Difficulty with prime focus

chrisc (Chris) · #1 26-04-2010, 02:53 PM

Hi everyone,

I recently bought a new Canon 450D SLR, and the appropriate T-ring and adapter to attach it.

At the moment the only thing I have to attach it _to_ is a little Celestron Astromaster 130mm newtonian with the world's most wobbly focuser assembly.

I assembled the various bits onto the camera for the first time last night, and wandered out into the front yard with the intent of grabbing a couple of shots of the moon to get the hang of it, and learn a bit more about which exposure/ISO settings, etc. were better/worse.

Unfortunately, I could not get a decent image in prime focus configuration no matter what I did. I'm wondering if the focuser travel on my scope is insufficient, or if I've done something obviously stupid along the way? :-)

The adapter is a Prostar variable length job, allegedly suited to both eyepiece projection and prime focus work. Eyepiece projection _did_ work, but I'd like to be get things working properly at prime focus.

Any suggestions or "you connected it up _that_ way?!?!" comments gratefully appreciated. :-)

Cheers,
Chris

dannat (Daniel) · #2 26-04-2010, 06:12 PM

chris- easiest if you take a pic & post it..however most newtonians do not have enough in-focus to achieve focus with an slr

however i think the astromaster EQ may have a piece of the focuser whivh screws off to reveal a t-thread, you just screw the t-ring onto it & attach to your slr

chrisc (Chris) · #3 26-04-2010, 07:13 PM

Hi,

I've just been outside fiddling with the telescope, camera & adapter... the problem is definitely that the camera's sensor just cannot get close enough. Seems like it only needs another few mm's of travel and it'd be okay! Bugger.

<<< pauses briefly to go look at the focuser more closely... >>>

And yep, you're right. The end of the focuser does, indeed, just screw right off! Hadn't occurred to me to try and take it apart before.

I'll go set up the telescope again and have a crack at prime focus with the end-bit removed and see how it goes. If it works I'll post the result in a little while.

Cheers,
Chris

chrisc (Chris) · #4 26-04-2010, 07:24 PM

But alas, no...

The diameter of the screw thread on the barrel of the focuser does not match any part of the T-ring or the various bits of the camera adapter that attach to it.

Chris

mswhin63 (Malcolm) · #5 26-04-2010, 08:15 PM

That particular scope does not have a Crayford 2" focuser so there is no threaded focuser like the 12"DOB which I have. You would need to consider replacing the focuser with a low profile focuser. Another option is a low powered Barlow. I would see if you can borrow one and test before buying.

mswhin63 (Malcolm) · #6 26-04-2010, 08:20 PM

Below is a post with pictures of a SW 2" Crayford with the removable 1.25 extension.

http://www.iceinspace.com.au/forum/s...1&postcount=55

chrisc (Chris) · #7 26-04-2010, 08:50 PM

Thanks for that... it has helped quite a bit, as my next worry was that I would have the same problem on my new scope (which will likely be a short fast Skywatcher 200mm newtonian with a standard 2/1.25" Crayford focuser).

Cheers,
Chris

adman (Adam) · #8 26-04-2010, 09:20 PM

Chris - a reasonably easy solution is to get some longer collimation bolts and springs to move the primary up the tube by the amount you need. I did this to my 200mm skywatcher reflector - no probs. Bunnings sells a fair selection of compression springs - but some of them you may need to cut to size.

Just be aware that if you do this, you will then have problems getting focus with your eyepieces - but this is easily fixed with an extension tube.

It sounds though that your focuser may need upgrading anyway - in which case just get a decent low profile one, and you won't need to move the primary....

Adam

chrisc (Chris) · #9 26-04-2010, 10:44 PM

I'd love to replace the focuser, however on these low-end Celestron telescopes, they've chosen to use a very non-standard mounting plate. :-(

Otherwise I would have chucked the supplied focuser and put a nice normal Crayford on it ages ago. :-)

Cheers,
Chris

mswhin63 (Malcolm) · #10 27-04-2010, 09:33 AM

Quote:

Originally Posted by chrisc

Thanks for that... it has helped quite a bit, as my next worry was that I would have the same problem on my new scope (which will likely be a short fast Skywatcher 200mm newtonian with a standard 2/1.25" Crayford focuser).

Cheers,
Chris

Hi Chris,

If you are going to buy SW again then that would be fine but in all reality it maybe better to purchace a scope that is design to install a camera inside.

Modification to a scope is also an idea as long as the mirror design is suitable. A simple change of focuser on you current scope but warranty would be the only concern.

I think when I purchaced my 12" DOB I was extremely lucky that the designs to alter are there but I still have minor problem of only a small amount of focus travel left under 5mm, Although being a collpsable DOB I can lower the extension a fraction if it a real problem.

Focuser replacement is my next approach, and will be the first modifacation to the scope too.

alistairsam · #11 02-05-2010, 01:04 AM

Hi Chris, Adam,

Do you'll have any pics taken at prime focus with your reflectors, have you'll added tracking for longer exposures?
I'm close to converting my dob to a fork mount, got the stepper control and drive sorted, will look for a low profile focusser as you mentioned to avoid moving the primary.
but thought it'd be good to see some pics taken with reflectors, especially long exposures.

chrisc (Chris) · #12 02-05-2010, 12:42 PM

Hi Alistair,

I can't physically get the CCD of my DSLR into the prime focus plane of my Newtonian, because the focuser doesn't have enough in-travel. Eyepiece projection works tolerably well, but on non-driven mount, mediocre snapshots of the moon are about all I've been capable of.

I'm in the process of upgrading to a new scope and mount (have the mount, and awaiting the shipping of my new OTA) and hopefully will post some photos shortly after everything is up and running (which I'm sure will be woeful, but with any luck, steadily improve) though they will be from an SCT rather than a Newtonian.

Cheers,
Chris

mswhin63 (Malcolm) · #13 02-05-2010, 02:16 PM

Quote:

Originally Posted by chrisc

Hi Alistair,

I can't physically get the CCD of my DSLR into the prime focus plane of my Newtonian, because the focuser doesn't have enough in-travel. Eyepiece projection works tolerably well, but on non-driven mount, mediocre snapshots of the moon are about all I've been capable of.

Hi Chris,

How much focus travel do you think you need to get it right?

adman (Adam) · #14 02-05-2010, 08:42 PM

Honestly, moving the primary is easier than upgrading the focuser!

just get some longer collimation bolts, and maybe some stiffer springs - both from bunnings, and you are away in about 20 minutes....

Adam

chrisc (Chris) · #15 02-05-2010, 10:08 PM

Best guess is that it's only a few mm away.

I may indeed go down the path of moving the mirror and see how it goes. A little reluctant to tinker with my poor little scope, though. :-)

Chris

mswhin63 (Malcolm) · #16 02-05-2010, 10:56 PM

Quote:

Originally Posted by chrisc

Best guess is that it's only a few mm away.

I may indeed go down the path of moving the mirror and see how it goes. A little reluctant to tinker with my poor little scope, though. :-)

Chris

I would try moving up the primary mirror might acheive the results you need.