Using 30D for short exposure

[that_guy (Tony)]

Hello, I'm wondering if Canon 30D is any good for short exposure on a dobsonian. Like for omega Centaurus.

Thanks,
Tony

[that_guy (Tony)]

Okay, I've got a reply from nettie through facebook and said that it would be impossible to get focus. Is it possible to move the mirror up with out surgically cutting it to bits??

[bmitchell82 (Brendan)]

? what telescope are you using first.!? thats not absolutely true in all cases!

[that_guy (Tony)]

hi brendan, its a dob and the ota is pretty long ...

[Bolts_Tweed (Mark)]

Gday Tony - You've got the right advice - first check that you cant unscrew a section of focuser tube to get your camera closer to the telescope.

Moving the primary howevr isnt usually a big deal - depends on the primary mount - usually it is on held in by 3 botls. If you are a handy kind of person it is just a matter of drilling 3 more holes about say 30 mm (guessing cause I dont know how far from focus you are) further up thetube (120 degrees apart usually but just match the location of the original holes) and rebolting it in. Note you may howeber need a focuser tube extension to reach eyepiece focus after you modify the scope but they are cheap from any of the duppliers like Lex at Mermaid or Ron at Underwood.

It will probably require recollimation but thats a whole other question and there is reams of info on this forum for that.

Mark

[bmitchell82 (Brendan)]

hey mate, the reason why i was asking what telescope it was is because without any modifications i can reach focus in my SW 10" dobsonian with a Canon 40d, GSO types of dobs will not reach focus without some type of modification, Saxon is the same as Skywatcher ect ect. so you see not all apples are the same so. What brand of telescope do you have

[jjjnettie (Jeanette)]

with my sw200, focus could be achieved with RobF's 350d but NOT with my 20D, go figure.

[cybereye (Mario)]

I can reach focus with my Pentax Km attached to my Skywatcher 8" newt (was a Dob (model number sw680)). I do this by unscrewing the eyepiece holder and screwing on my T adapter to the thread which is on the eyepiece holder holder...("holder holder" - does this make sense?!!!)

[Omaroo (Chris Malikoff)]

Tony - it's a Newtonian telescope, not a Dobsonian The "dob" bit is in reference to the mount, not the optics.

"Pretty long" doesn't really describe too well what you have - is it 6" (150mm) or an 8" (200mm) mirror? There might be a sticker on the OTA near the focuser that describes this - along with the focal ration (f/5 or f/6 probably).

I've had success with my old 200mm newtonian reaching focus with DSLR's (where they wouldn't before hand) by swapping the typical rack and pinion focuser that comes standard with Chinese scopes for a low-profile crayford model from Bintel. Most times this achieves the same result as moving your mirror up the tube - without the hassle. With the shorter focuser body you'll have enough in-travel to reach focus with your camera. The base shoe of the focuser can be changed for one that matches your tube diameter so that it sits nice and flush on the tube.

When you want to use the scope for visual, you'll need to also buy a 35mm (or so) extension tube to bring the eyepiece back out to where the focal point is.

[bmitchell82 (Brendan)]

very interesting with your camera JJJ, the 40d definately hits the mark with the 10" try finding the metal back distance for the 20d and the 350d and that will tell you if there is any variation, other wise there might be something else at hand here.!

[jjjnettie (Jeanette)]

Oh, I ended up cutting down the tube on that one Brendan, then sold the scope on. It was too long for me to handle safely at the time.
I have a shiny new RC8 now.

[that_guy (Tony)]

sorry for the lack of info, also when i meant a dob, i really meant to say was that the newt ota is longer than Alt az or GEM newts...
Also its an 8" Skywatcher Dobsonian...

[bmitchell82 (Brendan)]

then you should be able to get focus on it, unless the metal back (distance from the front of the camera to the sensor) is more than 10mm deeper than the 40d which i doubt.

grab the 1 1/4 nose piece it splits into two pieces
the remaining thread that you see on the outside of the collar that you put into the focuser and tighten down with the thumb screws, screw that piece into your Tthread to canon adaptor. with this configuration you will get focus. if you find yourself missing by a few mm, screw the collimation thumb screws out a bit, and re collimate they have about 20mm in them.

also you cant check if it will focus during the day because the objects are not at infinity.

Keep on asking questions and just make sure to add every single piece of information, we can then give you a answer quicker!

[that_guy (Tony)]

so no need to modify? also I need a t-thread and a camera adapter for prime focus??

[bmitchell82 (Brendan)]

shouldn't need to no, but its easy to see if you can reach focus. follow what i wrote down there, chuck the camera on your telescope on the dob base point it at a star and take a 1 second shot.
youll see straight away if your good to go

[that_guy (Tony)]

Went to talk Ray Suckling... Very helpful talk, decided to go to sas meeting and borrow and possibly borrow a barlow to see if it goes into focus like that.

[that_guy (Tony)]

Hi, how does eyepiece projection work? Is it good for planetary and lunar??