I recently bought a coma corrector - as you can see on the enclosed image I had to pull it slightly out of the focuser on the scope to achieve focus. The distance needed would that be something normally solved with one or more spacers?
Focus with coma corrector
Hi Christian,
I also bought a coma corrector for my 80mm refractor, and yes, I also had to buy a set of spacer rings from Orion USA. It was $43.00US and was definitely worth it. The fit is very good (no slop), and it saves extending the refractor's focus too much to the maximum.
https://www.telescope.com/Orion-2M48-Extension-Spacer-Ring-Set/productVariantId/0/p/134735.uts
Hope this helps a bit.
- - Steve
I also bought a coma corrector for my 80mm refractor, and yes, I also had to buy a set of spacer rings from Orion USA. It was $43.00US and was definitely worth it. The fit is very good (no slop), and it saves extending the refractor's focus too much to the maximum.
https://www.telescope.com/Orion-2M48-Extension-Spacer-Ring-Set/productVariantId/0/p/134735.uts
Hope this helps a bit.
- - Steve
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Thx Steve, I suspected that.
Will get myself a set of spacers.
Will get myself a set of spacers.
Christian Bennich:
Thx Steve, I suspected that.
Will get myself a set of spacers.
Those won't help you there. You would need 2" external spacers to recoup those 20mm which is what the CC is moving the focus out to.
andrea tasselli:Christian Bennich:
Thx Steve, I suspected that.
Will get myself a set of spacers.
Those won't help you there. You would need 2" external spacers to recoup those 20mm which is what the CC is moving the focus out to.
Thx Andrea - I am a bit confused on the whole spacer/focus and what’s needed where 🤓🤓
I’m still a noob.
what’s the difference between your suggestion and the one from Steve?
thank you in advance 🙏
Christian Bennich:
Thx Andrea - I am a bit confused on the whole spacer/focus and what’s needed where 🤓🤓
I’m still a noob.
what’s the difference between your suggestion and the one from Steve?
thank you in advance 🙏
The CC will push the focus out by something close to 20mm (depending on the CC of choice) and it is also physically long which means that normal newtonian focuser do not have enough out-travel to cope with the new focus position plus you need also a little wiggle room to make focus adjustments, say a couple of mm either side of focus. Therefore you'd need spacers (with 2" inner diameters between the end of the focuser and the flange of the camera/CC to add to the length of the focuser draw-tube, in other words you want to extended the draw tube for the amount required to achive focus, with margins, outside its furthest reach.
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andrea tasselli:Christian Bennich:
Thx Andrea - I am a bit confused on the whole spacer/focus and what’s needed where 🤓🤓
I’m still a noob.
what’s the difference between your suggestion and the one from Steve?
thank you in advance 🙏
The CC will push the focus out by something close to 20mm (depending on the CC of choice) and it is also physically long which means that normal newtonian focuser do not have enough out-travel to cope with the new focus position plus you need also a little wiggle room to make focus adjustments, say a couple of mm either side of focus. Therefore you'd need spacers (with 2" inner diameters between the end of the focuser and the flange of the camera/CC to add to the length of the focuser draw-tube, in other words you want to extended the draw tube for the amount required to achive focus, with margins, outside its furthest reach.
So “tube extenders” rather than spacers?
Christian Bennich:
I recently bought a coma corrector - as you can see on the enclosed image I had to pull it slightly out of the focuser on the scope to achieve focus. The distance needed would that be something normally solved with one or more spacers?
Christian,
Have you verified your backfocus first? It doesn't look to me like you have any installed. That would likely explain why you can't achieve proper focus yet. Once you get a baseline 55mm (verify this for your coma corrector but it's pretty standard) it will shift the focus point, hopefully for the better.
I'm not sure what Andrea is referring you. You can simply 3D print spacers. I have this myself. But in your case, this just isn't the solution to your problem yet....
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Have you verified your backfocus first? It doesn't look to me like you have any installed
He has the same exact setup I have when using a DSLR (Canon): T-adpter and camera. Incidentally, the right backfocus has all to do with correcting the field coma and NOTHING to do with achieving focus.
My current setup:
I can achieve focus without any issues - but as I showed in my first image - I have to pull the CC slightly out from the focuser tube.
I would like to be able to insert the CC fully into the tube, which I believe Andrea's solution would allow me to do.
