How to work out extension tubes

[PatrickO]

I'm trying to use a ZWO ASI 224mc with a William Optics Megrez 72mm f/6 APO refractor.

I'm not able to focus and think I need an extension tube.

Other than trial and error is there a way to work out what extension tube I need?

Many thanks.

[vlaiv]

You can measure exact focus position with a bit of tracing paper and moon (best thing to do on a full moon evening as not much else can be done).

Move tracing paper back and forth until you get sharp image of the moon on it - it will be small but you should have no difficulty judging where it is sharpest.

 

[powerlord]

First, do it in daylight.

You will then find it pretty straight forward, you'll see the image gradually coming into focus. If you run out of focuser, add some ext tube until you get there. don't try at night with stars, because until you are very close and start to see a large slightly brighter circle, you won't really know - and it gets annoying very quickly.

in daylight - easy peasy - note down the ext tubes used for the ota and the rough focus position and at night, it will be plain sailing!

[vlaiv]

11 minutes ago, powerlord said:

First, do it in daylight.

Problem with doing it in daylight is that you need to do it on a distant target - like at least a mile.

Closer your target - further away moves your focal plane. You want infinity focus - and that is closest position to lens - focusing on anything closer moves focal plane further out.

For something really close - like twice focal length - focus position shifts one full focal length behind the infinity focus (if you try to find focus position for 1m scope on object 2 meters away - you'll miss by whole meter ).

 

[PatrickO]

....

Edited April 5, 2022 by PatrickO

[PatrickO]

11 hours ago, vlaiv said:

You can measure exact focus position with a bit of tracing paper and moon (best thing to do on a full moon evening as not much else can be done).

Move tracing paper back and forth until you get sharp image of the moon on it - it will be small but you should have no difficulty judging where it is sharpest.

 

 

10 hours ago, powerlord said:

First, do it in daylight.

You will then find it pretty straight forward, you'll see the image gradually coming into focus. If you run out of focuser, add some ext tube until you get there. don't try at night with stars, because until you are very close and start to see a large slightly brighter circle, you won't really know - and it gets annoying very quickly.

in daylight - easy peasy - note down the ext tubes used for the ota and the rough focus position and at night, it will be plain sailing!

 

9 hours ago, vlaiv said:

Problem with doing it in daylight is that you need to do it on a distant target - like at least a mile.

Closer your target - further away moves your focal plane. You want infinity focus - and that is closest position to lens - focusing on anything closer moves focal plane further out.

For something really close - like twice focal length - focus position shifts one full focal length behind the infinity focus (if you try to find focus position for 1m scope on object 2 meters away - you'll miss by whole meter ).

 

Thanks guys. I've ordered a 65mm tube. I'll try that on a distant object in daylight.

[powerlord]

10 hours ago, vlaiv said:

Problem with doing it in daylight is that you need to do it on a distant target - like at least a mile.

Closer your target - further away moves your focal plane. You want infinity focus - and that is closest position to lens - focusing on anything closer moves focal plane further out.

For something really close - like twice focal length - focus position shifts one full focal length behind the infinity focus (if you try to find focus position for 1m scope on object 2 meters away - you'll miss by whole meter ).

 

yeh sorry, thought that went without saying. I do it upstairs where I can focus on some trees in the distance. I know I only need a little bit more to get the stars from experience, so I know if I can focus on them I'll be close enough to see stars in the night and focus from there. Once I've done it once for a particular combination of kit, I write it down (e.g. asixxx, 43mm bf, 80ed with focuser out at position 5.4) then I know a start position for the night to save a lot of faffing.

[rickwayne]

It actually does work to just hold the camera up to the focuser and move it back and forth, too. Hard to precisely measure without an assistant, but precision isn't really necessary.

But as Vlaiv says, you'd be surprised at how distant "infinity focus" is for a telescope. My experience with terrestrial photography led me to believe that eh, it's f/4 or so, everything from 100 feet away to infinity will be within the depth of field.

Yeah, no. Not so true at longer focal lengths.

[PatrickO]

Well got a 65mm tube and got focus on a barn about a mile away. Just need the cloud to clear to try out for real. Hoping to do some EEVA.