18 replies to this topic

[MichiP]

Hi, 

I know this topic has been discussed often, but it's hard to get a clear "this is it" answer. 
Which colli tool delivers perfect results on a 16" Ritchey-Chrétien? 

 

There is the Takahashi Collimation Telescope, the R.E.E.E.G.O. 2" LED Collimator and Hotech Laser Collimator and for sure many more, feel free to expand the list.

 

Is some also using the Howie Glatter laser collimator? 

 

What does really work with a 16" Ritchey-Chrétien?

 

Would be great to get insights from people who are calibrating their RC's.  :-) 

 

Thank you!

 

Best regards,

Mike 

Edited by MichiP, 02 July 2020 - 05:13 AM.

[polare70]

I use REEGO and I am very happy, easy to use and comfortable!

[Timo I]

I wouldn't use any collimation tool for exact RCT collimation. They are just not accurate enough for precise RCT collimation and rely too much on scope mechanics and optical axis being 100% lined up with each other.

Terry White has explained this info in more detail here: https://www.cloudyni...tien/?p=9958577

 

Here is a link for the DSI collimation method for all RCT scopes, which works very well: http://www.deepskyin...ure_Ver_1.0.pdf

[akulapanam]

I wouldn't use any collimation tool for exact RCT collimation. They are just not accurate enough for precise RCT collimation and rely too much on scope mechanics and optical axis being 100% lined up with each other.
Terry White has explained this info in more detail here: https://www.cloudyni...tien/?p=9958577

Here is a link for the DSI collimation method for all RCT scopes, which works very well: http://www.deepskyin...ure_Ver_1.0.pdf

Do you still need to finish on a star if you use a tool? YES

Do you still need a tool? YES

Best tools: Tak and glatter laser. The new Teleskop Express tool looks good too although it doesn’t have the Tak ability to magnify.

You guys are leaving a trail of frustration recommending the DSI method. If a person has a GSO scope collimating that way is virtually impossible because of the primary mirror mechanics. Also even most high end manufacturers where that method works well recommend against that method (Aluna, the original RCOS, Officina, DFM, etc...) are all in that camp. Personally I believe it has its place and I use it to finish the secondary BUT it should not be the first choice.

Edited by akulapanam, 02 July 2020 - 04:18 PM.

[Darth_Takahashi]

Cheshire eyepiece and a FarPoint laser collimator only because of its weight which nicely simulates my imaging train.

 

A sowing needle, piece of tin foil and a torch to make an artifical star.

 

Thats all folks. No magic tools needed.

 

Regards

 

 

Neil

[MichiP]

Thank you for sharing your thoughts. Seems I have to try out all the different solutions to find the best working one for me. 

I'm doing the collimation since years on the stars, but due to limitations in seeing, it would be great having a something that works during daytime especially in the winter month when it has -20°C or less

 

If I find "THE ONE" I'll let you know :-D 

[Timo I]

You guys are leaving a trail of frustration recommending the DSI method. If a person has a GSO scope collimating that way is virtually impossible because of the primary mirror mechanics. Also even most high end manufacturers where that method works well recommend against that method (Aluna, the original RCOS, Officina, DFM, etc...) are all in that camp. Personally I believe it has its place and I use it to finish the secondary BUT it should not be the first choice.

Touché, point taken...

I did not have time to the write down the necessary essay yesterday, so my short reply was quite inadequate. That is because "It depends..."

 

Let's suppose someone has bought his 1st RCT scope from some astronomy shop (probably GSO/AT/whatever-branded Chinese RCT). That device is quite well collimated as it left from factory, because these scopes tend to keep collimation fixed where ever that's adjusted. Here's my 10" GSO RCT scope's first light image without any reducer etc. As you can see it has almost perfect star field without any major flaws.

 

 

Then the little devil on your shoulder starts to whisper about lower left corner's slightly oblong star shapes.

Or maybe the free evaluating version of CCD Inspector software gives you something "less-than-perfect" test results for the same star field.

 

 

At that stage every fresh RCT scope owner should really stop there and think.

Also reading more basic information from RCT collimation from these forum messages (RCT collimation difficulties actually) would be highly advisable. All this before any new RCT owner makes any adjustment to his RCT collimation. This new RCT owner might even have some old collimation tools in his possession due to his previous scope history. Maybe one beam laser collimator, which he eagerly tries on the new scope's focuser.

He thinks "Well I have collimated Newtons or I have collimated SCTs or whatever scopes before... It seems that my laser tells that this little RCT has a very slightly mis-collimated secondary, so I can finish its' collimation in no time.

