Guidescope or Off-axis?

[orly_andico]

well i just tried guiding my C9 with a 50mm guide scope (with barlow).

utter failure.

don't know if the mount can't handle it (was really unbalanced because the weights would strike the tripod legs if I lowered them). My RMS guiding was about 0.22 which is pretty bad.

if weather's good tomorrow night I'll try the OAG again..

[MooEy]

Interesting article, thanks for sharing.

Anyway, the author did suggest that skies conditions are roughly 3 arcseconds, and that his calculations are good for 1 arcsecond. I guess mine assumes that seeing is perfect.

Orly: at those focal length i wouldn't really bother with a guidescope, esp on an scope with parts that moves on it's own...

~MooEy~

[orly_andico]

yeah... forgot to mention this in the earlier post.

the problem with SCTs is that the mirror shifts around by itself (it's possessed!)

the EDGE SCT's have mirror locks so that certainly helps - but mine is non-EDGE. and i don't feel up to modding it (yet) to add mirror locks.

[cloud_cover]

Buy my VMC200L It comes with fixed mirror, a quality dual speed crayford focuser and it has a flat field with and without its dedicated field flattener. Batteries not included

[Mariner]

Alan article seem to point out that guiding accuracy is solely dependent on F/num irrespective of focal length of the guide scope. Interesting...hor. Or have I misunderstood his article meaning wrongly?

If right, then can guide a C9.25 with a Orion 50 mm guiderscope, right?
180mm/50 =f3.6, wa perfect leh. Not far out from his 100mm

BTW, Mariner, care to share what you plan to image with your large scope?

More interested in taking shots of DSOs but I haven't really got round to doing it, simply getting the equipment ready first (still in process though).

[kochu]

Well,
I did some experiments in early 2011 with my 12 inch SCT and a toyscope , with 2x barlow , (I think around 700 mm fl) as guide scope and Canon 450 D. Guiding result turned out to be OK.
Previous posting below.
viewtopic.php?f=7&t=9171
Kochu.28-12-12

[mymoon]

Alan article seem to point out that guiding accuracy is solely dependent on F/num irrespective of focal length of the guide scope. Interesting...hor. Or have I misunderstood his article meaning wrongly?

If right, then can guide a C9.25 with a Orion 50 mm guiderscope, right?
180mm/50 =f3.6, wa perfect leh. Not far out from his 100mm

Yes apparently so for most local to this region amateurs I quote from the article:

"1 arcsecond RMS guiding is good enough for almost everyone, particularly with focal lengths under two meters (66 inches)

For users with big scopes on mountaintops I would drop this to 0.33 arcseconds RMS to be completely safe.

Solid guide scope mounting is absolutely necessary to reduce differential deflection problems. Nor does buying a perfect mount solve all your problems.
Mirror shifts, thermal shifts and gradients, and changing gravity loads mandate guiding with update rates of at least every 60 seconds or so. A “Perfect” mount is not an option, in my experience."

Hence autoguide according to his Table One and Table Two

Another urban legend bite dust.

I stand guided.

cheers

[cloud_cover]

Well, with my VMC200L and a Nikon D700, my resolution is 0.89arcsec/pix so 1 arcsec is fine, and that's at 1950mm.
Even then, deconvolution can be achieved by using Photoshop for small errors so again there's more leeway.
So..... take the pictures and enjoy the results Obsess over the perfectness of the stars only if there's nothign else left to do.... heh heh....

[orly_andico]

well during last night's C9 guiding experiment.. I was getting guiding RMS of about 0.22 on PHD. This was with a guide scope focal length of 300mm giving 6.6" per pixel.

So 0.22 * 6.6" = 1.45" RMS guiding.

not too far from the 1" the guy mentions. I suspect I can get tighter guiding with good balance (with a lighter scope I can guide at 0.38" RMS).

and my FL was about 1900mm (C9 + 0.8X reducer). Stars were still streaked. I think it's due to mirror flop.

[beginner]

More interested in taking shots of DSOs but I haven't really got round to doing it, simply getting the equipment ready first (still in process though).

Most DSOs can be imaged with short scope of 400 - 700mm range
And F/num is more important to image these dim objects.
At F/10 very difficult to image dim object, need very long imaging time.

Firstly you need a reducer, even with that your FL still too long, FOV small and the image circle reduce.
Tracking become very critical and polar alignment need to be accurate.
Mount performance is another area of concern.
Guiding become crucial and guiding is not always smooth sailing.

Ideally if you are new to imaging it would be better you experiment with shorter scope.