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# Off Axis Guider Focusing

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Hello all

i just bought one of these off axis guider. Can I ask if my presumption is correct. It says under the technical data that the middle of the prism to camera side is 6mm so that leaves 44mm to my Dslr sensor. So it's this same distance 6mm prism to camera side + 44mm will give me the required distance?

The center of the prism to the T2 connector is adjustable 26 -34mm

So I'm assuming this. 6mm +44mm = 50mm to get focus for the guide cam in my case a zwo 120mm so a 20mm spacer would work? I was looking at a helical focuser like baader but the focal length will be wrong if my assumptions are correct.

Any help much appreciated

cheers

Gerry

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6 + 20 + 26 = 52 (>50).

I think a 10-18mm extension tube would work, if the variable distance is 26-34mm

6 + 10 + 34 = 50

6 + 18 + 26 = 50

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24 minutes ago, Demonperformer said:

6 + 20 + 26 = 52 (>50).

I think a 10-18mm extension tube would work, if the variable distance is 26-34mm

6 + 10 + 34 = 50

6 + 18 + 26 = 50

Thanks is this the way everyone does it? Or is there a helical focuser I can use?

I am correct in assuming 50mm for the focus plane?

Thanks!

Gerry

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32 minutes ago, Demonperformer said:

6 + 20 + 26 = 52 (>50).

I think a 10-18mm extension tube would work, if the variable distance is 26-34mm

6 + 10 + 34 = 50

6 + 18 + 26 = 50

Ah I've just realised I have to add 12mm back focus for the zwo 120mm the sensor is 12mm in.

So if I'm correct 26 + 12 + 12 =50.

Youve added the extra 6mm but that is already included in the 26 I believe.

Hope im getting this right

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Although I have them on my OAG's, I rarely use the helical focusers. I prefer instead to use a Baader clamping eyepiece holder to manually fix the position. Once you have it, it stays the same, and super precise focus is not totally necessary or even always possible with edge of field stars.

They are a bit fiddly the first time you set up, but soon become second nature and well worth the initial effort.

Looking forward to seeing your results

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4 minutes ago, Tim said:

Although I have them on my OAG's, I rarely use the helical focusers. I prefer instead to use a Baader﻿﻿ clamping eyepiece hol﻿de﻿r﻿ t﻿o manually fix the position. O﻿nce you ﻿have it, it s﻿﻿﻿t﻿ays the same, and super precise focus is not totally necessary or even always possible with edge of field stars.

They are a bit fiddly the first time you set up, but soon become second nature and well worth the initial effort.

Looking forward to seeing your results

You mean something like this?

but the focal length is 28mm - 34mm won't that push me over my 50mm requirement for dslr?

Thanks

Gerry

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I have the same adapter, do you have it in your possession?

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There is a 12.5mm back focus on your camera.

So if the requirement is 44mm distance from chip to mirror, then you need to set your camera at 44-12.5mm= 31.5mm of spacing.

The range of adjustment inherent to the OAG is 26-34mm so you wont need any additional adapters or spacers.

HTH

• 1

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2 hours ago, tooth_dr said:

There is a 12.5mm back focus on your camera.

So if the requirement is 44mm distance from chip to mirror, then you need to set your camera at 44-12.5mm= 31.5mm of spacing.

The range of adjustment inherent to the OAG is 26-34mm so you wont need any additional adapters or spacers.

HTH

Is that right? Because the off axis guider I have bought says there is a additional 6mm to the center of the mirror prism so I was calculating 50mm is that wrong?

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3 hours ago, tooth_dr said:

I have the same adapter, do you have it in your possession?

No unfortunately but I have ordered a 10mm spacer so I can have a little flexibility. Thanks!

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7 minutes ago, Gerry Casa Christiana said:

No unfortunately but I have ordered a 10mm spacer so I can have a little flexibility. Thanks!

Sorry I meant the OAG, I have that same OAG as you have.  I use a DMK21 with a backfocus of 15mm and it works out of the box with this OAG.

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Hi Gerry

Your dslr sensor is 44mm from the bayonet.

The prism is a further 6mm forwards, so it's 50mm from prism to dslr sensor.

Guide cam has 12.5mm back focus.

So the T2 on the periscope needs to be 50 - 12.5 = 37.5mm from the prism centre.

However the range of adjustment is only 26 to 34mm (so perfect for tooth's 15mm, but not for your top selling ZWO - nice one TS !!)

So your 10mm  extension  is good as it leaves the periscope at only 1.5mm  extension, good for rigidity.

Michael

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Posted (edited)
23 hours ago, Gerry Casa Christiana said:

one of these off axis guider

Hi. Excellent choice; watch your stars approach pinpoints in one easy step!

No need to measure anything. Make it easy: get a helical focuser. Here is my oag11 on on Canon 700d with a cheapo t7m ali express guide camera and even cheaper helical focuser from the same source. Focus the main camera on a bright star somewhere in Cygnus then loop around 0.5s in PHD2 to focus the guide camera. Done:)

HTH and clear skies

Edited by alacant

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On 09/07/2018 at 23:16, alacant said:

Hi. Excellent choice; watch your stars approach pinpoints in one easy step!