Let's try it before taking this baby under the stars.."

 

I myself even had purchased this Takahashi collimation scope with the RCT scope, because it's a well known high-quality and very accurate collimation device. I had also my Howie Glatter holographic laser with many different attachments available in my usage too. (All these were very high quality collimation tools, just like CatsEye XLKP etc. Cheshires.) When I put them into this new RCT scope's focuser, won't these tools give me better collimation to my new RCT scope, because I know what I'm doing with them? Right? But what good did they make there with my RCT collimation at that stage... nothing, but ruined my nearly perfect RCT factory collimation

 

Eh... you can see where this leads, because all astronomy related forums around the globe are filled with queries for RCT collimation issues. And whenever more information comes from the message thread, then the story goes most often this way: "I have used my collimation tool X with my new RCT scope and now the star field has weird star shapes. I have tried to use my scope's collimator tools to get my secondary/primary mirror collimated... etc. and I'm getting nowhere. Please help me!"

 

My humble wish here is that every new RCT owner would at least read this DSI guide ("A Procedure for Collimating Ritchey‐Chrétien and Other Cassegrain Telescopes") and then just take some test pictures from star field with their RCT/camera combo (=without taking any adjustments to their RCT scope). DSI guide helps oneself to understand the secrets about RCT optics and guides you how you can use these aberrations into your advantage. Highly recommended, IMHO.

 

That quide has also an excellent paragraph called "Problems with the Traditional <collimation> Method", which describes exactly ALL the mechanical issues, which these Chinese RCTs might have and why the usage of any tradional collimation tool is highly likely to fail with these RCT dual-hyberbolic optics. In addition to these scope mechanical flaws, there's always the unavoided user-error possibility, when beginner is fixing his tradional collimator tool into scope's focuser. For example getting this laser view from RCT is highly dependant from the Howie Glatter laser's own position in the standard Chinese RCT focuser. When you will know, that your collimation tool is attached to the focuser as straight as possible without any tilt??? For example I cannot quarantee my Glatter laser alignment in my Starlight Instruments FeatherTouch focuser even with Glatter's own Parallizer adapter. I know, that there's always a slight movement tolerance for attaching these collimator tools to any focuser without threads and this "allowed" tolerance is a bit too much for accurate RCT collimation needs, I think. You can get up to 90% collimation results with these tools, but what does it help, if your RCT scope had before some 96% collimation accuracy from the (Chinese) factory line?

 

I have often referred RCT collimation to "walking on the wire couple of meters above the ground level". There's a very narrow area, which keeps RCT collimation in balance and when you slip away from that perfect collimation point (optimists say "area" ), then your perfect collimation starts to fade away very rapidly.  And with any separete collimation tools I have tried here with my RCT I could not get my RCT scope fully collimated (ie. better collimated than with this "frustrating" DSI method).  These generic collimation tools are good enough for getting your focuser lined up and check your basics with RCT collimation. You can probably get your RCT optics rougly (maybe up to 90 %) collimated with these tools but nowhere closer, because they usually fail for the reasons described in the DSI collimation method guide. (Disclaimer: you can be very glad, if you can disgree with myself here on RCT collimation tools, because that would help things a lot. But no such luck happened here...)

 

Anyway, RCT fine collimation can also be adjusted with a cardboard peep-hole and "walking the line" (=optical axis) in the front (and back) of your RCT scope. No additional tools needed (just your eyes, cardboard and your hex keys + nearby neighbours, who think you're crazy ;-), because viewing those internal mirror reflections from different distances away from your RCT scope is a very very sensitive way of checking your RCT collimation during daytime. (It's actually the same principle for any optical bench used in those assembly lines for RCT scopes, I guess.) For the reference, I have received this kind of results with my daytime collimation attempts only, but the mirror adjustments during daytime are much harder to understand than in the night collimation with the DSI method.

Anyway YMMV... (it took years of practise from me to get here from there)

 

PS. Quote: "If a person has a GSO scope collimating that way is virtually impossible because of the primary mirror mechanics."

 

Is this really true? I had forgotten that aspect, because I had separeted my focuser from the primary, before I found the DSI method guide.

Edited by Timo I, 03 July 2020 - 12:31 PM.

[MikeECha]

Hi, 

I know this topic has been discussed often, but it's hard to get a clear "this is it" answer. 
Which colli tool delivers perfect results on a 16" Ritchey-Chrétien? 

 

There is the Takahashi Collimation Telescope, the R.E.E.E.G.O. 2" LED Collimator and Hotech Laser Collimator and for sure many more, feel free to expand the list.

