Focal Lenght - stupid question

[ian_bird]

Hi

I know this is going to sound dumb - but can you use the focal length to measure from the front of the OTA to your CCD chip to get a rough idea of where to start finding your focus?

For example - I have just totally rejigged my setup. I now have a WO 98 FLT, an Riccardi 0.75 flattener and a new Atik 4000. So I don't have the faintest clue where I should start in terms of finding the focus range.

So it's 618mm focal length times 0.75 which makes 463.5mm. Does that mean the chip on the CCD should be roughly 463mm away from the front element on the OTA? And if so - what part of it?

I find this really difficult (the new rig bit). I can spend hours trying out different focus positions.

So could I take a tape measure - and get myself in the ball park?

Cheers

Ian

PS - Thanks for not laughing!

[ronin]

No, the focal plane of the reduced image will be "more or less" the same place, what happens is that the reducer simply produces a smaller image but not a vastly different location, it certainly cannot produce it up inside the tube which 463mm would be. Much the same means 10-20mm, maybe 25mm either way I would expect.

It will be a case of trial and error.

[jambouk]

Have a play in daylight and make a note of how many turns and in what direction you need to make to achieve focus; then write it down as you'll forget

Good luck.

James

[Gina]

I set up in daylight.  I'm fortunate in having the horizon visible to the east - a hill about a mile away.  I set the scope up roughly horizontal and set the focus midway on the focuser.  Then I move the scope up and down by hand while watching the camera image on the laptop.  I adjust exposure to give an ADU of a couple of thousand.  Then I adjust the scope altitude to give a gradient on the image.  From that it's possible to move the focus in and out to sharpen that gradient until finally the horizon with trees and houses comes into focus.  Then I clamp the focus and wait for it to get dark on a clear night.  Having set the focus for around a mile, it isn't that far out for stars at virtually infinity so only a small focus adjustment needed and we know the focuser needs moving inwards slightly.

[ian_bird]

Thanks guys (and girl).

Was worth a punt.

Interestingly TS offer an OTA tube shortening service with the Riccardi. However I think that is due to it's physical length if you can't mount it inside the focuser - which is what I am trying to do.

Thanks again.

Ian