I recently purchased an RC8 to compliment my 80mm Triplet. I was in the field last night working on fine-tuning the new scope and trained it on M31 as a target for testing purposes.
Being a typical refractor imager I never explored the collimation feature of SharpCap until now. This feature is noted to be experimental.
Questions-
1) Noting this is labeled to be experimental, is there a current measure in regards to both the realiability and accuarcy as related to its current functionality?
2) The manual references the use of a star field when using this tool. However, the example in the manual depicts the tool being used with an image of the Horsehead nebuala- Question: Is it best practice to use this function on a star field or a DSO?
3) It the manual it references the tools use on Newtonian and also discusses how coma correctors impacts the accuarcy of this tool. Since the RC design does not suffer from coma, it would follow then, this tool would lend itself well for the collimation of an RC scope. Is this an accuarate assumption? Can you speak to this?
4)- the attached image is plate-solved, my question is- based the the attached image, which is the center of the image- the galaxy core or the the central point of the X? Which leands me to my next question-
4) This is more of a clarifying question: If the glaxy core of M31 is the central point- If I am reading the manual correctly, does the X represent the Focuser Tilt, The Concetric Circles represent the tilt of the Primary Mirror and the Plus the tilt of the Secondary mirror?
4) Clarifying question- To use this feature, is collimation acheived when the PlUS and the X are centered on the central object along with acheiving a a consistent pattern of concetric circles?
Thank you for your help on this feature. To be transparent, I really like that this is a feature inlcuded in SharpCap. Is there an effort to perfect this tool beyond just an experimental phase?
Kind regards,
Brian
Questions for Robin-: The Collimation Feature
Questions for Robin-: The Collimation Feature
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Re: Questions for Robin-: The Collimation Feature
Hi Brian,
the feature is still labelled experimental because I never was really convinced that it helped much to actually adjust the collimation rather than just measure how good/bad it is. I suspect that the image of the horsehead was a result of me not having a handy screenshot when writing the documentation, so I would have gone through some images captured for other reasons with a Newtonian looking for one that displayed a reasonable circle/ellipse pattern to pop in the documentation. A star field would really be best.
The tool relies on off-axis stars being less sharp than those on axis. It doesn't really matter if this is coma or some other optical factor, as long as there is a signal that is related to the distance off the optical axis. I'm not really an RC expert, so not sure if the design has that sort of off-axis issue.
If you have shown the X reticule from scratch and not moved it then it points to the center of the image - the off center nature of the galaxy will be some sort of minor miscalculation in the plate solving/syncing/etc.
Assuming that we have a good signal, taking the 'X' reticule is the center of the camera sensor, we would take the '+' point as being optical center - ie where the optical axis of the telescope hits the sensor. The shape (ellipses rather than circles) comes as a combination of tilt of both mirrors and any focuser tilt - basically it shows that as well as not hitting the center of the camera, it is not hitting at exactly 90 degrees, meaning the focus varies across the image in the direction of the short axis of the ellipse.
Adjustments to get the 'perfect' pattern of circles centered on the 'X' would be a combination of
* Focuser tilt adjustment if you have it - not critical though, since you can adjust the mirrors to match the tilt
* Primary mirror adjustment
* Secondary mirror adjustment
There is no clear separation between what the primary/secondary adjustments do (at least not for a simple Newtonian and I am assuming an RC is the same). Adjusting either will move the optical center relative to the sensor (the '+') and adjust the tilt. However... since the light travels a longer distance from the primary to the sensor than from the secondary to the sensor, you get this effect :
Adjustments to the primary move the optical center ('+') more but affect the optical axis tilt less than the same adjustment to the secondary
So, essentially you would adjust the primary to center the '+', then adjust the secondary to try to get rounder contours (which would decenter the plus, hopefully by less), then the primary again to recenter, then the secondary and so on, each adjustment should get smaller as you home in on a combination that puts the optical axis of the telescope running through the center of the sensor at right angles to it.
cheers,
Robin
the feature is still labelled experimental because I never was really convinced that it helped much to actually adjust the collimation rather than just measure how good/bad it is. I suspect that the image of the horsehead was a result of me not having a handy screenshot when writing the documentation, so I would have gone through some images captured for other reasons with a Newtonian looking for one that displayed a reasonable circle/ellipse pattern to pop in the documentation. A star field would really be best.
