Moving mirror to achieve focus.

[sheeprug]

Hi All,

I have an OO VX14 dob and a Canon 750d DSLR. Obviously not the best rig for AP, but it's always fun trying and a good challenge. . I guess lots of short exposures and a highish iso is the way to go.

Anyway, main issue is focusing.  If I have the collimation screws as far unscrewed as I dare to get the mirror well up the tube I can just about achieve focus, but it's a but marginal.  As I see it there are two possibilities to improve this: find a low profile T mount, or drill some new holes in the tube a few mm up to get the mirror higher.

I'm sure this sort of problem has been solved many times before, so anybody wish to share experiences or ideas?

Oh, and happy Easter to you all!

SR.

[xtreemchaos]

how far is it out focus, if not by much say 10mm or so you could get longer collimation screws for the prime, thats how I sorted my 200p when needing more inward travel for my CC. Charl. ps happy easter mate

[faulksy]

27 minutes ago, sheeprug said:

Hi All,

I have an OO VX14 dob and a Canon 750d DSLR. Obviously not the best rig for AP, but it's always fun trying and a good challenge. . I guess lots of short exposures and a highish iso is the way to go.

Anyway, main issue is focusing.  If I have the collimation screws as far unscrewed as I dare to get the mirror well up the tube I can just about achieve focus, but it's a but marginal.  As I see it there are two possibilities to improve this: find a low profile T mount, or drill some new holes in the tube a few mm up to get the mirror higher.

I'm sure this sort of problem has been solved many times before, so anybody wish to share experiences or ideas?

Oh, and happy Easter to you all!

SR.

on the vx14 there is 2 sets of screw centres, 1 is for visual, 1 is for ap. these screws move the mirror cell up the tube by around 30mm

[sheeprug]

12 hours ago, xtreemchaos said:

how far is it out focus, if not by much say 10mm or so you could get longer collimation screws for the prime, thats how I sorted my 200p when needing more inward travel for my CC. Charl. ps happy easter mate

Yes that would probably do it.  Nice one!

SR.

[sheeprug]

11 hours ago, faulksy said:

on the vx14 there is 2 sets of screw centres, 1 is for visual, 1 is for ap. these screws move the mirror cell up the tube by around 30mm

You mean the ones on the mirror cell?  I'm already on the top one, so nowhere left to go at the moment. 

SR.

[xtreemchaos]

yer worked for me , every mm you move the prime up the tube = 2 mm of inward focus, let us know how you get on. charl.

[happy-kat]

Do you need a t mount? Can you screw your t ring straight to the focuser tube and gain another mm or two.

[John]

Interesting that you have this issue. With Orion Optics scopes the focal plane is usually well outside the tube wall, to the extent that an extension tube or focuser spacers are needed to bring eyepieces to focus

 

[Stu]

2 hours ago, John said:

Interesting that you have this issue. With Orion Optics scopes the focal plane is usually well outside the tube wall, to the extent that an extension tube or focuser spacers are needed to bring eyepieces to focus

 

That's my experience too John.

@sheeprug perhaps you could post some pictures of the mirror position and focuser etc to see if it gives any clues?

[knobby]

A Barlow might help short term 

[Tonynexevo925]

17 hours ago, faulksy said:

on the vx14 there is 2 sets of screw centres, 1 is for visual, 1 is for ap. these screws move the mirror cell up the tube by around 30mm

I didn't know this... good info for the future ??

[Tonynexevo925]

I used an extension tube to focus most 2" EPs until I fitted the moonlite focusser. 

With the moonlite and all spacers fitted I now get focus with everything from Nagler 3.5mm up to Panoptic 41mm - very pleased. May also help with camera use?

Haven't used for astrophotography so glad to get that useful piece of info from Faulksy. 

[John]

32 minutes ago, Tonynexevo925 said:

I used an extension tube to focus most 2" EPs until I fitted the moonlite focusser. 

With the moonlite and all spacers fitted I now get focus with everything from Nagler 3.5mm up to Panoptic 41mm - very pleased. May also help with camera use?

Haven't used for astrophotography so glad to get that useful piece of info from Faulksy. 

I use a Moonlite with the spacers on my Orion Optics dob. Great to get eyepieces to reach focus but I'd need to remove the spacers if I wanted to get a camera to focus I reckon. Then I'd need an extension tube again for visual use.

[sheeprug]

2 hours ago, Stu said:

That's my experience too John.

@sheeprug perhaps you could post some pictures of the mirror position and focuser etc to see if it gives any clues?

Ok.Ill see what i can do. In fact I'll re-examine the whole setup later this evening in the light of some comments here. I do need to use a 50mm extension tube for visual use with the mirror in its current position.  

SR.

 

[dph1nm]

On 17/04/2017 at 08:21, xtreemchaos said:

every mm you move the prime up the tube = 2 mm of inward focus

Err, why?

NIgelM

[xtreemchaos]

the light gos down the tube to the prime that's the first mm the up to the secondry the 2nd mm by moving the prime up the tube you shorten the light path 2mm for every 1mm you move the prime,. charl.

8 minutes ago, dph1nm said:

Err, why?

 

[dph1nm]

21 minutes ago, xtreemchaos said:

the light gos down the tube to the prime that's the first mm the up to the secondry the 2nd mm by moving the prime up the tube you shorten the light path 2mm for every 1mm you move the prime,. charl.

It's a Newtonian - so the light path down the tube from the top is irrelevant, the light is parallel there - moving the mirror has no effect on that. The focal length is simply the distance from the primary to the focus. Move the mirror up the tube 1 mm and the focal point moves out 1mm.

NIgelM

[xtreemchaos]

cool, I'm just as happy to be wrong as right mate, its just the way things worked out when doing my scope.... charl.

[sheeprug]

Hi all,

Just to close this one out, I got hold of some 60mm M6 stainless steel machine screws to replace the 50mm ones in the mirror cell an that seems to have provided a crucial few mm.  Thanks to xtreemchaos for that idea.

So that's that speed bump overcome. Now onto the next one....

SR.