Article: Modifying a Newtonian Tube for DSLR Photography

[Ian Robinson]

Originally Posted by glenluceskies

Cut the tube, cut the tube, cut the tube!!!

I agree with Ian, Brendan and everyone else from now until the end of time who will give the same advice I've been through this and you will not regret it.

Forget the focuser method, just cut the tube using the terrific article by Alchemy.

And then go and buy an extention tube to bring your eyepiecees into focus.

And there is absolutely nothing wrong with using silicone to adhere the secondary mirror to that GSO holder. I did this for my first 6" newtonian I build 25 years ago, and seriously, it never moved 1 millimetre in 15 years!

I wouldn't bother shortening the tube at all , really, an extra couple of inches below the bottom of mirror cell will be more beneficial than going to the trouble of getting a good square and smooth cut when shortening the tube ( a pain in the butt , been there done that) AND it'll help reduce turbulence in the tube around the main mirror (air flow in the gap between the mirror and the tube past the mirror will have more chance of becoming laminar) and it'll help cut stray light from entering the tube from the bottom (helping with contrast).

[bmitchell82 (Brendan)]

Ian,

If stray light is a problem. make a skirt to go over it or put a shower cap over the end like most people do in any case. I think people get to wizzed up about "contrast" ect ect if you can show me the difference in contrast and quantify it scientifically and show that it makes a difference with amature DSLRs and low end equipment then ill eat my boots.

Simple fact is that contrast is is harmed more by diffraction than by maybe a little bit of light that may reach the focuser though the rear of the tube which is highly unlikely due to the angles it needs to make to reach the focuser from the rear, which doesn't change in any fact because your only removing 15-40mm then replacing the tube end. Even when removing this amount, it doesn't matter if its not truly square as all the other adjustments will take care of it.

My 10" SW dob that is being converted to a Astrograph the mirror is visible from the rear with a open set up, tube currents are virtually non existent once everything is cooled to ambient. and the resolution and contrast is fine.

To be honest, if i was to be putting a CCD + Filter wheel + OAG + a good focuser your going to need more than a low profile focuser and a few long bolts! For example my FT 3" focuser stands 110mm from the tube to the top which is close to 40mm taller than the standard SW focuser. Now if i was going to run previous stated equipment I'd need approximately 70mm back focus. So your bolts will have to be close to 100mm up the tube... that is silly and asking for issues.

If your going to do it properly do it straight up don't stuff around with bolts that can expand changing focus and collimation or shift. Its fine for a quick fix until everything else arrives but not in the long run.

Fast newts are extremely sensitive to collimation errors with coma correctors inline. When using them for photography you need to be very accurate with your collimation or else you loose resolution and light.

This i can quantify and show you the differences made. if you wish.

[Cosmic (Daniel)]

Here we go, gave this a bash last night everything went well thanks heaps Mike for your article. Having that as a reference was great!

The only problem now is that I now have 7.4 inch effective light collecting scope haha. Ill get to changing the secondary mirror at some stage.... that should be ok for now right? or will I end up with imaginary problems because the 2ndry effectively not big enough?

Doing this was great end up cleaning the primary mirror as well and got to know the scope dynamics better than before. Anyone going down this path don't be scared, and take your time. Its all worth it to get that DSLR in focus

Dan

[alistairsam]

Originally Posted by bmitchell82

Ian,
Fast newts are extremely sensitive to collimation errors with coma correctors inline. When using them for photography you need to be very accurate with your collimation or else you loose resolution and light.

This i can quantify and show you the differences made. if you wish.

Hi Brendan,

I've read your posts and agree with not going with long bolts, expansion or contraction would lead to significant misalignment.

But what if there was a balance between moving the mirror slightly up, using a proper low profile focuser and not increasing the secondary too much?
if you're imaging DSO's, you'd want to minimize secondary obstruction to get as much light as can be obtained.
so as long as your secondary covers the light cone by one or two mm, you would not lose on effective aperture.

i've been thinking of fabricating a very low profile focuser, around 20mm in height at most and without a tube that extends inward. at the same time strong and stiff enough to support a dslr. not using traditional designs, but slightly different engineering principles.

if i were to manage this, then in theory, i should be able to achieve focus by keeping my secondary as is, and moving my mirror up very slightly, and keep central obstruction to below 20%.

below are two comparisons for an 8" F4 comparing two secondaries with the program NEWT.

63mm secondary, 50mm focuser
-vignetting of 75% ray at front aperture - yes
-obstruction by diagonal - 32%
-angular field of view for 100% illuminated area - 1.527 degree

and for

50mm secondary, 32mm low profile focuser
-vignetting of 75% ray at front aperture - none
-obstruction by diagonal - 25%
-angular field of view for 100% illuminated area - 0.7074 degree

could you comment on the effect of field of view, and how significant the 7% reduction in obsruction would be for DSO's?

just trying to understand the concepts.

thanks