i was guiding limited or my guiding was seeing-limited ... how was your seeing that night?

 

PS:check out http://www.wilmslowastro.com/tips/autoguiding.htm:

"

For me seeing is probably the limiting factor at say 2 arcseconds on most nights. Working this backward means I am wasting my time with a guider resolution greater than 30 arcseconds a pixel. Working this out for the STV which has 7.4 micron pixels gives...

arcseconds/pixel = (pixel size in um) / (focal length in mm) * 206

fl = (pixel size in um) / (arcsec /pixel) * 206

fl = 7.4 / 30 * 206

fl = 50mm

"

 

 

[D4N]

That night wasn't brilliant seeing, it is a bit early in the year still so lots of moisture in the air and thermals.  In the winter the skies get pristine where I live as it gets so cold all the moisture freezes out of the air and the lack of daylight limits diurnal temperature variations so there are few thermals.

That link was very helpful, it seems to suggest that a 50mm prime lens is actually ideal for my setup.  That made me a lot happier  

[D4N]

Ok I had another go last night and had a good drift alignment so PHD2 was doing very few corrections to stay on target.

RMS was steady at 2" with the very occasional blip but I suspect any more than that wouldn't have been possible as the transparency wasn't good enough.

Kinda happy with the new setup, it is very easy to find stars with such a fast lens

[uhb1966]

Same here, i am using a cheapo orion "finder type" scope for my SSAG, which has 50mm aperture and as stated 162 mm fl. Was thinking about getting sth with longer fl, but i am now (after reading the above link) quite sure that i'm almost all the time seeing-limited.

PS: there is also an article from the phd author on the web, where he explains that he's also been using guide scopes with short fls. He suggests setting mnmo to 0.05 ... wouzld be interesting if you could share your phd2 settings? Here are mine:

[D4N]

I just shut my obsy down for the night but my agression is about half yours and hysteresis is twice yours.

I do need to tweak the settings though as it is still mostly set how I had it on my OAG so it could be optimised for the shorter focal length.