Article: Modifying a Newtonian Tube for DSLR Photography

[iceman]

Hi all

Alchemy has kindly written an article on Modifying a Newtonian Tube for DSLR Photography.

You can read the article on the IceInSpace Projects & Articles (http://www.iceinspace.com.au/?projects) page, or directly by clicking on the link below:

Modifying a Newtonian Tube for DSLR Photography (http://www.iceinspace.com.au/index.php?id=63,441,0,0,1,0)

Thanks to Alchemy for writing the article. If you'd like to submit a review or article to IceInSpace, please Contact Me (http://www.iceinspace.com.au/?contact).

[iceman]

Article uploaded.

[erick]

Slow down!! You are uploading them faster than I can read them all!! :D

[Gargoyle_Steve]

I think it's all the cloudy weather Eric, folks can't do any observing / imaging so their creative minds have found other outlets. All the better for those of us looking for solutions to problems that others have already solved!

[Tilt]

Seems fairly straight forward.

I would really like to image through my GSO 10" with a DSLR, anyone else modified their Newt?

Michael

[AdrianF]

Instead of cutting the tube could you maybe make up some gadget to move the primary up the tube? Maybe longer screws/springs?
That way if its decided to sell the scope later it would be an easy reversal.
Anyone tried this?
Over this weekend I will have a look to see if possible.

Adrian

[adman]

Yep - works well - longer bolts, but you will also need stiffer springs. Bunnings has an assortment of compression springs, you will probably have to cut them to size and file the ends flat.

Adam

[Benno85]

Ok, sliiiightly off topic, but....

I'm looking at using a low profile focuser as opposed to cutting the tube. The focuser I'm looking at is 32mm racked in height with a draw tube travel of 38mm, giving a total of 70mm maximum "out" distance.

The article says to subtract distance A from distance B to arrive at the "sweet spot distance" which I'll call distance X. Given the 70mm total distance a low profile focuser will allow me, if distance X on my setup falls within 70mm then am I right in my assumption that I should be able to reach focus without having to cut the tube?

My apologies if that was a bit long winded and complex, believe me I had a brain burnout whilst writing this reply!!

Cheers,

Ben.

[JD2439975]

Not sure on the maths Ben but if you're thinking of a GSO low profile focuser I would NOT recommend it.

I had one, it was rubbish for more than a light eyepiece & certainly can't support the weight of a DSLR, flexure would be your bane.

On my GSO 10" it turned out I had to cut the tube anyway...live & learn.

Justin.

[Benno85]

Thanks for thoughts Justin, this isn't the first time I've heard of the GSO focusers not handling a DSLR too well. A Moonlight or Feathertouch is out of the question for a while so I guess I'll bite the bullet and cut the tube. Did you change your secondary mirror to accommodate for the change in the light cone?

Cheers,

Ben.

[JD2439975]

No, no need to, I still had full mirror clips showing & beyond.

[bmitchell82]

hey fellas, your already asking the questions that i have answered and shown to work.

Justin just because you can "see the mirror" doesn't mean anything really, you are loosing a lot of your light cone with the standard Minor axis secondary being sub 60mm in a 10" your losing out before you start.

My flats show the big difference and I know that i need to go to the 88mm MA secondary if i want to fully illuminate my 40d sensor minus the vignetting due to the MPCC. For example a VC200L has a 46% Central obstruction where as the SW Dob has a 26% central obstruction, difference is one is made for photography and has a fully illuminated and corrected where as the dob is tuned for visual.

Most of the factory large mirror newts are generally not designed for photography and it shows in the GSOs with them not coming to focus with DSLRS let alone CCD's with Filter wheels and Active optics.

http://www.iceinspace.com.au/forum/showthread.php?t=59178

I have done a lot of upgrades now to the 10" with the only things left in it that are Sky watcher being the Tube, Mirror, dob mountings, quick release foot and the finder scope itself. Bugger all really.

If you want to do it do it right straight up don't fiddle around with long primary bolts. if they where good then you would see them on places like Gemini and Mt Palomar ect ect. they introduce flexure and a whole heap of other nasties unless you go stupid and put massive bolts in there ect ect. thats just my 2 cents worth.:thumbsup:

[JD2439975]

Damn it Brendan, not another expense! :mad2:

I still owe for the Moonlite...this hobby is sucking me dry mate, could have supported half a dozen girlfriends at this rate. :lol:

[bmitchell82]

... im a uni student :D...

[Benno85]

Confirming my fears there B-Man, now I'll have to start petitioning SWMBO for some parts for Christmas. I'm thinking that maybe a JMI focuser might be enough for my needs, I've just gotta work out how much closer I need to be to the secondary in order to reach focus.

[bmitchell82]

for 50 bucks you can sort your fears out 100%... that and a bit of silicone and everything is all better! :)

a 2 inch focuser is alright try to get 2.5 or 3" is the best.

[Benno85]

Hmmm now you've got me intrigued...where would my hard earned fiddie be spent? Longer bolts and stiffer springs perhaps??

[bmitchell82]

cut the tube be done with it! :) take your 50 bucks to either bintel/andrews depending on which one is cheaper get yourself a new 70mm secondary. get some silicone, take the old secondary off, stick the new secondary on. don't be scared :) i just pulled mine off for the second time and re collimated the whole kit from complete pull down within 5 minutes :D.

win win bigger full illumination, giving you fainter nebulosity from the center out to the edge :) nice and cheap uni student style!

[Ian Robinson]

Neither - just move the mirror cell up the tube and closer to the focusor and invest in a 1-2" long extension tube for when you want use to eyepieces that need the extra backfocus.

[stephenb]

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.:thumbsup::thumbsup:

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!

[Ian Robinson]

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]

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]

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 :thumbsup:

Dan

[alistairsam]

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