10" GSO dob and prime focus photography

[Grahame]

Has anyone else had problems achieving focus using this kind of setup? the only way I have been able to get focus with my canon camera so far is using a 4X powermate. Im sure there is another way to do this. Does anyone have any ideas on what might work?

Thanks,
Grahame.

[gbeal]

Grahame,
I think if you trawl a bit you find a few threeads on this very subject.
In my case, (although with a Mak/Newt, not a newt, but essentially the same) I got the primary as far as I could forward on the collimation screws, and the secondary as far aft as I could while still keeping it under the focuser, and then got a T ring that fitted into the 2" opening of the focuser. You might like to try this, but before you did, why not establish where the actual focus point is. Try a piece of lunch paper, while aiming the scope at the moon. Work out then if it is possible, bearing in mind at least 46 - 50mm of distance between the lens flange and the chip.
Gary

[peter_4059]

Grahame,

I've tried a Universal Parracor with my 400D and 10" F5 GSO and it comes to focus without moving mirrors. I've heard the Baader MPCC also works and have one on order. I'll let you know when I have it.

Peter

[Grahame]

Gday peter, I actually have one of the MPCC's and had no luck with it. I'll post a shot of it shortly as I had it set up. Hence my amazement on it not working as I though it would :)

Grahame.

[peter_4059]

Interesting - a friend up here has a skywatcher f4.7 10" newt and a 400D and achieves focus no problem with the MPCC. As Gary says you can measure where the focal plane is. I'm interested in how you had it set up.

[Grahame]

Hi Peter, here is a photo of the camera and adapters ready to be mounted in scope. I will also try the focus trick next chance I get too.

[peter_4059]

Thanks for the pics Grahame - that's what I expected. Not sure why this doesn't come to focus. My MPCC is due in about 2 weeks.

[Grahame]

The only thing i can think that would be different with this scope is the dual speed crayford style focuser it has (its the deluxe version from andrews comms) apaart from this i can see no reason why i should not be able to achive focus at prime :( i'll let you know the results with my tests tomorrow night.

[peter_4059]

If the point of focus is within 10mm you can move the primary mirror forward 10 mm on longer colimation bolts/springs quite easily. I did this to achieve focus with the DSI II/filterwheel.

[Grahame]

hm.. I will try this as I believe that it is only just out of focus, hopefully within the limits of moving the primary forward as suggested :)
my car blew a raidatior today; worse still my scope is in the back seat so the weekend will have to be my testing/tweaking time now, thanks for your advice too, i never thought to just move the mirror forward!

Grahame.

[Grahame]

after some mucking about with moving the primary and re-collimating, focus was still not achieved with the above setup. Today I have purchased a low-profile GSO crayford style focuser which should get my camera into the sweet zone at last.
Thanks to all who responded to my call for ideas!

Regards,
Grahame.

[iceman]

Good luck Graheme, I look forward to hearing whether it worked.

[Grahame]

just following on from previous post, I have installed the low profile focuser and re-collimated the main mirror ~10mm up the tube and FINALLY have achieved focus. Using the MPCC the following pics have resulted (all are unprocessed and unguided)

now to fine tune the system :-)

Thanks to all who helped out with ideas too!!

Regards,
Grahame.

[peter_4059]

Glad it worked out Grahame. The initial images are looking good. Look forward to seeing more.

Peter

[PCH]

Hey Grahame,

those images are really good. If you have a spare minute, I'd be really happy to hear some details of your technique/settings etc.

Cheers,

[iceman]

Fantastic work, Grahame! Well done!

Glad to hear it worked without moving the mirror up the tube.

Can you post pics of the focuser?

[JD2439975]

Mike & grahame

I purchased one of these to replace a rack & pinion type but it doesn't come without problems. :sadeyes:

Attached are some pictures of the focuser...

Pic 1 - Just an overview.

Pic 2 - Note the two, stepped machined surfaces, if the locking screw is just clear (loose) of the main surface, focuser travel is 31mm.
Only if the locking screw is very loose to clear the second surface will you get the advertised 38mm of focuser travel.

Pic 3 - Note the spacing between the focuser tube & body, a slight misalignment can be seen bought about by the tightening of the knob tensioning screw...but it gets worse.

Pic 4 - Tightening the locking screw (a little overtightened but it illustrates the point) the misalignment between tube & body becomes extreme, can you imagine what that would do for a flat field across your chip not to mention eyepiece collimation.
Furthermore if the focuser is mounted knobs down (regular configuration) with a heavy eyepiece or camera attached it will sag under it's own weight.
Inverting the focuser (knobs up) solves the weight sag problem but you can't use the locking screw, even a light tighten adds misalignment.

Pic 5 - The only solution I can think of is to insert a teflon strip between the roller bearings as they don't seem capable of doing the job they're meant to do.

Just a few things I feel people should be aware of if they have or are thinking of getting one of these focusers.
As I've learned, precision comes at a price & the price of this one is too low. ;)

If anyone has another bright idea of how to cure this problem please, please, please let me know. :shrug:

JD

[peter_4059]

I had heard of problems with these low profile focusers with mis-alignment. The MPCC delivery has been delayed and I'm now talking myself into a parracor instead since I know that works with my scope as is.

[Grahame]

JD2439975, I agree with your findings, I have had exactly the same probs as yourself. The teflon strip is a good idea to stop the misalignment issue - I'll have to track some down and see how it goes :)

Attached are a few pics of the mounting on my scope - because there appears to be no focuser for a 10" scope the 8" one has to be used, I added some weather proofing tape to fill the gaps because of the different diameter of the focuser and tube.

Grahame.

[peter_4059]

Grahame,

My paracorr arrived last week and I tried it with the 400D briefly on the dob mount - reached focus no problem. Now looking forward to some clear nights to see what it can do.

Peter