Coma Corrector and Back Focus

[Starwiz]

I have the Baader Mark-III MPCC Coma Corrector.  It says it requires a back focus of 55mm.

Does this mean 55mm from the camera sensor to the coma corrector or 55mm from the camera sensor to the telescope's focuser?

Thanks

John

[runway77]

it's from the coma corrector to the sensor. In my case, with my scope, I needed 56,5mm to achieve optimal coma correction.

[Starwiz]

28 minutes ago, runway77 said:

it's from the coma corrector to the sensor. In my case, with my scope, I needed 56,5mm to achieve optimal coma correction.

Many thanks.  Now I can start playing with spacers.  😀

John

[michael8554]

Depends if you have the thin lockring in place or not:

 

 

 

[Starwiz]

22 hours ago, michael8554 said:

Depends if you have the thin lockring in place or not:

 

 

 

Thanks.

With that spacing, I can't focus.  Does that mean I have to move my primary mirror forwards somehow?

John

[michael8554]

3 hours ago, Starwiz said:

With that spacing, I can't focus. 

You mean you can't focus with 57.5 mm but can with 55mm ?

Michael

[Starwiz]

56 minutes ago, michael8554 said:

You mean you can't focus with 57.5 mm but can with 55mm ?

Michael

I can't focus with either.

John

[michael8554]

Well this is new !

You have an OTA in-focus problem, why are we talking about Coma Corrector spacing ?

Michael

[Starwiz]

18 hours ago, michael8554 said:

Well this is new !

You have an OTA in-focus problem, why are we talking about Coma Corrector spacing ?

Michael

Without my new CC, I've not had a problem achieving back-focus of 55 mm.

I've just checked it again.

So I need 57.5 mm back focus from the CC to the sensor as I'm not using the lock-ring.

I can achieve focus with a back-focus distance of 36.5 mm from the CC to the sensor with 12 mm of inward travel left in the focuser when it's in-focus.

To get the required back-focus for the CC, I need to add an additional 21 mm of spacing, but I have only 12 mm of inward travel to focus with if that makes sense.

Thanks

John

 

[alacant]

26 minutes ago, Starwiz said:

have only 12 mm of inward travel to focus

Hi

Push the cc into the focus tube. The position of focus is the same as without cc.

Or, post a photo of the setup.

Cheers

[Starwiz]

2 hours ago, alacant said:

Hi

Push the cc into the focus tube. The position of focus is the same as without cc.

Or, post a photo of the setup.

Cheers

Thanks.  I will try another experiment in the daylight tomorrow.

John

[Starwiz]

On 29/12/2020 at 15:43, alacant said:

Hi

Push the cc into the focus tube. The position of focus is the same as without cc.

Or, post a photo of the setup.

Cheers

So, without the CC in place, my set-up is as follows:

ASI1600mm-pro Camera  (6.5mm)

ZWO EFW (20mm)

12mm Spacer

16.5mm Spacer

Distance from sensor to focuser = 6.5+20+12+16 = 55mm

Focusing on a distant building on the horizon - without the CC it's in focus with the focuser about mid point in the available travel.

 

On inserting the CC, with the same spacing as above, the focuser had to be wound fully in, but it was still not enough to obtain focus.

 

 

Image with the CC and focuser wound fully in.

Thanks

John

 

 

 

Edited December 30, 2020 by Starwiz

[alacant]

On 29/12/2020 at 15:15, Starwiz said:

without the cc

So, fit the original focuser barrel collar (the one without the thread) and push the cc and spacers into the focuser.

Make sure that the spacers are correct for the 58mm of backfocus of course. As you have it, and assuming 20mm for the fw, you're not gonna be that far off; a 20mm m48 extension instead of the 16.5mm should get you close enough.

Cheers.

Edited December 31, 2020 by alacant

[Starwiz]

8 minutes ago, alacant said:

So, fit the original focuser barrel collar (the one without the thread) and push the cc and spacers into the focuser.

Make sure that the spacers are correct for the 58mm of backfocus of course. As you have it, and assuming 20mm for the fw, you're not gonna be that far off.

Cheers.

Thanks,  I will have another look at this tomorrow in the light.

John

[Starwiz]

On 30/12/2020 at 18:20, alacant said:

So, fit the original focuser barrel collar (the one without the thread) and push the cc and spacers into the focuser.

