Focusing with Newtonian reflector.

[Guest Tuomo]

Hello!

 

I just had my first DSLR camera and I tend to use for AP. I know, there are MANY problems when using visual Newtonian reflector for AP. I bought low profile Canon EOS - 2" adapter and Im able to reach focus on treeline about 100m away. There is about 5mm left in focuser until its all the way down. Forecast promised cloud for 2 weeks, so I cant check focus with stars or moon. My question is: How far away terestial objects should I try to focus to "simulate" moon or stars. How far is focal plane of stars? Or is it so that if I can reach focus with trees I can reach focus with stars too....This is crazy and maybe even stupid question  

[Aaron F Johnson]

I have built phone cameras with attatched eye pieces for use in afocal astrophotography and used the neighbor's roof at 100' away to set the focal distance, several camera eyepiece combos and no issues. As long as your target is beyond the minimum magnification of your scope (number most likely available online) and you are reaching focus and beyond you should be grand...when the sky's clear...there all yours...

[John78]

5mm doesn't sound a lot - 100m is going to be nowhere near infinity, what tube is it?  The other problem with the focuser wound all the way in is it protrudes a lot into the tube generally, which takes bites out of your stars which looks horrible in the final picture.

[Aaron F Johnson]

I would say at this point you have try it either way...5mm is not much but if already at infinity on the tree line it would be enough. Be sure to post either way and best of luck...

[ChrisLX200]

You can probably gain a little more by using the collimation screws on the primary, loosening them all by the same amount will move the primary upwards and you'll get additional backfocus as a result.

ChrisH

[Aaron F Johnson]

Never even thought of that...could make the difference in some cases. Cool...

[Guest Tuomo]

43 minutes ago, John78 said:

5mm doesn't sound a lot - 100m is going to be nowhere near infinity, what tube is it?  The other problem with the focuser wound all the way in is it protrudes a lot into the tube generally, which takes bites out of your stars which looks horrible in the final picture.

Its 12"/1500mm/F5 Dobsonian. I think 5mm left in focuser when using SLR and visual newtonian is quite common since focal point is so close at focuser. If I can reach focus I might concider myself lucky. 5mm  at focuser makes everything sturdy. This will NOT (102/500mm will be) be my primary imaging scope. I just wanted to test AP with 12" Newtonian too.

[Guest Tuomo]

29 minutes ago, Aaron F Johnson said:

I would say at this point you have try it either way...5mm is not much but if already at infinity on the tree line it would be enough. Be sure to post either way and best of luck...

Thanks. I will post pics about moon for sure.

[Guest Tuomo]

Like they say: First light. This time it was with my EOS and 12 incher.

Goal: Manage to focus EOS and 12" Newtonian reflector with stars.
Summary: Fail, epic fail.

There was small window to test my new setup and I ran out and made everything ready for clouds to roll over. Here is the result:

As you can clearly see, there is Capella

And here are (part of it) the ever famous Pleiades, aka. seven sisters, aka. subaru, aka. seulaset.

 

Joking aside. No AP with my Newtonian. Not enough backfocus.

[Aaron F Johnson]

Sorry to hear that, after the investment in a lowpro adapter its got to be frustrating. I plan on doing the same setup and am hoping I don't wind up with a12" scope only usable for AP because of mods...you could go afocal with a T threads zoom if you have one lying about but all that weight hanging out there isent great either, not the intended configuration but workable none the less at least for the moon.

[John78]

Yeah if your at 5mm travel left at 100meters it was going to be a tall order you'll need about at least 12mm and 20 to be safe.  You could reach focus with a 2x Barlow but then you'll be 3000mm focal length which is a shade impractical for anything.

 

Chopping the end off the tube and moving the mirror up is the best option, then you'll need an extension tube to reach focus with an eyepiece but that's easy compared to focusing with your camera which is currently impossible ?

