41 replies to this topic

[flolic]

You can easily screw up primary-secondary distance just by driving primary collimation screws too far in or out. In this RC system every millimeter counts, and 1mm difference in mirror spacing translate to roughly 10mm difference in FL.

 

On my scope calculation shows that I also have FL of 1586mm (7.4um camera pixels, measured pixel scale of 0.962 arcsec/pixel). I must measure focal plane distance from rear cell to be sure.

Edited by flolic, 15 September 2015 - 03:14 AM.

[rainycityastro]

You can get the spacing just right by using a ronchi screen at eyepiece position and viewing a real star, slightly in and out of focus, you will see 3-5 perfectly straight lines ideal spacing position.

 

This method is recommended by Planewave for their collimation procedure.

 

When I did this test on my RC, I saw perfect straight lines, so I didnt bother changing anything. 

[darkairmouse]

Aligning primary with collimator

 

Adjust primary collimation screws so that the reflected image of cross hair is perfectly aligned with the collimator. See the center of attached picture. Ignore the laser spots for now.

 

When you say "Adjust primary collimation screws so that the reflected image of cross hair is perfectly aligned with the collimator." did you mean the 4 short bars must be with the same length, also their left/right and top/bottom end are on the same ring;  or they just overlap with the reticle on the target but not necessarily being the same length?

Thanks!

[rainycityastro]

Yes,  left/right and top/bottom have to be on the same ring. 

 

If they are not of the same length, there might be some mechanical asymmetry in the system. I believe the inner edge of the cross hair is obscured by the secondary baffle ( I could be wrong on this). If so, only the outer edge which is unobscured needs to be considered and they need to be on the same ring. 

[Dan McConaughy]

I have a 16" f/12 R-C and follow the method suggested by RCOS, where I use the Takahashi collimation scope.  This method is so close that it requires only a very little collimation adjustment of the primary on a star, which I can do in a couple minutes.

 

How is the Hotech collimation device and procedure better than the 'Tak method'?

 

Thanks

 

Dan McConaughy

[rainycityastro]

If you are getting great results with a Tak Collimation scope, then congrats! Your optical axes are closely aligned to your mechanical axes.This is the way a scope ought to be when it ships out of a factory and reaches the customer.

 

The Tak collimation scope helps align the secondary so that it is orthogonal to the axis of the focuser tube.  It assumes a few things here:

1. That the dot was accurately placed on the center of the secondary.

2. Primary did not suffer from any miscollimation (decentering or axial tilt) after shipping.

 

If you were to have a small accident (scope got shocked due to rough travel etc.), it is possible that the Tak collimation scope will lead to errors.

 

The only way that I know of aligning all optical elements in a scope without assumptions on mechanical alignment is through the use of a mechanism similar to what the Hotech provides. This is why many manufacturers today are actually using Hotech to do the initial alignment in the shop. They then use a sharpie to mark the center of the secondary (RCOS was doing something like this) and make other alignment marks to make it possible to put the scope back without a complicated star test.

[darkairmouse]

Thanks Ram!  I was wondering whether should I align the inner or outer end of those bars if they are not of the same length. I think that is the secondary holder/baffle that casts the shadow.

Finally I've got a clear picture about how should the collimation work with the Hotech, and just collimated my VMC200L again and it was quicker and easier, finally!!

Still need to confirm with star test (when weather permits). 

 

P.S. I have used Tak collimation scope on my AT6RC and the result was good enough. But I have never been able to use that Tak on this VMC200L - no center mark on the secondary is just one of those reasons, the others remain unknown....

Edited by darkairmouse, 21 November 2015 - 10:49 PM.

[anismo]

This seems to be the only documentation for RC collimation with this. I got the Hotech ACT hoping to collimate my AT8RC and have run into problem

 

I started with collimating it just as I would do a SCT which requires removing the secondary. The problem was that in AT8RC, to remove the secondary, you need to remove the top and bottom dovetails.. so the scope cannot be mounted and it has to be kept on a wedge.

After fiddling with it, I got the scope and collimator coalligned.  Since I was following the SCT collimation method from hotech's video, it didnt require any primary adjustment (since the co-align itself got the reflected cross hairs to be in aligned properly).

Any adjustment to the focuser (I have the moonlite CSL 2.5) or the primary was causing both the primary mirror reflection cross as well as the relfection of the secondary and the hotech reflection mirror to move. 

The main problem turned out to be for secondary adjustment. The three dots could never converge in my case to even attempt the adjustment of the secondary. 

 

====

 

I restarted by mounting the scope onto to my spare 8SE mount (but this time I had to leave the secondary on). I also had the co-align done pretty quickly  and ran into the same "convergence of the 3 dot issue". Did anyone ever complete collimating an RC8 scope with this? I would appreciate any help on this.

 

 

 

 

 

 

[rainycityastro]

Hi Anis, 

Removal of the secondary is only possible in some SCTs, not on the RCs. I had the same issue. So for RCs, it becomes a slightly iterative process.