 

Is some also using the Howie Glatter laser collimator? 

 

What does really work with a 16" Ritchey-Chrétien?

 

Would be great to get insights from people who are calibrating their RC's.  :-) 

 

Thank you!

 

Best regards,

Mike 

I am in the process of collimating my AT6RC. I took it apart to clean the mirror and to understand how it was designed so I can understand how do the collimation. 

 

I have been using is a long Cheshire collimation EP and I iterate between adjusting to secondary circle mark to be concentric with the Cheshire center spot and trying to get the other reflection concentric. I use the hall of mirrors to gauge how much I adjust the secondary screws at once. That keeps adjustments under control.

 

Last night I did star checks. All over the field star diffractions are round as far as I can tell, they are evenly bright and show a very tinny Poisson's dot on the center. I also tried to use Metaguide but the seen was very bad and the stars never made a useful shape. But the red square was always hovering on the center area.

 

One thing that I can not get to happen as indicated by a copy of Astroteck's collimation instructions for this scope is to get the outer most bright ring (the end of the focus tube) with the outer most black ring (the end of the optical tube).

 

This observation seems to coincide with the DSI principle of align the optical axis and forget about the mechanical condition.

[Aucello]

I've owned a GSO 16 inch RC telescope for 4 years now, and I'm just now finally figuring out the collimation process.  My scope came from OPT under the brand name TPO. I was roughly 2 years into under-performance issues photographically (good, but not great) before I decided it was time to break down the OTA and reset the mirrors. The first thing I learned was that the secondary mirror was loose in its cell and likely had been since I took possession. It has an inclination to do this periodically, so I have now shimmed it.

 

I also have made some modifications to the secondary mirror assembly. I was never satisfied with the central Phillips screw so I made a replacement bolt (same 8mm , 1.25 pitch thread) that is longer (greater range for mirror spacing) and has a hex head for adjustment using a wrench. I was also underwhelmed with the short central spring that permits mirror tilt and inter-mirror spacing adjustments.  I milled an aluminum plate to attach inside the central housing of the spider and it provides a 3-point spring flotation system to support the secondary mirror assembly. The stronger, longer springs give me more stability and range with which to move the secondary mirror cell.

 

My collimation tools are A) a Howie-Glatter laser (red) with 3 attachments - the dot, the ringed hologram and the barlow; B) a 2" diameter cutaway draw tube; and 3) a Rhonchi screen eyepiece. My general collimation process is performed with the entire ensemble of attachments that I would use for astrophotography, such as telescope extension rings and electric focuser. The only thing missing is the camera. My process is as follows:

 

1.  With the secondary mirror assembly removed I place a piece of white plumber's tape over the central hole of the spider.  This acts as a translucent screen onto which I can see the laser dot while standing in front of the OTA. I put the laser/dot attachment into the focuser and observe how close the dot is aligned to the central hole on the secondary. If it is not accurately centered, I center it using the focuser tilt adjustment screws on the rear cell of the telescope.  The focuser tilt plate moves independently of the primary mirror position in the larger GSO RCs. This establishes a central mechanical axis onto which the optical axis will be placed.

 

2.  I attach the secondary mirror assembly. To check its rough alignment I use the laser/dot to see how closely the dot appears to the central circle of the secondary mirror.  I adjust the secondary mirror tilt (e.g., 3 tilt adjustment screws behind the spider) to center the dot.

 

3. I next check the primary mirror tilt using the laser/ring hologram attachment.  I project the rings onto a flat white surface (inside the dome of my backyard observatory) and look for concentricity of the rings.  Adjustments are made using the 3 primary mirror tilt screws on the rear plate of the telescope until I'm satisfied with the results.

 

4.  I bring the optical axis into alignment with the mechanical axis using a 2" cutaway draw tube into which I place the laser/barlow attachment.  When aligned, I will see the shadow of the secondary mirror centered on the white face of the barlow attachment. If not centered, I adjust the secondary mirror tilt screws until it is centered.

 

5.  I typically go through several more iterations of primary mirror alignment (using the laser/ring hologram) and optical alignment (laser/barlow) until I see no further adjustments needed.

 

Now comes the tough part: inter-mirror spacing. I initially do this photographically using plate solving.

 

1. With the camera focus having been established, I first take a short (2 minutes duration) picture of a general star field, making sure not to include any overly bright stars.  Such bright stars seem to create problems for plate solving software.

