14 replies to this topic

[uptomato]

I do astrophotography with my refractor, and a dslr. I would love to get a better view of smaller targets such as galaxies, and my reflector is decently sharp when collimated. But I have left it there for a while, because I can’t use the dslr with it because I can’t reach focus. I have a cheap Barlow that came with the scope, but that can’t even resolve craters on the moon, let alone spiral arms. So I’m not buying a Barlow ow anything else, the only option I have found is cutting the scope ( I can cut it but am not sure) this telescope is not really usable now anyway so it isn’t a massive problem if I break it. I would love to know some details about this and if there is any workarounds.

[Bigeye1988]

I assume you can't get your camera close enough to focus. If that's the case and you can get a good estimate about how much further in travel you would need you could remove the mirror cell, drill new mounting holes for it further up the tube and move the primary mirror that much close to the secondary there by moving the focus out. At least if it still isn't satisfactory you can put the mirror back in it's original position. No perminent altercations.

[BQ Octantis]

What scope?

 

You can swap out the Crayford in some Newts…

 

BQ

[Alvan Clark]

You just need to drill new holes for the primary mirror mount and move it up the tube however much you need. Also maybe you can buy and replace your focuser if a new one has a lower profile.

[Jon Isaacs]

So I’m not buying a Barlow ow anything else,

 

 

Planetary imagers all use Barlows or Powermates, even with long focal length scopes like SCTs.  But DSLRs are not well suited for planetary imaging.  

 

Just because your inexpensive Barlow doesn't do the job, that doesn't mean a quality Barlow will not work. The bigger problems are the accuracy of the tracking and the freedom of from vibration and tilt of the focuser.

 

Jon

[cuzimthedad]

Moving to the DSLR imaging forum...

[uptomato]

Thanks, I’ll cut it about an inch

[wongataa]

A thin t adapter (1mm thick) allows me to achieve prime focus with my Dobsonian telescope and DSLR.

 

Try a thin t adapter.  If that doesn't work get a lower profile focuser.

[smiller]

There are several ways to try to get that camera into focus:

 

1) some coma correctors provide additional back focus by pushing out the focus point, and they can often be enough to give you the necessary spacing.  if you’re going to use that camera and want to get sharp images from edge to edge you are going to want a coma corrector anyway.  
 

These numbers I tried to collect for use in Dobsonians (which are newts obviously) may not be correct so take them with a grain of salt:

 

 

2) You can try to move the mirror forward by using longer collimation bolts and a spacer along with each original spring.  Whether this is possible, and the exact plan on how to do it is dependent on the design of your Newtonian.  I did it on my Orion Goto Dob and it was actually fairly easy to do. I moved it forward about 20 mm.   it was non-destructive, and it would be pretty easy to move it back in the future, if I wanted to.

 

3) get a low profile focuser.

 

4) you can sometimes get a few millimeters by collimating your scope with the mirror as forward as possible given the travel range of your collimation screws.

 

5) Get a low profile T-ring.  For some camera models there are low profile rings that are available that instead of adding about 10 to 12 mm only add about 1-3 mm of space.  I was able to find one for my Canon rebel T3i.

 

6) Use a Barlow lens, but position it (I.e. adjust the spacing) so that you minimize the amount of magnification yet it still pushes the back focus out just enough sometimes you can find a balance there where are you are not magnifying the image too much but you get the imaging to focus.

 

7) and finally I know this is not the solution you’re looking for, but maybe buy a used mirrorless camera and use that as your camera instead of the current DSLR as they have minimal back focus requirements.  Then you have a camera that works well as a land camera, and as an Astro camera.

 

 

Many people use some combination of the six techniques above to solve the problem.

 

I think one key thing to do is to measure where your back focus is and know how much you need. I believe you can do this by pointing the telescope at the moon and holding a piece of paper behind the focuser without an eye piece in it, and find the spot at which the image of the moon comes into focus on the paper. Measure the distance from that to the top of the focuser with it rack in as far as possible, and that gives you how much back focus you have.  You can then look up the back focus of your camera and measure the offset your T ring adds, and you can decide an approach to close that gap.
 

Edited by smiller, 18 April 2025 - 08:58 AM.

[wongataa]

7) and finally I know this is not the solution you’re looking for, but maybe buy a used mirrorless camera and use that as your camera instead of the current DSLR as they have minimal back focus requirements.  Then you have a camera that works well as a land camera, and as an Astro camera.

You still need a short t-ring for this as well as a standard t-ring will but the sensor 55mm from the mount face DSLR or mirrorless.

[uptomato]

I have now cut the scope, and can reach focus, but it is still unusable, I pointed it at Jupiter to test focus, and it was good. But, there is a little, looks like shadow shape, white smudge coming off of it and all of the stars, I would say I'm reasonably collimated. ill try and draw a picture of it, well pardon my bad example but it's like this.

 

[uptomato]

See that little smudge next to Jupiter and its moons, that's on every star and we cant have that.

[BQ Octantis]

Astigmatism.

[uptomato]

I've never seen it before is this on the primary or secondary

[Marcus1]

I have a c8 edge hd and wanted to try to capture what I was seeing so decided to try to get my canon eos m3 back into action. I ordered a Celestron T-adapter #93644 along with a Fotasy adjustable T/T2 adapter BOOP25XGVI for a total of $48. I expected focal plane issues but the T-adapter is in two pieces that screw together with the same thread. So there should be adequate adjustment possibility. There is thundery migrainous cloud cover, but some trees in the distance. I slapped the camera on the C8, tweaked the focus and there were trees on the camera display. I am amazed that this worked on first try by a noob. There will be rain for next three days, and a Bonnie Raitt concert before clear skies return.