Dobsonian + DSLR + TC ?

[Honest_Gaza (Garry)]

Reading various threads on the limitations of using a DSLR on a Dobsonian, it seems that general consensus is that the camera sensor needs to get closer to the Scopes Mirror.

There seems to be 3 main ways of achieving this :

• Moving the Primary Mirror (seems a bit drastic)
• Low profile Eyepiece mount (requires some mods / hole drilling......hopefully I'm correct with this one)
• Add a Barlow lens to magnify the image (simple but not always enough magnification)

It is the last one that has me thinking about a possible solution and not sure if anyone has tried it.


Does the addition of a Tele-Converter on the DSLR get you the "perceived" extra distance that you need for the Camera sensor to focus ?


Therefore...Camera + TC + Adapter + T-Ring + Barlow

[naskies (Dave)]

Originally Posted by Honest_Gaza

[*]Moving the Primary Mirror (seems a bit drastic)

I can get my Skywatcher flex tube Dob to focus simply by not extending the secondary mirror all the way, i.e. about an inch short of the normal visual position.

It's one (of several) reason why I went with a truss instead of solid tube version.

The downside of using a barlow/teleconverter is that the focal length may become too long for convenience. An APS-C (crop) sensor will just fit in the sun/moon at a focal length of ~ 1600 mm... so even with just a 2x extender you probably wouldn't be able to fit them in with a single shot.

[Honest_Gaza (Garry)]

Yes but I think the issue at this stage is one of "focus" not "field of view".

[trek1701 (Mark)]

Check Chris’s helpful comment on this post back in May


http://www.iceinspace.com.au/forum/s...ad.php?t=91048


Cheers Mark

[stephenb (Stephen)]

Garry,

As you've discovered there are three methods of observing which I use on my 10" Dob are visual (i.e. an eyepiece), a DSLR and a CCD camera. Unfortunately each one focuses at a different point on the focuser. Most off-the-shelf newtonians are setup for the eyepieces and, as you're aware, they don't allow enough focus travel to accommodate all three.

However I achieved all three by simply replacing the collimation screws on the primary mirror and inserting some longer springs.

Now my ccd camera (DBK41) focuses about 5mm from the bottom of the focuser travel, the DSLR is around the middle of the focuser travel, and the eyepiece focuses at the extreme top of the focuser travel. Admittedly I need to slide the eyepiece out about 7mm but it works fine.

I never entertained the idea of cutting the tube shorter.

[rmuhlack (Richard)]

i'll second stephenb's suggestion. I modded my 8" dob recently to allow prime focus DSLR imaging simply by replacing the collimation/locking screws and using longer (and stiffer) springs.

For my GSO/Bintel dob the collimation screws use a 6mm thread, so I purchased six 50mm long 6mm screws, and three new compression springs from my local Bunnings. 10 minutes to fit, less than $20 spent, couldn't be easier

DSLR now comes to focus with about 10mm of focus travel. For visual use I now simply use a 35mm extension tube with my eyepieces.

[stephenb (Stephen)]

Richard, I used the exact items from my local Bunnings

Once I removed the 'black knob' collimation screws and replaced them with the 6mm x 50mm long screws, I then used the 'black knob' screws in place of the 'white knob' locking screws.

I should use a 35mm extension but I haven't had the chance to get to BinTel at this stage.

[Honest_Gaza (Garry)]

Originally Posted by trek1701

Check Chris’s helpful comment on this post back in May


http://www.iceinspace.com.au/forum/s...ad.php?t=91048


Cheers Mark

Thanks Mark....will look at this as an option.

Stephen and Richard....at this stage I'll leave the collimations screws "as is"...would hate to bugger it up this early on in the piece....but thanks for the suggestions.

[swannies1983 (Dan)]

Originally Posted by trek1701

Check Chris’s helpful comment on this post back in May


http://www.iceinspace.com.au/forum/s...ad.php?t=91048


Cheers Mark

+1. I replied in that thread stating how I didn't need to make any modification with my Skywatcher Newt. With the dslr attached, I have about 5mm inwards travel of the focuser. Not sure if this means I would or wouldn't be able to use a coma corrector.

[Wookie (Ryan)]

Hey Garry,

I'm in the same boat.
I have a bintel (Gso) 10" and I can not get close to focus with my dslr.
I can achieve focus with a 2x Barlow with about 10mm of inward travel to spare.
I assume its worse for mine since it has a1250mm focal length.
I was considering a low profile focuser but the 50mm x 6mm bolts look like an attractive first path.
I'll keep you posted.

Ryan.

[swannies1983 (Dan)]

Bummer that you couldn't get focus Ryan.

[Honest_Gaza (Garry)]

I'll put this on hold for now....don't want to spend too much money on further accessories until I'm convinced they are going to work.

[Honest_Gaza (Garry)]

OK...taken off hold again.

Purchased the T-Ring and EOS Adapter....DSLR couldn't focus, but no surprises there.

Followed the earlier instructions of 6mm x 50mm screws....re-collimated...but can't focus with the DSLR. Found Jupiter up in the sky but when trying to use the DSLR to focus, all I get is what I believe is one of the mirrors. A ball and a telescopic sight.

When I swing the telescope away from Jupiter, the "telescopic sight" disappears....so it does seem to be light related.

Suggestions ? Have I extended the mirror too far up the 50mm screws....or not enough ?

[stephenb (Stephen)]

By "telescope sight" I assume you mean something that looks like crosshairs with a round obstruction in the centre? That is your secondary mirror the four spider vanes simply way out of focus. Your DSLR is not in focus then. You need to wind the focuser from one end of travel to the other and see if there is a difference.

Think of it this way.... The distance from the surface of the primary mirror, up the tube, bouncing off the secondary diagonal mirror, up the focuser and onto the surface of the CCD camera chip - that distance is what needs to be correct for the image in the camera to be "in focus" Or what we call the Focal Length. This distance will be different for an eyepiece.

So when you wind the focuser all the way out, you are lengthening this distance. Winding it in, you are shortening the distance. It would appear you do not have enough travel in the focuser to find the correct distance for the camera.

If you wind the focuser one way or the other does the "telescope sight" vary in size? Does it get better in one direction? If you wind it out and it gets better ( better means the telescope sight gets smaller and the big blurry light behind it (which is Jupiter out way out of focus) gets smaller, then you need to push the primary mirror back down the tube.

[rmuhlack (Richard)]

I second stephen's assessment - sounds like you were just out of focus

With the 6 x 50mm screws I found that I achieved focus with the DSLR + MPCC when the focuser was (from memory) at about the ~15mm position ie about the middle of the focuser's travel. However I agree with the previous suggestion - rack the focusser all the way from one end of its travel to the other

[Honest_Gaza (Garry)]

Thanks Guys....did further adjustments of the 50mm screws yesterday in anticipation of fine tuning in the evening....and wouldn't you know it, cloudy skies in Sydney for the whole evening.

Maybe tonight, although storms are forecast.

I'm comfortable with the theory of the focuser replacing the normal function of a zoom lens...just not sure what movement is required in the primary mirror in terms of distance. With the mirror sitting at the end of the 50mm screws, this positions it approx 20mm further up the tube (and therefore closer to my camera sensor) than would have been possible with the original collimation screws.

[Honest_Gaza (Garry)]

< mental note to oneself >
When moving the Primary Mirror, and deciding you need to recollimate....make sure the Barlow is removed first.

Otherwise, no amount of adjustments will get the laser to provide a pin-sharp dot