Re: [APML] Genral Focus Questions and a pathetic plea for help

[Jerry Lodriguss <jerry@astropix.com>]

Hi Bill,

Please do not take offense at this, but are you really sure you are 
correctly focusing the ronchi screen?

I ask because I have seen some people misinterpret exactly how it ought to 
be done.  Some people think it is focused when they can see the edge of the 
knife blade cut into the star, but it ain't.

Let me see if I can describe the process, please forgive me if you are, in 
fact, intimately familiar with this and are doing it right.

If the scope if way out of focus, all you will see is the out of focus star 
and a large number of lines that make up the ronchi screen.

As you get closer to focus, there will be less lines.

As you get closer still, there will be the one line that your eye is lined 
up with the star.  As you move the telescope in R.A. or Dec (the ronchi 
screen lines should be at a right angle to this direction), this line on 
the ronchi screen acts as a single knife edge.

As this knife edge cuts into the light cone, you can see it move across the 
star, but it's still not focused exactly.

When the knife edge is at the focal plane (and the scope is at "focus"), 
and the scope and star are moved, the star will gray out all at once (more 
or less), and you won't be able to see the silhouette of the knife edge 
coming in from either side.

This may seem like there is no room for doubt, but in my experience, there 
is. There may be a range where you can move the focuser and the star will 
gray out all at once. This is from the depth of focus at whatever the 
f/stop is. It's still  "in focus".

Also, the seeing can greatly effect what the image looks like when the star 
grays out.

Now, having said all of that... if you got better results with just the 
ground glass in the camera, then you might want to consider getting a 15x 
magnifier, and focusing it on the bottom of the ground glass, and using 
that to focus.

Take a look at:

http://www.astropix.com/HTML/I_ASTROP/I08/I0812/I0812.HTM

for some more info.

Jerry


At 06:05 PM 6/2/2003 -0500, you wrote:
>Gentlemen,
>
>I have been using a 253mm f/4 Newtonian @ prime focus and using a STI
>ronchi screen set-up to focus this and have had nothing but terrible
>results. Richard has provided me with multiple versions to no avail. The
>last trip out was the back breaker. I forgot the focuser on Wends. And
>decided to focus using the ground glass on the Nikon F. The following
>day I opted to drive for quite a distance to retirieve the focuser from
>home and use it on Thursday. The results ????
>
>Wends - Nearly perfect, or at least to my un-critical eye
>
>Thursday - Every star has the central obstruction clearly visible.....
>arghhhhh!
>
>
>So at this point is it safe to assume that focusing on the ground glass
>of the camera is preferable to using a ronchi screen ?!?!?!
>
>What has been done to date is Kent Kirkley and I have measured the Nikon
>F and it matches the Nikon Spec. The Focus Aide seems to agree with this
>reading. Conversation with Richard @ STI have been not too encouraging.
>Statements have been made questioning the Newtonian that I have. That it
>is somehow different. How does this compute ?
>
>Short of getting a Hutech (not available until the end of the month -
>maybe), any comments or suggestions ?
>
>I am, at this point thoroughly confused on this subject.
>
>Regards
>
>Bill
>--
>William R. Mattil       | Fred Astaire wasn't so great.
>wrmattil@ix.netcom.com  | Ginger had to do it all backwards
>(972) 399-4106          | and... in high heels.
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