No need to measure anything. Make it easy: get a helical focuser. Here is my oag11 on on Canon 700d with a cheapo t7m ali express guide camera and even cheaper helical focuser from the same source. Focus the main camera on a bright star somewhere in Cygnus then loop around 0.5s in PHD2 to focus the guide camera. Done:)

HTH and clear skies

Hi thanks

What make of helical focuser is that and it's focal length? Do FLO sell it? I'm in Italy so would like to get it from somewhere closer

Thanks

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3 hours ago, Gerry Casa Christiana said:

somewhere closer

Germany? This with this look about the same.

3 hours ago, Gerry Casa Christiana said:

focal length?

It's an empty focuser with no optical components.

HTH

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Posted (edited)
1 hour ago, alacant said:

Germany? This with this look about the same.

It's an empty focuser with no optical components.

HTH

No sorry what I mean what does it add to the imaging train in length. What I thought I needed to do is keep the focal plane to 50mm  44 from Canon sensor plus 6mm from the middle of the OAG mirror. Am i wrong?

Many thanks!

Edited by Gerry Casa Christiana

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Posted (edited)
20 minutes ago, Gerry Casa Christiana said:

Am i wrong?

No, sounds OK if only theoretical as it's almost impossible to measure with any accuracy. The focuser gives you a few mm either side of focus; 50mm is at the focuser's mid travel. But remember it depends on how much you push in the guider camera. Indeed, with the fat zwos, you have to focus by moving the guide camera on the prism stalk; very difficult as you can't use a focuser.

HTH

Edited by alacant

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4 hours ago, Gerry Casa Christiana said:
5 hours ago, alacant said:

No sorry what I mean what does it add to the imaging train in length. What I thought I needed to do is keep the focal plane to 50mm  44 from Canon sensor plus 6mm from the middle of the OAG mirror. Am i wrong?

The helical focuser goes on the guidecam, not the DSLR.

It would replace the 10mm spacer you've bought.

Michael

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Just out of curiosity , is this being used on a Newt scope ? I ave an older C8 and an older Celestron OAG and I’m glad I didn’t have to worry about the extra spacers or measurements . Just hook up and all set . I see the new Celestron OAG comes with all adapters needed for any setup . What brand are you using ? Looks like a Orion OAG .

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16 hours ago, alacant said:

No, sounds OK if only theoretical as it's almost impossible to measure with any accuracy. The focuser gives you a few mm either side of focus; 50mm is at the focuser's mid travel. But remember it depends on how much you push in the guider camera. Indeed, with the fat zwos, you have to focus by moving the guide camera on the prism stalk; very difficult as you can't use a focuser.

HTH

Ah yes mine does have a stalk and I think that's why I think I can't use the focuser because it says with mine it has 26-34mm adjustment on guidecam focusing. Then add 12mm of back focus for my zwo 120mm even if I start with it at the shortest length 26mm + 12mm = 38mm so my helical focuser would need to be under 12mm thick. That's my thinking. Anyway it's arrived yesterday I'll hook it up with the 10mm spacer and see if I can achieve focus.

Thanks

Gerry

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11 hours ago, celestron8g8 said:

Just out of curiosity , is this being used on a Newt scope ? I ave an older C8 and an older Celestron OAG and I’m glad I didn’t have to worry about the extra spacers or measurements . Just hook up and all set . I see the new Celestron OAG comes with all adapters needed for any setup . What brand are you using ? Looks like a Orion OAG .

Yes definitely going to be used on a Newtonian. It's a TS optics. Here

I got it because it was on offer 99€ bargain I thought

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Posted (edited)
15 minutes ago, Gerry Casa Christiana said:

zwo 120mm

That clarifies it then; you can't use a focuser. Grit your teeth, replace one of the allen head grub screws with long m4 screws -so can tighten one of the screws at least without a hex wrench- and focus by moving the camera/turret toward or away from the dslr on the prism stalk. Tighten the long screw finger tight when you're at focus. Take your time with the others and make sure you retain the camera orientation along the flat on the prism turret.

HTH and good luck.

Edited by alacant

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12 hours ago, michael8554 said:

The helical focuser goes on the guidecam, not the DSLR.

It would replace the 10mm spacer you've bought.

Michael

Thanks Michael

Yes I understood that but from what I read online you work out the focus distance of you off axis guider from center prism of off axis guider to the center of your dslr. It's the same measurement the other way towards your guide cam.

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1 minute ago, alacant said:

That clarifies it then; you can't use a focuser. Grit your teeth, replace one of the allen head grub screws with long m4 screws -so can tighten one of the screes at least without a hex wrench- and focus by moving the camera/turret toward or away from the dslr on the prism stalk.

HTH and good luck.

Ah thought so good idea about the m4 screws I'll look for that. Something with a head on would be good.

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Just now, Gerry Casa Christiana said:

a head on would be good

Just thread a nut until it hits the head of the bolt; instant thumbscrew:)

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