Make sure that the spacers are correct for the 58mm of backfocus of course. As you have it, and assuming 20mm for the fw, you're not gonna be that far off.

Cheers.

Many thanks for the suggestion.  I couldn't do it with the barrel collar as the CC wouldn't fit, but I got it working by pushing it straight into the focuser with no collar.

I've achieved a back-focus of 58mm by using the following:

Camera = 6.5mm

EFW = 20mm

Spacer = 16.5mm

OAG = 15mm

Unfortunately, the OAG camera won't focus with this arrangement.  The OAG is mounted next to the CC but even with the prism adjusted fully inwards, it still requires another 5mm inward travel of the focuser to get it in focus, which of course puts the main camera out of focus.

How essential is it to hit 58mm back-focus?  If there's not much leeway, I'll be going back to using a guide scope with a rather expensive spacer in the main optical train.

Thanks

John

 

 

 

[alacant]

3 hours ago, Starwiz said:

The OAG is mounted next to the CC but even with the prism adjusted fully inwards,

Hi

It should reach focus easily. The prism position is irrelevant. 

Post a photo so we can see what oag/camera you have.

Cheers

Edited December 31, 2020 by alacant

[Starwiz]

16 hours ago, alacant said:

Hi

It should reach focus easily. The prism position is irrelevant. 

Post a photo so we can see what oag/camera you have.

Cheers

Thanks, here it is.

With the focus position as shown, the main camera is in focus with a back-focus of 58mm from the sensor to the CC.

The guide camera focus can be adjusted by loosening the screw and moving it toward or away from the OAG.  As shown, it is fully inserted into the OAG with no more play, but to get the guide camera to focus, I need to move the main focus barrel inward another 5mm towards the telescope, which of course puts the main camera out of focus with the 58mm back-focus distance.

John

 

[alacant]

35 minutes ago, Starwiz said:

the OAG.  As shown, it is fully inserted into the OAG with no more play, but to get the guide camera to focus, I need to move the main focus barrel inward another 5mm towards the telescope

 

- Remove the extension/adapter from the camera/OAG to reveal a male m42 thread on the OAG (shot below)

- Screw the camera directly into tho OAG.

That's it.

 

To make life -a lot- easier, you may want to upgrade to this camera in one of these focusers.

 

 

[AlanP_]

Thinking of buying this coma corrector for my canon 600d and TS Photon 6" f/5. Where do you get a seperator to make it 55mm? I cant find it online.

[Starwiz]

1 hour ago, alacant said:

 

- Remove the extension/adapter from the camera/OAG to reveal a male m42 thread on the OAG (shot below)

- Screw the camera directly into tho OAG.

That's it.

 

To make life -a lot- easier, you may want to upgrade to this camera in one of these focusers.

 

 

 

SUCCESS!

I didn't know that bit came off.  Both cameras now in focus.

Thanks for the recommendations, I've been thinking of getting a better guide camera for a while as the ASI120mc isn't the best for this.

Many thanks again for your help.

John

[Starwiz]

11 minutes ago, AlanP_ said:

Thinking of buying this coma corrector for my canon 600d and TS Photon 6" f/5. Where do you get a seperator to make it 55mm? I cant find it online.

It depends what you have in your image train and the thread sizes.  I was able to make up the 58mm (optimum 57.5mm) from the sensor to the CC by using a combination of EFW, 16.5mm spacer and the OAG that I already had.  But you should be able to calculate what you need and find them online somewhere.

e.g.   https://www.celestron.com/products/m42-spacer-kit

I'm looking forward to trying it out on the stars now.

John

[AlanP_]

31 minutes ago, Starwiz said:

It depends what you have in your image train and the thread sizes.  I was able to make up the 58mm (optimum 57.5mm) from the sensor to the CC by using a combination of EFW, 16.5mm spacer and the OAG that I already had.  But you should be able to calculate what you need and find them online somewhere.

e.g.   https://www.celestron.com/products/m42-spacer-kit

I'm looking forward to trying it out on the stars now.

John

Hopefully you get clear skies soon! 

[alacant]

46 minutes ago, Starwiz said:

It depends what you have in your image train

Hi

It's a DSLR so all that is needed is this.

Cheers

Edited January 1, 2021 by alacant