Re: [ap-ug] "Drew's Law" was Side by side guiding


Nov 12, 2011

 


----------------------------

#57556 Nov 12, 2011

In the spirit of C. Northcote Parkinson, reducing things to a simple law that is sort of correct :-)



"Multiply your image scale in arc seconds/pixel by 400 to find the maximum length subexposre in seconds possible with a guidescope."



As some examples (all with an STL11K and 9 micron px):



Tak FSQ+Reducer (FL 385mm) Image scale 4.82"/px x 400 = 1926 I.e. you can take a 30 minute sub.



AP 160 (FL 1205mm) Image scale 1/54"/px x 400 = 615 You can take a 5 and probably a 10 min usb but not a long (20 or 30 minute) Narrowband sub.



Planewave 12.5" (FL 2541mm) Image scale 0.73"/px x 400 = 291. A 5 min sub is about all you can take before you start to see elongation.



Note with a smaller px camera (ST-10) you will see elongation sooner and note this assumes you have good seeing (FWHM 1.5 to 2.0"). If your seeing is in the 2.5 to 3.0" range the seeing will blur minor elongation so you can go a bit longer.



Anyhow that's about what I find.



----------------------------

#57560 Nov 12, 2011

In a message dated 11/12/2011 12:46:35 PM Central Standard Time,

ancient.sull@... writes:



> In the spirit of C. Northcote Parkinson, reducing things to a simple law

> that is sort of correct :-)

>

> "Multiply your image scale in arc seconds/pixel by 400 to find the maximum

> length subexposre in seconds possible with a guidescope."

>

> As some examples (all with an STL11K and 9 micron px):

>

> Tak FSQ+Reducer (FL 385mm) Image scale 4.82"/px x 400 = 1926 I.e. you can

> take a 30 minute sub.

>

> AP 160 (FL 1205mm) Image scale 1/54"/px x 400 = 615 You can take a 5 and

> probably a 10 min usb but not a long (20 or 30 minute) Narrowband sub.

>

> Planewave 12.5" (FL 2541mm) Image scale 0.73"/px x 400 = 291. A 5 min sub

> is about all you can take before you start to see elongation.

>

> Note with a smaller px camera (ST-10) you will see elongation sooner and

> note this assumes you have good seeing (FWHM 1.5 to 2.0"). If your seeing is

> in the 2.5 to 3.0" range the seeing will blur minor elongation so you can

> go a bit longer.

>

> Anyhow that's about what I find.

>



If your imaging scope and your guide scope both have really non-sagging

focusers, then the best way to couple them is to attach the guide scope directly

to the body of the imaging scope via a second set of rings. Those rings

must float free of the mount's cradle and must only be connected right to the

main imaging scope.



If you go back a century, one of the most effective ways to produce a

guiding system back then was to attach a second lens-in-cell directly to the

primary refractor cell in a figure 8 configuration. The second lens can be

smaller, and has the same focal length as the main lens. The guider focuser is

attached directly to the imaging focuser (again figure 8) and both drawtubes

are securely locked down so that no sag can occur on either one. The guider

system is open tube, no worries about stray light because no imaging is done

with that. Using this system, astronomers back then could take many

hour-long exposures with no differential flex. Using crosshairs and pushing buttons

manually.



Attaching a guide scope with rings widely spaced directly to the tube of an

imaging refractor will almost accomplish the same thing. Of course, this

cannot be done with any sort of Cassegrain systems - way too much flex inside

those tubes regardless of focuser type.



Rolando



[Non-text portions of this message have been removed]


S
e
n
i
o
r
T
u
b
e
.
o
r
g