 

2.  I convert the fits file into a JPEG and upload the photo to www.astrometry.net.  The plate solving results (e.g., arcseconds of picture height and arcseconds of picture width) enable me to determine the focal length and ration of my system using a common algorithm.  The 16" GSO RC telescopes are specified as f8 systems.  My initial plate-solving results typically result in a shorted focal ratio (e.g., f7.89). Since this value is smaller than the RC specification, it suggests that I need to extend the focal length.  BEWARE - in a RC system you must DECREASE the inter-mirror spacing, contrary to intuition, in order to INCREASE the focal length. That is to say, you must move the secondary mirror closer to the primary mirror by turning the secondary central screw clockwise, pushing it forward. These aren't teenie-tiny adjustments either.  I had to complete a minimum of 2 complete revolutions of the central screw in order to see a meaningful change in plate solving results. After making any adjustment to the secondary mirror position YOU MUST recheck the optical axis alignment using the laser/barlow attachment to recenter the shadow of the secondary mirror cell.  Afterwards, take a photograph of your adjusted system for the next plate-solving exercise.

 

3.  After 6-8 such iterations of secondary mirror adjustments and plate-solving, I incrementally adjusted my focal ratio from f7.89 to f7.92, f7.98 and f8.00.  A confirmatory 5 minute photograph of the adjusted system gave me the best pinpoint star images I have recorded to date with this telescope.  But are we done?  NO!!!  Plate solving completes the COARSE adjustments necessary to achieve accurate the inter-mirror spacing.  The FINE adjustments are made as described below.

 

4.  The simplest way to confirm that we have the optimum spacing between the mirrors in an RC telescope is with the use of a Ronchi eyepiece.  I obtained my Ronchi eyepice from Gerd Neumann of Hamburg, Germany.  You simply won't find a better Ronchi eyepiece for the price.  Simply said, the inter-mirror distance is perfect when the photographic focal plane and the Ronchi null image reside in the same plane.  A Ronchi eyepiece when aimed at a bright star typically provides dark parallel lines within the disc of the star. These are visually seen as you peek through the eyepiece and slide the draw tube in and out.  As you move the Ronchi eyepiece farther away from the focus point of the star you will see more and more lines crowded together (they get skinnier) within the star image.  The closer you move the Ronchi screen to the focal plane you will see fewer and fewer lines, and they get wider.  When no lines appear, you are at the focal plane and you have achieved the Ronchi null image.

 

5.  I bring my camera to focus, but taking a photograph isn't necessary. I measure the distance between my camera sensor and a fixed point in the optical train, such as the rear plane of the focuser, and I make a note of that value.  Let's call that measurement Distance 1.  Next I remove the camera and place the Ronchi eyepiece such that the plane of the Ronchi screen is at the exact same distance (Distance 1) behind the focuser.  Now I look into the Ronchi eyepiece. If I see lines then I know I need to further adjust the inter-mirror distance. To figure out which way to adjust the secondary mirror, slide the Ronchi eyepiece forward then backward.  If the null image (the Ronchi lines disappear) appears when you slide the eyepiece forward (a measured value less than Distance 1), then you need to further increase the distance between the mirrors by making slight clockwise adjustments to the central screw of the secondary mirror.  If the null image is seen when you slide the Ronchi eyepiece backwards (a measured value greater than Distance 1), then you need to decrease the focal length of the telescope by making slight counterclockwise adjustments to the central screw of the secondary mirror. Always remember that after making an adjustment to the secondary mirror position YOU MUST recheck the optical axis alignment using the laser/barlow attachment to recenter the shadow of the secondary mirror cell.

 

6.  Keep repeating this process until you observe the Ronchi null image resides at the exact position for which you achieve optical focus.

 

With all good wishes,

 

Rick

[Timo I]

A huge thanks for writing all that down there, Rick!

What is really important with these RCT scope (and which Rick also emphazised there) is the secondary mirror alignment in the center of the tube. I never got that solved out in my RCT scope, but I got quite near the optimal position with trial and error. Rick's method described above is much more controlled way of managing this critical central position in the tube for secondary mirror. And all it needs is this simple modification, quite brilliant, indeed:

 

Quote:

I also have made some modifications to the secondary mirror assembly. I was never satisfied with the central Phillips screw so I made a replacement bolt (same 8mm , 1.25 pitch thread) that is longer (greater range for mirror spacing) and has a hex head for adjustment using a wrench. I was also underwhelmed with the short central spring that permits mirror tilt and inter-mirror spacing adjustments.  I milled an aluminum plate to attach inside the central housing of the spider and it provides a 3-point spring flotation system to support the secondary mirror assembly. The stronger, longer springs give me more stability and range with which to move the secondary mirror cell.

