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Author Topic: Comp Article: modfying a newtonian telescope for DSLR focusing  (Read 12582 times)

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Offline Hoohoo

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Getting focus with DSLR and a newtonian telescope

As many beginners in astronomy notice when first buying some serious equipment is that newtonians is pretty much the best option when thinking of "aperture for money".
And very often these same beginners want`s to start up with astro-photography, so they buy a DSLR, a T-Ring and a adapter just to end up in theyre first imaging session without the abbility to focus. When i started imaging i found myself in just that position(autumn 2006), so how do you solve this problem?

In this article i will start  with the cheapest and less permanent options first, so that you can have the benefit of trying most alternatives before starting the more serious modifications(those that will void the warranty).

Ooh, one more thing, i do not, in any way, to be held responsible for any damage on your equipment caused by trying these modifications.

So let`s start:

Method 1:
NB!! Make sure that when adjusting the collimation screws, the mirror is at the lower end of the OTA, this will prevent the mirror from tilting over when/if you adjust too far in.

The first, and probably the easiest thing to do is moving the mirror further in. This is done by loosening the collimation locking screws at the mirror end of the Optical Tube Assembly(OTA).
When that is done, you adjust the collimation screws so that the mirror goes as far into the tube as possible. You may have to get some new locking screws in order to use the full lenght of the adjustment screws, but you should be able to get those at any hardware-store.
One more thing to think of: Make sure that the adjustment screw is screwed at least one thread on(360degrees rotation), or it may loosen during transport.

Method 2:

This is a no-risk-method to try out, but it may not work with all equipment.
I will now show/tell how to get some extra in-focus by modifying the T-Ring(the part that is mounted dirctly to the DSLR).
The T-Ring is built up by two separate rings, one outer ring that have the camera threads on it, and one inner ring that has the threads for the nosepiece-adapter.
What you want to do is dismantle these two rings, wich is done by unscrewing the 3 locking screws found around the sides in 2mm holes. My T-Ring uses flathead screws.
Now you have two separate rings, it is (of course) the ring with the camera threads we are going to use.
What you should do now is to mount that ring onto your focuser, outside the eyepiece holder.
This may require you to remove the locking screw that normally holds the eyepiece to the focuser.
Now that you have mounted the camera threads onto the telescope it should be adjusted so that it is centered on the focuser, use your eyes for that on. The last thing to do is to mount your camera onto the scope and adjusting the angle, this is done by supporting the camera with one hand while loosening the ring from the scope, then rotating the camera to the desired angle and tightening the ring again.

See picture below on how i did it.

You are now ready to try out if you need further modifications, i`m sure you know how to try this so i wont bother explaining.

Method 3:

If you still is not able to get focus you either have to give up or start some more serious modification.

NB!! This method WILL most likely void the warranty.

This is the last method that i will go "deeply" into, simply because after this one i got focus and did not want to do the more heavy modifications. I also do have to say that even this will give you alot more in-focus, it also will degrade the focuser very much.

This method involves cutting down the focuser.... and that is why the focuser get`s less presise. When cutting the focuser you allow much more movement to the focuser tube wich in turn allows a HUGE image shift.

Because of the focusers of different kinds have different build-ups i will not go very deep into explaining how to do this, only giving you a step-by-step direction:

1: Start with taking the focuser of the OTA, make sure the Primary mirror is in the upper end of the OTA to prevent the nuts to fall down on it. Also make sure that they cant hit the secondary mirror.
2: Dismantle the focuser so you have the tube and "body" in separate pieces.
3: Cut the "body" as much as you think you need for getting focus.
4: Move the rack(on rack&pinion focusers) as far to the eyepiece side as possible, this requires to make som new holes to attach the screws in.
5: Use som grinding paper to grind the edge of the "body" straight.
6: Assemble the focuser and mount on OTA.
7: You are now ready to try it out.

!!NB Read the warning above twice.

Method 4:

This method involves moving the primary even further, by cutting the telescope tube...

Cutting the tube is not really a hard job, the hardest thing to do is getting a clean straight cut all the way.

The worst part is knowing how much to cut off, the way i found is the best is to take of the focuser, then hold your camera where the focuser used to be and move it in and out to see where the focus point is.

To find how much to cut:

Take the distance from the camera when mounted on the focuser when focuser is as close too focus as possible, subtract the distance from where you got focus when holding the camera over the secondary mirror, the number returned will be how much you need to cut. It would be wise to cut about 5mm more just in case.

"One drawback could be that the reflected light from the primary partially will miss the secondary."
In order to make sure that all of the light hits the secondary i`ve come up with this formula:

"Fm1-m2min = F-((M2*F)/A)"

Where:
Fm1-m2min is the minimum distance between the primary and secondary
F is the focal length
M2 is the size of the secondary mirror as seen from the primary.
A is the size of the aperture

Just remember to plot all of the information in the same form (mm, inches, meter), or it will fail misarably.

The drawback is that this is not in any way reversible, once the tube is cut there is no way back.

