Focusing... major help needed

[EzyStyles]

Hi Guys,

After waiting for a clear night, i decided to give the 350D a shot in the scope. I can't get inner focus helppp. Is there anyway besides cutting up the scope to get inner focus? On the 8" F/4 Newt scope using a 2" prime focus adapter with the t-mount.

Thanks.

[Lee]

I believe you will need to get the sensor of the camera and the primary mirror closer together - lower profile focuser, or move the mirror up the tube.... I am actually planning to chop up a 6 or 8" f/4 to image with before long!
I think some Barlows can help get focus?? Not sure though......
Not much help.... sorry....

[janoskiss]

Get extra long collimation screws or chop an inch or two off the tube. ;)

[Lee]

the long collimation screws sounds like a cool idea....

I asked a similar question last year.... link in case it helps.....

http://www.iceinspace.com.au/forum/showthread.php?t=3916

[janoskiss]

I only mentioned them becayse saw a set a while ago being offered on ebay with pair of binoviewers (by IIS member but cannot remember who).

[EzyStyles]

cheers guys thanks. I don't want to cut up my scope . I'll try the long collimation screw method. where do i get them from?

[bojan]

It is not a good idea to move primary closer, you may have vignetting due to secondary being too small ....
Low profile focuser is a way to go.

[janoskiss]

Good point, bojan.

[Lee]

There is a market out there for imaging Newt's out of the box! I'll buy one.....

Until then - Is there a focuser around "low enough" to bring the average DSLR to focus in a newt???

[bojan]

This may be a challenge... :-( All depends on how much closer you have to go with the camera.
There are some Crayford design which are quite low profile, but even that may not be enough in some cases.

[ving]

if you dont want to chop your tube you could get a new tube rolled... i wouldnt think it would cost that much. :)

[Lee]

I think from memory my D70 *just* misses inwards focus in my Dob.... will have to quantify it and go from there.....

[EzyStyles]

thanks for all your suggestion and help guys.

I was playing around with the t-mount and prime focus adapter. There is a small ring inside the t-mount holded by 3 small screws which can be taken out. I unscrew the 2 inch barrel from the prime focus adapter and replaced it with the ring inside the t-mount. Its abit shorter now hope it helps. might also pop into bunnings for some longer collimation screws. don't really want to replace my crayford 10:1 focuser as it is quite new still.

[ving]

bah! i still like my sugestion... get a new tube the correct length and bung all your gear on that. it'd be really easy (as long as kyou can find someong to make you a tube, maybe some aluminium manufacturer).

hope your plan (above) works tho. the best idea is always teh one that costs nothing :D

[EzyStyles]

i do like a new tub roll idea. but making it will be quite hard though. eg: cut focuser hole, screw holes, spider holes etc etc. + painting inside black. my way is cheaper :P just get longer screws.

[bojan]

Longer screws are very much OK for the test.
Then you will see if the vignetting is acceptable or not when you have the focus where you need it.
Once you know that, it will be easy to decide what to do.... Keep the longer screws where they are then (this is the same thing as the shorter tube btw) or you need to go for bigger secondary or lower profile focuser.

[EzyStyles]

thanks bojan. having a bigger secondary will that affect the quality of the image?

[Starkler]

Yep low profile focuser would be the ideal.

Bringing the focal plane way outside the ota can do what bojan said.
Do yourself a favor and download the program "newt" which can do all the calculations for you of how big your secondary needs to be to fully illuminate the ccd chip.

[bojan]

Slightly , but not even close as much as vignetting.
You can say that the major contributor to the image quality is the outer peripherial parts of the mirror, not the middle part:-)

[bojan]

Eric, there is another vignetting issue... you may be OK at the middle of your 350D sensor, but not at the edges...
another argument towards larger secondary (while having low profile focuser as well).
Newt software (http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm (http://home.att.net/%7Edale.keller/atm/newtonians/newtsoft/newtsoft.htm)) proposed by Geoff may help a lot here. It certainly helped me :-)

[EzyStyles]

cheers . downloaded the program. what a great piece of software.