 

So whar Rick tells above, is that he has there an additional secondary mirror alignment for the aluminium plate alone (?), because this traditional tilting system makes mirror cell to move sideways and this way then alters this critical secondary mirror position in the tube. In your system the mirror cell gives only course adjustment for the secondary positition in the tube and then there's additional tilting screws for secondary mirror tilt alone (missing in that image from my scope). Right?

 

There's also exact procedure for using that Ronchi eyepiece for determining the inter-mirror spacing exactly. I have never seen that written, so thanks for this too!

Highly appreciated!

 

Quote:

With the secondary mirror assembly removed I place a piece of white plumber's tape over the central hole of the spider.  This acts as a translucent screen onto which I can see the laser dot while standing in front of the OTA. I put the laser/dot attachment into the focuser and observe how close the dot is aligned to the central hole on the secondary. If it is not accurately centered, I center it using the focuser tilt adjustment screws on the rear cell of the telescope.  The focuser tilt plate moves independently of the primary mirror position in the larger GSO RCs. This establishes a central mechanical axis onto which the optical axis will be placed.

 

Determining a central mechanical axis is also very important in the collimation process, because otherwise you might run into situation where your optical axis points for example to the corner of your imaging sensor. And when you try to aling mirrors (optical axis) with the DSI method, you will not get perfect results from this method either. This causes the "frustration for the DSI method" referred above. (In practise optics have been adjusted with the principle for minimizing RCT abberrations as it is described by DSI. But despite of this optical adjustment stars do not look exact pinpoints in the star field. Whate every RCT user needs to understand, is that there's two axis in a RCT scope: central mechanical and mirror optical. These two need to be aligned together somehow, without that your collimation attempts will not be fully successful.

Edited by Timo I, 05 July 2020 - 01:32 AM.

[akulapanam]

Touché, point taken...

I did not have time to the write down the necessary essay yesterday, so my short reply was quite inadequate. That is because "It depends..."

 

Let's suppose someone has bought his 1st RCT scope from some astronomy shop (probably GSO/AT/whatever-branded Chinese RCT). That device is quite well collimated as it left from factory, because these scopes tend to keep collimation fixed where ever that's adjusted. Here's my 10" GSO RCT scope's first light image without any reducer etc. As you can see it has almost perfect star field without any major flaws.

 

 

Then the little devil on your shoulder starts to whisper about lower left corner's slightly oblong star shapes.

Or maybe the free evaluating version of CCD Inspector software gives you something "less-than-perfect" test results for the same star field.

 

 

At that stage every fresh RCT scope owner should really stop there and think.

Also reading more basic information from RCT collimation from these forum messages (RCT collimation difficulties actually) would be highly advisable. All this before any new RCT owner makes any adjustment to his RCT collimation. This new RCT owner might even have some old collimation tools in his possession due to his previous scope history. Maybe one beam laser collimator, which he eagerly tries on the new scope's focuser.

He thinks "Well I have collimated Newtons or I have collimated SCTs or whatever scopes before... It seems that my laser tells that this little RCT has a very slightly mis-collimated secondary, so I can finish its' collimation in no time.

Let's try it before taking this baby under the stars.."

 

I myself even had purchased this Takahashi collimation scope with the RCT scope, because it's a well known high-quality and very accurate collimation device. I had also my Howie Glatter holographic laser with many different attachments available in my usage too. (All these were very high quality collimation tools, just like CatsEye XLKP etc. Cheshires.) When I put them into this new RCT scope's focuser, won't these tools give me better collimation to my new RCT scope, because I know what I'm doing with them? Right? But what good did they make there with my RCT collimation at that stage... nothing, but ruined my nearly perfect RCT factory collimation

 

Eh... you can see where this leads, because all astronomy related forums around the globe are filled with queries for RCT collimation issues. And whenever more information comes from the message thread, then the story goes most often this way: "I have used my collimation tool X with my new RCT scope and now the star field has weird star shapes. I have tried to use my scope's collimator tools to get my secondary/primary mirror collimated... etc. and I'm getting nowhere. Please help me!"

 

My humble wish here is that every new RCT owner would at least read this DSI guide ("A Procedure for Collimating Ritchey‐Chrétien and Other Cassegrain Telescopes") and then just take some test pictures from star field with their RCT/camera combo (=without taking any adjustments to their RCT scope). DSI guide helps oneself to understand the secrets about RCT optics and guides you how you can use these aberrations into your advantage. Highly recommended, IMHO.

