Focus issues (again).

[Daz Type-R]

Is there any mathematical way in which I can work out what combination of riser plates / extension tubes I need so I can use my ep and power mate without having to keep swapping things around?

For example while trying to use my TeleVue 13mm Ethos and my TeleVue 2x PowerMate....

For clarification, I'm using inward and outward terminology, so inward = focus tube going into the OTA, outward = focus tube withdrawing from the OTA.

13mm on its own, not enough outward focus.

13mm with 50mm extension tube, perfect, plenty of outward, ok amount of inward focus left.

13mm with 2x PM and 50mm extension, not enough inward focus.

13mm with 2x PM, nearly at full outward focus but just focuses.

On the Moonlite, the shoe to fit the OTA and the 1" adapter. Will adding the other 1/2" adapter and removing the 50mm extension work?

[tuckstar]

Sell the power mate and buy a 7mm eyepiece ;-)

[Doc]

I agree with that.

 

I never use barlows or powermates, I find they degrade the view, you are putting a lot more glass in the optical path.

[Tweedledee]

Just wondered if one of these would help. It is a variable extension tube combined with a fine helical focuser, I have one and it is fine bit of kit which I may need on my dob. I'm sure there are others available with different ranges...

http://www.teleskop-express.de/shop/product_info.php/info/p4340_TS-2--Rotating-Helical-Focuser-with-M48-Connection-AND-variable-extender.html

[Tweedledee]

I'm sure there is a mathematical way to work it all out, but I wouldn't like to grapple with the formulae

[+Graham]

Change the 50mm extension for a 25mm extension. 

Focus for both should then be achievable. 

[+Bino-viewer]

I agree with that.

 

I never use barlows or powermates, I find they degrade the view, you are putting a lot more glass in the optical path.

Not true with a Powermate. Barlows OTOH....

 

Is there any mathematical way in which I can work out what combination of riser plates / extension tubes I need so I can use my ep and power mate without having to keep swapping things around?

For example while trying to use my TeleVue 13mm Ethos and my TeleVue 2x PowerMate....

For clarification, I'm using inward and outward terminology, so inward = focus tube going into the OTA, outward = focus tube withdrawing from the OTA.

13mm on its own, not enough outward focus.

13mm with 50mm extension tube, perfect, plenty of outward, ok amount of inward focus left.

13mm with 2x PM and 50mm extension, not enough inward focus.

13mm with 2x PM, nearly at full outward focus but just focuses.

On the Moonlite, the shoe to fit the OTA and the 1" adapter. Will adding the other 1/2" adapter and removing the 50mm extension work?

Case or trial and error really to see what works best for you.

If you can just focus with the 13E & 2x Pm, then surely you're in the ballpark aren't you ?

Can you reach focus with the 13E on its own if you unseat it a few mm ?

I know thats not a good idea with a Dob, but if you're really close, you could maybe put a shim under the focuser to raise it a wee bit ?

 

(I have the same issue with my 2" 31mm wide-angle - i'm about 3 or 4mm short of out focus, and thats using a 4.5 inch draw tube !  

It means i have to use a 2" extender on the draw tube. But then, when i put the bino on, i run out of in focus, so have to remove the 2' extender.

I seem to be constantly screwing and unscrewing various bits of gear all night. Just the way it is....but the end views are worth it   )