[Guest Tuomo]

19 hours ago, John78 said:

Yeah if your at 5mm travel left at 100meters it was going to be a tall order you'll need about at least 12mm and 20 to be safe.  You could reach focus with a 2x Barlow but then you'll be 3000mm focal length which is a shade impractical for anything.

 

Chopping the end off the tube and moving the mirror up is the best option, then you'll need an extension tube to reach focus with an eyepiece but that's easy compared to focusing with your camera which is currently impossible ?

Hmm. Thanks for the tip! Im thinking about getting 50mm collimation screws. Originals are 25mm long so gained extra is about 25mm. You think that should work?

When I raise primary about 20mm the focal point moves same amount out from the focuser. Do I really need extension (no prob, already have one) there is still plenty of forward focus left?

[D Wright]

When you move the primary up the tube, for evey 1mm you move it relates to 2mm on focal plane. So you do not have to move it very far. I had the same problem on my 10" newt and I cut 15mm off the end of the tube. I can get focus with DSLR and still focus with EP without the need for an extention tube. I thought about using longer collimation screws but was concerned about mirror slop as the mirror is heavy.

[John78]

Yeah these longer FL scopes you probably can move the mirror and still focus eyepiece and cameras because have more focuser traveller than my little 130P...

 

Give it a go - if you can do it on the screws then its risk free to try.

[dph1nm]

1 hour ago, D Wright said:

When you move the primary up the tube, for evey 1mm you move it relates to 2mm on focal plane

Err, why? The FL of the mirror is the same wherever it is. If I move the mirror by 1cm then surely the focal plane moves by 1cm in the same direction?

NigelM

[D Wright]

Not sure why but thats what I was told.

[ChrisLX200]

Don't think it applies to flat secondaries - maybe to SCT/RC optics.

ChrisH

[Guest Tuomo]

17 hours ago, D Wright said:

When you move the primary up the tube, for evey 1mm you move it relates to 2mm on focal plane. So you do not have to move it very far. I had the same problem on my 10" newt and I cut 15mm off the end of the tube. I can get focus with DSLR and still focus with EP without the need for an extention tube. I thought about using longer collimation screws but was concerned about mirror slop as the mirror is heavy.

 

16 hours ago, dph1nm said:

Err, why? The FL of the mirror is the same wherever it is. If I move the mirror by 1cm then surely the focal plane moves by 1cm in the same direction?

NigelM

 

12 hours ago, ChrisLX200 said:

Don't think it applies to flat secondaries - maybe to SCT/RC optics.

ChrisH

Yep. Its SCT.

[Guest Tuomo]

17 hours ago, D Wright said:

When you move the primary up the tube, for evey 1mm you move it relates to 2mm on focal plane. So you do not have to move it very far. I had the same problem on my 10" newt and I cut 15mm off the end of the tube. I can get focus with DSLR and still focus with EP without the need for an extention tube. I thought about using longer collimation screws but was concerned about mirror slop as the mirror is heavy.

How did you cut your OTA? With hacksaw or angle grinder? And did you measure needed cut with the feeling or made some kind of calculations where new focal point would be?

[Guest Tuomo]

So here it is. New longer bolts and springs assembled and ready to go. Now I just need clouds to move.

[D Wright]

On 01/11/2016 at 05:56, Tuomo said:

How did you cut your OTA? With hacksaw or angle grinder? And did you measure needed cut with the feeling or made some kind of calculations where new focal point would be?

I cut the tube with a jigsaw and filed it smooth. To calculate the amount to cut off I used the highly technical method of guesswork.

[Guest Tuomo]

19 hours ago, D Wright said:

I cut the tube with a jigsaw and filed it smooth. To calculate the amount to cut off I used the highly technical method of guesswork.

Damn...I knew I should not have skipped that lecture at university. 

[Guest Tuomo]

Alas!

Cant focus with new collimation screws. Missing just few millimeters. I think I let it go because I needed extensions for eyepieces. Its a shame. because this 12" would have been perfect for moon because moon filled entire DSLR screen.