SCT focus

[matt]

Just a quick question to remove any doubt.

With a newt it's pretty easy to see which way the focuser is moving and therefore gauge what is inside and outside focus.

But, assuming you have a star sharply in focus in the eyepiece of an SCT, which way do you turn the stock focuser knob to take the image inside and outside focus?

It was my understanding a clockwise rotation achieves inside focus, and vice-versa. Is this right?

Thanks guys

[casstony]

You could work this out by withrawing the eyepiece a little and refocusing: the way you turn the knob to refocus would be towards outside focus.

Tony

[matt]

So it's not as straight-forward as saying defocusing in an anti-clockwise position is either outside or inside focus???

[xstream (John)]

That's what I would of thought Matt.

• Clockwise shifts mirror up (inside focus)
• Anti-clockwise shifts mirror back (outside focus)

At least that is the way it is on a Meade.

[matt]

Thanks John

Easy

[xstream (John)]

Sorry to give you a bum steer Matt. It's the opposite way round.
I stand corrected.

[matt]

Ohhhhhhhhhhh-K...

Hmmmmmmmmm

[Dennis]

If I remember correctly, moving the primary mirror up the tube towards the secondary (decreasing separation) makes the 'scope focus at infinity.

Moving the primary down the tube away from the secondary (increasing separation) makes it possible to focus on terrestrial objects.

Which way the focus knob turns to achieve this, I dunno. Will check when I next have the ‘scope out.

Cheers

Dennis

[RAJAH235]

T'all depends on whether it's right or left handed thread..... L.
ps. Tony's idea is good.

[Dennis]

And from the C9.25 Manual, we have a diagram of the focuser as shown below. I no longer have this knob as I replaced mine with a Hutech mirror lock assembly.

So, it appears that:

Turn focus knob CCW to move primary closer to secondary.
Turn focus knob CW to increase primary / secondary separation.

Does that make sense?

Cheers

Dennis

Edit: Reading the manual further (a novelty for me!) we have:

"Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object".

[matt]

Cheers Dennis

Sorry to boil it down to its bare bones but I interpret that to mean CCW for outside focus ... CW for inside focus???

That sound about right?

Cheers

[Dennis]

Hmm, let me think of this one…..ain't no expert so I'm thinking on my feet..

• Inside focus suggests that the light rays had not yet come to a focus (still converging).
• Outside focus would suggest the light rays had already been focused and were now past the focal plane (now diverging).

So, based on the above:

• Turn focus knob CCW to move primary closer to secondary and continue past the point of sharpest focus = Inside focus.
• Turn focus knob CW to increase primary / secondary separation and continue past the point of sharpest focus = Outside focus.

Therefore, from the point of sharpest focus, I make it:

• Turn focus knob CCW for Inside focus.
• Turn focus knob CW for Outside focus.

Anyhow, that’s how I interpret the meanings.

Cheers

Dennis

[Rigel003 (Graeme)]

Dennis is correct and the manual is clear. CCW moves the mirror forward and takes you inside focus. CW moves the mirror back and takes you outside focus.

[matt]

Thanks guys

Sorted

[Rigel003 (Graeme)]

As an aside, many sources recommend winding the mirror back as far as it will go (with many clockwise turns of the focus knob - taking it way out of focus) before moving SCTs any large distance e.g. car trips to country sites. It seats the mirror against the back of the tube assembly, keeps it more stable and puts less strain on the primary baffle in the case of any severe knocks.

[matt]

Thanks Graeme

That's a good tip to keep in mind when hauling the scope out to a nice dark site