where is focal length measured from?

[galileo]

Hi there,

I'm building an Istar 6" f12 refractor and getting to the 'cutting the tube to length' stage having made a collimation cell and a focusser adaptor.

In order to cut it approx. correct (preferably not too short) before I make my final cut I need to know where the 1800mm is measured from, is it the front of the lens or the back or the middle?

Thank you

[boomboom]

I used the middle. I also made sure that I could use binoviewers at native magnification too….only took 3 cuts to get it right!

Matt

[galileo]

Thanks Matt,
Not quite ready to cut yet, I want to find a position that will allow me to get enough back focus to allow me to put a dslr on it too.

Should have an initial assembly over easter hopefully.

Going to cover the tube and dew shield with carbon fibre sheet, it's relatively inexpensive and is a great alternative to painting which will chip and flake off. It will look the business as well and nobody will know it weighs half a ton

It's very psychological the aesthetics of a scope isn't it, get someone to look through a drainage tube then thro a fancy carbon fibre one with the same optics, bet they swear the carbon fibre is the better one. 

[boomboom]

I wrote on my full scale ray trace tube lenght 1530mm...and think I took another 100mm off that.

Good luck with the vinyl wrap. I managed an 8x50 finder everything else ended in....ummm screwed up piles of vinyl! I couldn't do it and had to resort paint.

Matt

[Ales - iStar Optical]

Hi guys,

when ever I design a new iStar refractor I allow 275mm to 285mm from back plate to focus. If you want to use big focuser or use the scope for some imaging, you can go with approximately 300mm distance. I have attached drawing of our latest version of Phoenix WXT 204-6LT to show you how this is done on original iStar refractors. Please remember that focus is not measured from center of last lens surface (surface 4 in case of doublet) but from center of the doublet.
I hope this info helped a little,

best regards,

Ales

Attachments:

00 Phoenix WXT 204-6 Assembly.pdf (19.72 KB)

[boomboom]

Thanks Ales,

Makes sense as I ended up with 300mm as my final focus point from the backplate, which reaches focus perfectly with binoviewers.

The only thing I had trouble with was the illuminated field size…? I just guessed at 37mm after reading the few vague ideas on the subject. The IFS seemed to be based on the field stop of your largest eyepiece? ES 40mm 68 in my case and 37-38 seemed to be the best of all worlds.
Can you illuminate (pun intended) what you think on this somewhat mystical subject?

I ask as I'm planning on my next Istar ATM scope…an 8" f something or other on an ATM 2" pillow Block mount. I'm collecting the bits for as I write….2" pillow blocks arrived today

Matt

[galileo]

Thank you for your feedback guys, can approach the cut this weekend with a bit more confidence now, will post pics on the build thread.