The tool relies on off-axis stars being less sharp than those on axis. It doesn't really matter if this is coma or some other optical factor, as long as there is a signal that is related to the distance off the optical axis. I'm not really an RC expert, so not sure if the design has that sort of off-axis issue.
If you have shown the X reticule from scratch and not moved it then it points to the center of the image - the off center nature of the galaxy will be some sort of minor miscalculation in the plate solving/syncing/etc.
Assuming that we have a good signal, taking the 'X' reticule is the center of the camera sensor, we would take the '+' point as being optical center - ie where the optical axis of the telescope hits the sensor. The shape (ellipses rather than circles) comes as a combination of tilt of both mirrors and any focuser tilt - basically it shows that as well as not hitting the center of the camera, it is not hitting at exactly 90 degrees, meaning the focus varies across the image in the direction of the short axis of the ellipse.
Adjustments to get the 'perfect' pattern of circles centered on the 'X' would be a combination of
* Focuser tilt adjustment if you have it - not critical though, since you can adjust the mirrors to match the tilt
* Primary mirror adjustment
* Secondary mirror adjustment
There is no clear separation between what the primary/secondary adjustments do (at least not for a simple Newtonian and I am assuming an RC is the same). Adjusting either will move the optical center relative to the sensor (the '+') and adjust the tilt. However... since the light travels a longer distance from the primary to the sensor than from the secondary to the sensor, you get this effect :
Adjustments to the primary move the optical center ('+') more but affect the optical axis tilt less than the same adjustment to the secondary
So, essentially you would adjust the primary to center the '+', then adjust the secondary to try to get rounder contours (which would decenter the plus, hopefully by less), then the primary again to recenter, then the secondary and so on, each adjustment should get smaller as you home in on a combination that puts the optical axis of the telescope running through the center of the sensor at right angles to it.
cheers,
Robin
Re: Questions for Robin-: The Collimation Feature
Thank you Robin for your response and providing clarification in regards to the use of this feature. This information is very helpful in moving forward with trying to acheive collimation with my RC8.
In my situation, I found a machinist on a local forum who developed a decoupler for the RC style scopes. The decoupler seperates the the focuser from the primary mirror allowing for the focuser and the primary mirror to be adjusted independetly from eachother.
An added bonus, when slewing the OTA, because the focuser is no longer attached to the primary mirrir cell, the pull on the primary mirror cell from weight changes from the focuser changing positions no longer impacts the primary mirror collimation.
I think the collimation tool of SharpCap is very interesting because, after getting additional clarification on how to use it, this now becomes a very user friendly feature because the data is represented clearly, provides data on all three tilt planes and can be easiuly implemented in the field for making any final adjustments before implementing the imaging sequence. I would love to see this feature developed.
Thank you again
Brian
In my situation, I found a machinist on a local forum who developed a decoupler for the RC style scopes. The decoupler seperates the the focuser from the primary mirror allowing for the focuser and the primary mirror to be adjusted independetly from eachother.
An added bonus, when slewing the OTA, because the focuser is no longer attached to the primary mirrir cell, the pull on the primary mirror cell from weight changes from the focuser changing positions no longer impacts the primary mirror collimation.
I think the collimation tool of SharpCap is very interesting because, after getting additional clarification on how to use it, this now becomes a very user friendly feature because the data is represented clearly, provides data on all three tilt planes and can be easiuly implemented in the field for making any final adjustments before implementing the imaging sequence. I would love to see this feature developed.
Thank you again
Brian
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Re: Questions for Robin-: The Collimation Feature
Hi Brian,
wow, that's certainly an impressive looking setup. I will be interested to hear how well the collimation results by following the SharpCap procedures stack up in the final images - if they are giving the correct results (not sure why they woudln't, but have never tested on an RC before) that would be great.
cheers,
Robin
wow, that's certainly an impressive looking setup. I will be interested to hear how well the collimation results by following the SharpCap procedures stack up in the final images - if they are giving the correct results (not sure why they woudln't, but have never tested on an RC before) that would be great.
cheers,
Robin