Focus reducer second attempt

[Dennis G]

:help: I previously submitted a request for help to sortout a problem with a focal reducer. I wrote in that submission that a single lense similar to a filter arrived with a note to say the extension would be sent when in stock. The lens screwed to the Toucam adaptor will not come to fucus when racked fully in. I want to capture the whole moon.
I have the ability to machine odd bits, and while waiting for the extension and hoping it would solve the problem of focus, I made a series of threaded tubes to accept the lens and screw to the Toucam. I even made a new shorter 1.25mm adaptor for the focuser; gaining 8 mm reduction.
My findings were most disappointing. The shortest tube, just sufficient metal to scew the lens to the Toucam reduced the object slightly, (a power pole insulator) at about 100 m and would focus with about 3 mm of focal tube to spare. Longer tubes reduced the same object considerably with similar focal results, but again anything further away would not come to focus. I gained nothing,the tube length dosn't change the focal length, which leaves me thinking that I must either shorten my truss tubes or reduce the focuser base. I'd be willing to do iether IF I KNEW JUST HOW MUCH. I don't wish to mess up the original which handles my eyepieces fron 25mm to 6 mm.
Can some kind member provide an answer?
Dennis G:help:

[[1ponders]]

Ok here's the problem as I understand it Dennis.

If you buy a focal reducer, say a 0.63X, and screw it into your standard ToUcam adapter then that is approximately what you will get. But because it is the distance from the reducer to the chip that determines the reduction different 1.25" adapters give different reductions because they are varying lengths. Now when you add an extension to the adapter you "increase" the reduction from 0.63 to 0.5 for example, by moving the reducer further from the chip.

Now here is the bummer bit.

The usual result of adding the reducer to your webcam is you loose infocus. ie the focuser has to be racked in to achieve focus, and depending on how long your adapter is will depend on how far it has to be racked in. A 0.63 may need 30mm or more of intravel to come to focus using a "normal" adapter. Now the even bummier bit is if you add any sort of extension to that (say 30 mm) you will need even more in travel, up to 80mm or more. Which is pretty difficult with a Newtonian unless you move your mirror up (?) the tube.

Now, when you machine yourself a nice short adapter you are moving the focal reducer closer to the chip which will be "reducing" the reducer effect (see first paragraph). So how do you get around this? I'm sorry but I really can't help you there, all I know is that with 3" of intravel with one of my refractors I still cannot get enough infocus when using an extension with some of my "webcams". Even the SCT needs to be really cranked in.

Sorry if that is bad news, but at least you know what is happening now.

[[1ponders]]

BTW if you want to fully capture the moon with a ToUcam you will have a bit of a challenge. What scope are you using? There is an adapter that might be able to do it here (http://www.telescopes-astronomy.com.au/toucam_philips.htm). Check down near the bottom of the page for the "40mm plossl with T thread" It may solve your problem, but I have never used one so I don't know what it's infocus is like.

The only other easy way I can think of getting the full moon onto the chip is to buy a different camera with a larger chip (1/3" or 1/2' inch ).