 

That quide has also an excellent paragraph called "Problems with the Traditional <collimation> Method", which describes exactly ALL the mechanical issues, which these Chinese RCTs might have and why the usage of any tradional collimation tool is highly likely to fail with these RCT dual-hyberbolic optics. In addition to these scope mechanical flaws, there's always the unavoided user-error possibility, when beginner is fixing his tradional collimator tool into scope's focuser. For example getting this laser view from RCT is highly dependant from the Howie Glatter laser's own position in the standard Chinese RCT focuser. When you will know, that your collimation tool is attached to the focuser as straight as possible without any tilt??? For example I cannot quarantee my Glatter laser alignment in my Starlight Instruments FeatherTouch focuser even with Glatter's own Parallizer adapter. I know, that there's always a slight movement tolerance for attaching these collimator tools to any focuser without threads and this "allowed" tolerance is a bit too much for accurate RCT collimation needs, I think. You can get up to 90% collimation results with these tools, but what does it help, if your RCT scope had before some 96% collimation accuracy from the (Chinese) factory line?

 

I have often referred RCT collimation to "walking on the wire couple of meters above the ground level". There's a very narrow area, which keeps RCT collimation in balance and when you slip away from that perfect collimation point (optimists say "area" ), then your perfect collimation starts to fade away very rapidly.  And with any separete collimation tools I have tried here with my RCT I could not get my RCT scope fully collimated (ie. better collimated than with this "frustrating" DSI method).  These generic collimation tools are good enough for getting your focuser lined up and check your basics with RCT collimation. You can probably get your RCT optics rougly (maybe up to 90 %) collimated with these tools but nowhere closer, because they usually fail for the reasons described in the DSI collimation method guide. (Disclaimer: you can be very glad, if you can disgree with myself here on RCT collimation tools, because that would help things a lot. But no such luck happened here...)

 

Anyway, RCT fine collimation can also be adjusted with a cardboard peep-hole and "walking the line" (=optical axis) in the front (and back) of your RCT scope. No additional tools needed (just your eyes, cardboard and your hex keys + nearby neighbours, who think you're crazy ;-), because viewing those internal mirror reflections from different distances away from your RCT scope is a very very sensitive way of checking your RCT collimation during daytime. (It's actually the same principle for any optical bench used in those assembly lines for RCT scopes, I guess.) For the reference, I have received this kind of results with my daytime collimation attempts only, but the mirror adjustments during daytime are much harder to understand than in the night collimation with the DSI method.

Anyway YMMV... (it took years of practise from me to get here from there)

 

PS. Quote: "If a person has a GSO scope collimating that way is virtually impossible because of the primary mirror mechanics."

 

Is this really true? I had forgotten that aspect, because I had separeted my focuser from the primary, before I found the DSI method guide.

Very nice essay.  I agree with your point that people are way too quick to try and recollimate.  Many issues are really tilt, a lack of a flattener, mis-spacing on a flattener, or a refusal to replace their focuser with a Starlight or better.  BUT at some point something will happen: a significant temperature change, an accidental bump in transport, a larger camera that shows issues more readily, etc...

 

The question is then what do you do about it.  My view is that you can get to that 90-95%+ range with the tool and then finish on a centered star without trying to adjust multiple mirrors with like 1/16th turn or smaller tweeks.  I also feel like the issues that DSI talks about just aren't that common with GSO truss 2.0+ and maybe in general.  Remember, a lot this was marketing at a time when GSO was just beginning to get popular.  Might be help to look at the issues people have using tools:

• lack of an upgraded focuser
• lack of a custom screwed adapter for their Tak scope
• not checking collimation with the scope at multiple angles
• not understanding just how centered the dot needs to be in the Tak
• trying to use a cheap laser or a 1.25" laser. Also a cheap focuser or 2" adapter comes into play.  
• not screwing the focuser down to its stops.  With the V2+ truss GSO scopes and a true 3", 3.5", Starlight or better you can forgo the entire laser centered on secondary if you are fully screwed down and you checked your focuser for tilt before attaching it to the scope.  Note: teleskop express agrees with this view
• not being able to differenate between sensor tilt and collimation error

Many of these items above will also impact the DSI method with the only difference being at least with a tool you can detect the problem

 

Yes, that is true that the primary is really rough.  For comparison a 1/8th turn on a GSO is about equal to a entire turn on my Officina.  The CFF I looked at in Linz seemed even finer.  Small adjustments are key with the DSI.  Also there is a nasty tendency to pinch your optics or worse snap screws/damage the threads on the primary.   

[Timo I]

I know this topic has been discussed often, but it's hard to get a clear "this is it" answer. 
Which colli tool delivers perfect results on a 16" Ritchey-Chrétien? 