I am using the Skywatcher 200PDS with a 2" tube https://skywatcher.com/product/bkp-p200-ds/ - so I would assume that I could buy these 2
https://www.robtics.nl/en/adapters/7807-robtics-t-t2-threaded-5mm-extension-tube-spacer-ring.html
https://www.robtics.nl/en/adapters/7810-robtics-t-t2-threaded-15mm-extension-tube-spacer-ring.html
to achieve the additional 20mm on my tube.
- Canon EOS 50D
- T2 DSLR adaptor (M42x0,75) - https://www.astroshop.eu/t2-rings-lens-mounts/omegon-camera-adaptor-t2-ring-compatible-with-canon-eos/p,2416
- Coma Corrector - https://www.teleskop-express.de/shop/product_info.php/info/p6706
- Light pollution filter - https://www.astroshop.eu/broadband-filters/omegon-filters-light-pollution-filter-2r/p,15630
I can achieve focus without any issues - but as I showed in my first image - I have to pull the CC slightly out from the focuser tube.
I would like to be able to insert the CC fully into the tube, which I believe Andrea's solution would allow me to do.
I am using the Skywatcher 200PDS with a 2" tube https://skywatcher.com/product/bkp-p200-ds/ - so I would assume that I could buy these 2
https://www.robtics.nl/en/adapters/7807-robtics-t-t2-threaded-5mm-extension-tube-spacer-ring.html
https://www.robtics.nl/en/adapters/7810-robtics-t-t2-threaded-15mm-extension-tube-spacer-ring.html
to achieve the additional 20mm on my tube.
Christian Bennich:
My current setup:
- Canon EOS 50D
- T2 DSLR adaptor (M42x0,75) - https://www.astroshop.eu/t2-rings-lens-mounts/omegon-camera-adaptor-t2-ring-compatible-with-canon-eos/p,2416
- Coma Corrector - https://www.teleskop-express.de/shop/product_info.php/info/p6706
- Light pollution filter - https://www.astroshop.eu/broadband-filters/omegon-filters-light-pollution-filter-2r/p,15630
I can achieve focus without any issues - but as I showed in my first image - I have to pull the CC slightly out from the focuser tube.
I would like to be able to insert the CC fully into the tube, which I believe Andrea's solution would allow me to do.
I am using the Skywatcher 200PDS with a 2" tube https://skywatcher.com/product/bkp-p200-ds/ - so I would assume that I could buy these 2
https://www.robtics.nl/en/adapters/7807-robtics-t-t2-threaded-5mm-extension-tube-spacer-ring.html
https://www.robtics.nl/en/adapters/7810-robtics-t-t2-threaded-15mm-extension-tube-spacer-ring.html
to achieve the additional 20mm on my tube.
Where is your backfocus solution? TS GPU requires 55mm according to the page you just linked.
The backspace is necessary for this to work properly and is the distance from the sensor to the coma corrector. But you have a Canon EOS which has 44mm of backspace built in. In the camera world they call this flange distance and is the distance from the sensor (or film) to the front surface of the lens mount. Your T ring likely (but not always) adds the extra 11mm needed to get 55mm of backspace. Either way you are going to be within 1-2mm of this which you can fine tune later. So in this case, don't worry about backspace yet, it's close enough.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
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Joe Linington:
The backspace is necessary for this to work properly and is the distance from the sensor to the coma corrector. But you have a Canon EOS which has 44mm of backspace built in. In the camera world they call this flange distance and is the distance from the sensor (or film) to the front surface of the lens mount. Your T ring likely (but not always) adds the extra 11mm needed to get 55mm of backspace. Either way you are going to be within 1-2mm of this which you can fine tune later. So in this case, don't worry about backspace yet, it's close enough.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
Damn, 44mm?? I stand corrected. In that case, then yes, 3d printed spacers are easy to come by. If you don't have one, more than likely a friend has one. I did this exact solution for my coma corrector since I didn't have enough outward travel. It's a 10 minute print. If needed, I have an STL.
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Christian Bennich:
My current setup:
- Canon EOS 50D
- T2 DSLR adaptor (M42x0,75) - https://www.astroshop.eu/t2-rings-lens-mounts/omegon-camera-adaptor-t2-ring-compatible-with-canon-eos/p,2416
- Coma Corrector - https://www.teleskop-express.de/shop/product_info.php/info/p6706
- Light pollution filter - https://www.astroshop.eu/broadband-filters/omegon-filters-light-pollution-filter-2r/p,15630
I can achieve focus without any issues - but as I showed in my first image - I have to pull the CC slightly out from the focuser tube.
I would like to be able to insert the CC fully into the tube, which I believe Andrea's solution would allow me to do.