One more little tip about focusing: Try google "Hartmann Mask" ;)

Kim
Kim

Skywatcher 190MN
Skywatcher HEQ5 Pro
Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

Offline NickH

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #1 on: 09:27:08, 09 May, 2007 »
Nice one Kim, that will surely help a lot of people

 :big_clap:
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No time....always bloomin cloudy...need to build the Obsy..

Offline Hoohoo

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #2 on: 12:27:54, 09 May, 2007 »
Thanks Nick, and heres the pictures i promised last night.

Kim


Kim

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Offline the fordster

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #3 on: 09:52:46, 11 June, 2007 »
Great article Kim - This is a question that comes up many times and I'm sure it'll be a lot of help!

Offline Hoohoo

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #4 on: 13:37:46, 11 June, 2007 »
Thanks Andy. I will update the "tube-cutting"-section in a couple of weeks when done with the eksams.

Kim
Kim

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Offline Hoohoo

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #5 on: 13:21:03, 23 June, 2007 »
Hi again, i`ve just updated the tube-cutting section, any comments is gladly accepted.

If you have any questions feel free to post here or send me a PM and i`ll (or others) will help you out.

Kim
Kim

Skywatcher 190MN
Skywatcher HEQ5 Pro
Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

Offline dciobota

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #6 on: 19:21:24, 17 September, 2007 »
  Wow, very nice writeup Kim, this should be a stickie in the projects section I think.  :big_clap:

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Offline Hoohoo

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Re: Comp Article: DSLR focusing with newtonian telescope
« Reply #7 on: 15:11:15, 18 September, 2007 »
  Wow, very nice writeup Kim, this should be a stickie in the projects section I think.  :big_clap:



Well thank you Daniel.

Feel free to move it.
One thing tough, i think the name of the topic is a bit misleading, should maybe have changed it to make it more obvious that it is about modification.

And since i`m already writing a supply, there also was a picture in this article, as these get deleted by admins after a while just make a request and i`ll PM it.

Kim
Kim

Skywatcher 190MN
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Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

Offline dciobota

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #8 on: 02:56:50, 19 September, 2007 »
 Here you go Kim, alteredt the title slightly and put the pic back in (hosted on my website).

 :)
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Offline Hoohoo

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #9 on: 00:22:40, 21 September, 2007 »
Looks great now, the new title is much more describing.

Kim
Kim

Skywatcher 190MN
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Offline zeroy

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #10 on: 20:03:45, 22 November, 2012 »
Great article, im currently using method 2, I have the Tring attached to the 2" to 1.25" adapter directly on the focuser. One method that isnt described is to buy a low profile Focuser that can suit the Newtonian. I am still to find one affordable for my Meade N6.
Meade LXD75 N6-AT
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Offline psjshep

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #11 on: 21:22:55, 22 November, 2012 »
Kim has recently returned to UKAI.... will be interesting to see how you fair compared with his write-up those years ago  :yes:

Phil
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Offline Hoohoo

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #12 on: 22:29:19, 22 November, 2012 »
Hi Zeroy, good to hear that you got some help from the article :)

When I wrote it I only covered the options that I had tried myself. Most of them in desperation as being a student, it was just too expensive to consider a new focuser(I did design one at the time as material could be supplied by the school, never built it though). New focuser would of course be the best solution, but as you have already noted, quite expensive.

I think I still have the pictures belonging to the article somewhere if anyone is interested...

Cheers

EDIT: For the "method 2", i did a small improvement on it. If you can get access to a lathe, turn a thin groove in the eyepiece mount where the locking screws go, that will make it MUCH more secure....
Kim

Skywatcher 190MN
Skywatcher HEQ5 Pro
Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

Offline zeroy

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #13 on: 11:42:26, 23 November, 2012 »
New focuser would of course be the best solution, but as you have already noted, quite expensive.


Indeed. And not only that but Im reading a lot of problems can arise from using those low profile focusers. Stuff like having Prime focus but not enough back travel for eyepieces or barlows, etc... Still, the focuser on the Meade N6-AT is so bad (image shift mainly) that I am considering a Moonlite or JMI ...

Here are the pic of the method 2 for me, shows the Tring directly attached to the Focuser 1.25" adapter (i get zero vignetting btw)



Meade LXD75 N6-AT
Canon 1100D

Offline Hoohoo

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Re: Comp Article: modfying a newtonian telescope for DSLR focusing
« Reply #14 on: 12:03:48, 23 November, 2012 »
Well, the lack of backfocus is something I would not worry too much about, all you need to compensate is an extender and problem solved :)
I did adapt my original focuser into a low profile one, but it led to crazy amounts of image shift, so much that i could loose the object completely(1200mm FL).

Good to hear that the method works so well, I loved it myself as it was easy to change from camera to eyepiece(for aligning). And having the adaptor constantly attached ment one less thing to have floating around the car.
Kim

Skywatcher 190MN
Skywatcher HEQ5 Pro
Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

 

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