There is the Takahashi Collimation Telescope, the R.E.E.E.G.O. 2" LED Collimator and Hotech Laser Collimator and for sure many more, feel free to expand the list.

Is some also using the Howie Glatter laser collimator? 

What does really work with a 16" Ritchey-Chrétien?

Regarding these questions OP presented here...

 

As you can read from above, there's pleanty of tools which could work at least partially with these RCT scopes, but they all depend on different things (mostly their user's ability to interpret the selected tool's results and limitations). I really like Howie Glatter's laser (preferably 2" sized one) with its' holographic attachments as one part of my collimation equipment. That kind of accurately collimated laser tool is needed for several steps described above. Then you would need additionally some more accurate tool for refining your collimation results obtained with that laser. That fine tool would be the Tak scope in my opinion too. If possible, this Tak scope should also have a custom adapter made for your specific RCT's focuser. I have used Tak's own 2" adapter for my RCT scope, but that gave me also some play when attaching it to my Starlight focuser (not a huge one though).

 

When someone has a factory collimated RCT in his hands, then he could check out his RCT's collimation with these expensive tools with certain reservations discussed above. This kind of adjustment should get a RCT scope into above 90% collimation accuracy (but it's highly likely that factory collimated RCT is already in that kind of collimation accuracy). Also using these collimation tools even slightly carelessly there's also a real possibility for ruining the factory collimation results, especially if the collimation adjustments have been too coarse and random. What is usually missing there, is user's knowledge about these tool limitations (+long time experience for RCT optics and its' high collimation accuracy requirements). Just like akulapanam noted above: "Small adjustments are key with the DSI." And that same applies to all collimation tools mentioned here for a RCT scope. I would say that this kind of very minor adjustment need can also be lost in the collimation tool's own play inside the scope's focuser. All RCT collimation tolerances are just that small. (Please remember, I'm supposing here there's no need to re-center secondary or make any additional user tweaks for such RCT scope.)

 

In the past I had usually cross-checked my RCT scope's collimation results with all tools and methods I had in my possession (the more, the merrier . If those tools gave me "a green light" on average, then I liked to evaluate the same collimation status with my plain eye views to my RCT scope (="walked the optical axis" in front and back of my RCT scope). When you know you have this kind of basic 90% collimation for your RCT in tact, then you would get almost perfect results for the night time images too. So in that perspective, you could get away without that DSI method in the end. But I think it's still a good way to continue past daytime collimation methods/tools and get higher accuracy from your RCT scope, which these separate tools cannot achieve. (In my own situation, where I had fully disassembled my RCT scope for different tweaks, DSI proved to be an invaluable method for final collimation adjustments.)

[Chris W]

When I was writing  The Astrophotography Manual, back in 2016, I foolishly decided to write a chapter on collimating a RCT. I gave myself a budget of 5 pages. 25 pages later, I reached a conclusion.

 

Very much like some of the earlier posts, there is a great deal of misinformation out there, or procedures that rely on mechanical attributes that have nothing to do with mirror alignment. Some get lucky, most do not.

 

My dilemma was that there were so many methods that I had to evaluate most of them, otherwise the one that I had missed would immediately be seized upon.

 

I break down the collimation into several phases: 

 

First phase is make your primary mirror as robust as possible. On my GSO RCT, I replaced the short push screws with longer, stainless pointy ones. These create a dent and resist lateral movement. Some of the adjustments are ridiculously tiny and you need some lubrication to reduce static fraction. My top primary bolts are locked and all adjustments are made with bottom ones. All primary adjustments are made with scope vertical, to avoid any kind of lateral movement. (The initial position of the primary mirror is made with the mirror dangling on the pull bolts, so it finds a natural center, equally depth gauged through the push bolt holes and then locked up with the pointy push bolts) 

 

[added] - check the primary/secondary distance/focal length

I used Pinpoint to platesolve to work out the scale of a roughly collimated image. My RC was short, at 1900mm against the 2000mm spec. To increase the focal length, you bring the mirrors closer together. I released the secondary lock ring and wound it out a few turns and repeated until I got close to 2000 mm. With the mirror spacing correct, I found the field was noticeably flatter.

 

I concluded different approaches for coarse, medium and fine tuning.

 

Coarse tuning

I used a HG laser to align the focuser tube to point at the center of the secondary mirror. I abandoned the Tak scope. It was redundant - as I found the natural diffraction rings reflected off the secondary and back onto HG laser also had the image donut of the secondary, which I could center with the refractions rings with secondary adjustments.