I am using the Skywatcher 200PDS with a 2" tube https://skywatcher.com/product/bkp-p200-ds/ - so I would assume that I could buy these 2
https://www.robtics.nl/en/adapters/7807-robtics-t-t2-threaded-5mm-extension-tube-spacer-ring.html
https://www.robtics.nl/en/adapters/7810-robtics-t-t2-threaded-15mm-extension-tube-spacer-ring.html
to achieve the additional 20mm on my tube.
Where is your backfocus solution? TS GPU requires 55mm according to the page you just linked.
That's a good and relevant question ;) - I would assume that the T-Adaptor would give me that.
But this is one of the areas that are still super confusing to me.
Joe Linington:
The backspace is necessary for this to work properly and is the distance from the sensor to the coma corrector. But you have a Canon EOS which has 44mm of backspace built in. In the camera world they call this flange distance and is the distance from the sensor (or film) to the front surface of the lens mount. Your T ring likely (but not always) adds the extra 11mm needed to get 55mm of backspace. Either way you are going to be within 1-2mm of this which you can fine tune later. So in this case, don't worry about backspace yet, it's close enough.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
Thanks Joe - I'm learning all the time
Christian Bennich:
I am using the Skywatcher 200PDS with a 2" tube https://skywatcher.com/product/bkp-p200-ds/ - so I would assume that I could buy these 2
https://www.robtics.nl/en/adapters/7807-robtics-t-t2-threaded-5mm-extension-tube-spacer-ring.html
https://www.robtics.nl/en/adapters/7810-robtics-t-t2-threaded-15mm-extension-tube-spacer-ring.html
to achieve the additional 20mm on my tube.
Maybe I didn't make myself clear, of which I do apologise. You would need spacers with in ID of 2" of the right length (~20mm) in between the focuser draw tube and the CC/Camera assembly (otherwise said, you need them to be fitted onto the CC body). Or you need to find a an extender tube (ID of 2") that would thread between the drawtube and the lock-down flange of the same so that in effect your drawtube is made longer and thus better supporting your imaging train. I have the same issue as you have so I know by direct experience what the problem is like, if I may be so bold in saying so.
Of the two solutions, the latter is the much better option if you can find the right hardware to implement it. In my case, I couldn't.
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Joe Linington:
The backspace is necessary for this to work properly and is the distance from the sensor to the coma corrector. But you have a Canon EOS which has 44mm of backspace built in. In the camera world they call this flange distance and is the distance from the sensor (or film) to the front surface of the lens mount. Your T ring likely (but not always) adds the extra 11mm needed to get 55mm of backspace. Either way you are going to be within 1-2mm of this which you can fine tune later. So in this case, don't worry about backspace yet, it's close enough.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
Damn, 44mm?? I stand corrected. In that case, then yes, 3d printed spacers are easy to come by. If you don't have one, more than likely a friend has one. I did this exact solution for my coma corrector since I didn't have enough outward travel. It's a 10 minute print. If needed, I have an STL.
So in your picture @Brian - I have added an arrow where I think the spacer goes AND an arrow on my image where I would believe the spacer should be in my set up.
Thank you guys for your help here - I feel VERY noob
Yes, that's correct.
Christian Bennich:Joe Linington:
The backspace is necessary for this to work properly and is the distance from the sensor to the coma corrector. But you have a Canon EOS which has 44mm of backspace built in. In the camera world they call this flange distance and is the distance from the sensor (or film) to the front surface of the lens mount. Your T ring likely (but not always) adds the extra 11mm needed to get 55mm of backspace. Either way you are going to be within 1-2mm of this which you can fine tune later. So in this case, don't worry about backspace yet, it's close enough.
I'll let others speak to the 20mm focus shift. I just wanted to ease your mind on one of the numbers being thrown around. But 3d printed spacers seems like a good option.
Damn, 44mm?? I stand corrected. In that case, then yes, 3d printed spacers are easy to come by. If you don't have one, more than likely a friend has one. I did this exact solution for my coma corrector since I didn't have enough outward travel. It's a 10 minute print. If needed, I have an STL.
So in your picture @Brian - I have added an arrow where I think the spacer goes AND an arrow on my image where I would believe the spacer should be in my set up.
Thank you guys for your help here - I feel VERY noob
Correct!
Additionally, I shall point out, the backspace requirements for most of field correctors around is found to be 55/56mm is because Nikon made their T-Thread adapters (to a 42mm flange) this way back in the olden days. Canon standard T/T2 Thread adapter follows the same specification and add 11/12mm to the backfocus (in addition to 44mm of the camera).