 

Medium tuning

I used the excellent DSI guide, referenced above, on an open cluster to set up the primary and tweak the secondary, using slightly defocused stars. You can use this method to iterate between primary and secondary adjustments, but, depending on seeing conditions, it can take quite a while. When you are close, it is very difficult to differentiate between a primary or secondary issue and you can go around in circles.

 

Fine tuning.

One of the simplest methods I found to get the secondary just so, was to use the hall-of-mirrors test. I think this was alluded to earlier, by a different name. In daylight, you sight down the scope and look at the multiple reflections. You adjust the secondary so the reflections are symmetrical about the orthogonal spider vanes. If this is the case, the mirror planes have to be parallel. This is very sensitive and works well, as confirmed retrospectively by a DSI star test. (My RCT does not have the added complication of lateral adjustments on either - just as well, as I have limited time on Earth.

 

I often find that if my RCT has been in storage for a while, I just need to do this final fine tune.  

 

I also tried the GoldFocus system. It is easy to use if you are just setting the secondary but not self evident for dual mirror adjustments. After a little prodding, Jeff added a dual-mirror adjustment process. It does work and is very sensitive. For best results it still requires good seeing conditions and a good deal of preparation/planning/concentration. It too is iterative and after a while, it is tempting to call it a night. I prefer to use it is as a handy confirmation tool.

Edited by Chris W, 06 July 2020 - 02:35 PM.

[Aucello]

As a follow-up to my previous post, I thought I'd provide some pictures of my secondary unit modifications.

 

Image 1.  This is a side-by-side comparison of my 3-point (springs) flotation device and central bolt versus what was provided with the telescope. As you can see I have created a more stable, solid suspension system for the secondary unit that provides greater range of distance for moving the unit.  The springs on my device were later shortened to approximately three-fourths of their original length.

 

Image 2.  This shows my 3-point flotation device and central bolt in the mounted position on the telescope spider housing.

 

Image 3.  The 3 socket screws seen here represent the attachment points for mounting my flotation device.  I drilled these 3 holes in the spider plate to match up with the threaded holes in my device plate.

 

Image 4.  With my flotation device mounted to the spider assembly, I do a final check of the mechanical axis alignment to ensure that the focuser tilt plate remains on-axis with the central hole in the spider. Here you can see the dot from the Howie Glatter laser as it travels from the focuser, through the spider central hole, and projects onto my hand.

 

With all good wishes,

 

Rick

[Darth_Takahashi]

Hi Rick,

 

I'm not totally sure what the springs do? The adjustment bolts already push against the back of the secondary to align it?

 

The central bolt modification with the machined collar to ensure everything is centered is a nice idea and one that I will definately implement.

 

Thanks for sharing the images.

 

 

Neil

[luxo II]

Is some also using the Howie Glatter laser collimator? 

Mike, although not RC's, I used a Glatter laser collimator with the holographic cross pattern to precisely re-align the primary mirror of my Rumak after adjusting the mirror spacing (which meant moving the primary mirror back about 1.5mm). The secondary mirror is aligned the usual way visually and with a star test.

 

For the primary though, the challenge is to get the optical axis concentric with the centre of the secondary. With the collimator inserted in the back of the scope I placed a sheet of wax paper over the front so the lines could be seen. Two sets of crosses were visible on the paper:

 

- one set of crosses straight from the collimator, and

- a second set that bounced from the secondary to the primary and then to the wax paper.

 

It also helped to draw a circle corresponding to the secondary and accurately mark crosses (in ink) on the paper to show where the laser lines should be when centred.

Edited by luxo II, 08 July 2020 - 05:46 PM.

[MichiP]

Hi everybody,

 

Thank you so much for your insights and time to write those great to understand explanations. 
I will definitively start with the DSI guide and see where I end up and if other tools are needed to get the final adjustments. 

 

Cleas skies

Mike

[Aucello]

Hi Neil:

 

Thanks for your comments. In theory, the springs are not essential.  But their purpose is to provide a holding force that keeps the assembly near horizontal, prohibiting the secondary assembly from significant tilt or wobble as adjustments are being made to the central positioning screw/bolt. My experience is that the spring(s) contribute stability to the positioning of the the secondary unit.  I'd strongly recommend using them.

 

Rick

[werper]

I've had very good success with a Howie Glatter laser with the grid hologram for "rough" collimation and a Takahashi collimation telescope for fine collimation on my TPO RC8.  The only mod I've done is replace the stock focuser with a Moonlite CS, which I ordered when I bought the scope - never ever used the stock focuser.