 

The 3 dots are not converging because the backfocus distance appears to be too close. It may be correct for your camera but for the partially reflective mirror you may need to add another spacer between focuser and the reflective mirror. This step is crucial.

 

 

 

 

This seems to be the only documentation for RC collimation with this. I got the Hotech ACT hoping to collimate my AT8RC and have run into problem

 

I started with collimating it just as I would do a SCT which requires removing the secondary. The problem was that in AT8RC, to remove the secondary, you need to remove the top and bottom dovetails.. so the scope cannot be mounted and it has to be kept on a wedge.

 

 

After fiddling with it, I got the scope and collimator coalligned.  Since I was following the SCT collimation method from hotech's video, it didnt require any primary adjustment (since the co-align itself got the reflected cross hairs to be in aligned properly).

Any adjustment to the focuser (I have the moonlite CSL 2.5) or the primary was causing both the primary mirror reflection cross as well as the relfection of the secondary and the hotech reflection mirror to move. 

The main problem turned out to be for secondary adjustment. The three dots could never converge in my case to even attempt the adjustment of the secondary. 

 

====

 

I restarted by mounting the scope onto to my spare 8SE mount (but this time I had to leave the secondary on). I also had the co-align done pretty quickly  and ran into the same "convergence of the 3 dot issue". Did anyone ever complete collimating an RC8 scope with this? I would appreciate any help on this.

 

 

p2.jpg

 

p3.jpg

 

p4.jpg

 

p1.jpg

[anismo]

Thank you Ram!. I did pull the reflective mirror back just by hand) and I noticed that they dots do converge but only to a straight line at the closes point and then they diverge away They never did converge to a single point. 

 

The problem with not being able to remove the secondary was that I couldnt get the reflected cross from the reflective mirror (which would other be obstructed) to collimate the focuser. I did do that after removing the whole secondary assembly earlier. But the main issue now is the non convergence of the dots.

The sky is supposed to clearup for couple of nights. I will check how far the collimation is off now (I noticed the secondary center dot is no longer in center when seen through a cheshire.. so I expect it to be pretty off now). 

 

I am ready to move to a different scope at this point.. Are the 10'' truss version of RCs easier to do this collimation procedure? I am also considering getting a Edge 9.25 or Edge 11 but they seem to have their own quirks..  sigh

[rainycityastro]

If the dots dont perfectly converge, your collimation may not still be spot on. It is important that the dots converge to a single point.  When they go through again and cast an image on the collimator surface(2nd pass) that amplifies the error and gets a very high degree of precision. 

 

Collimating the 8RC should not be that difficult. If you are able to 1) mark 4 points around the circumference of the tube front precisely equidistant both front and back of the tube and co-align collimator with tube and 2) get the primary aligned precisely to the tube as I described earlier, you are nearly there. It helps at this step to close the secondary with a shower cap or something like that to reduce additional reflections. 

 

The truss scopes might be a bit easier to collimate as you can see where the rays are going. But I used to have a 8" RC that I collimated very precisely using this approach. Dont worry about the fact that the 8RC doesnt have independent focuser tilt adjustment. That is not necessary if your scope itself is in good alignment.

 

I suggest you dont give up on the RC. It took me numerous attempts at collimation and a lot of time before I finally succeeded in achieving repeatable results: I bought numerous devices (several glatter laser collimators, tublug, artificial star) and researched the heck out of the topic. I found the hotech the only one that I could trust in the end.

[akulapanam]

If you are getting great results with a Tak Collimation scope, then congrats! Your optical axes are closely aligned to your mechanical axes.This is the way a scope ought to be when it ships out of a factory and reaches the customer.

The Tak collimation scope helps align the secondary so that it is orthogonal to the axis of the focuser tube. It assumes a few things here:
1. That the dot was accurately placed on the center of the secondary.
2. Primary did not suffer from any miscollimation (decentering or axial tilt) after shipping.

If you were to have a small accident (scope got shocked due to rough travel etc.), it is possible that the Tak collimation scope will lead to errors.

The only way that I know of aligning all optical elements in a scope without assumptions on mechanical alignment is through the use of a mechanism similar to what the Hotech provides. This is why many manufacturers today are actually using Hotech to do the initial alignment in the shop. They then use a sharpie to mark the center of the secondary (RCOS was doing something like this) and make other alignment marks to make it possible to put the scope back without a complicated star test.

I disagree with you on the Tak scope. It absolutely can be used for primary missalignment just a Cheshire. The goal is to get the secondary centered and then the primary reflection centered in the eyepiece. The glatter holographic laser approach also works well.

I'm also confused on your TEC ADL. That scope is a cdk just like the planewave with a spherical secondary. On planewave, Tak Mewlon, and Orion U.K. versions they don't even let you move the primary because it is unnecessary.

[anismo]

If the dots dont perfectly converge, your collimation may not still be spot on. It is important that the dots converge to a single point.  When they go through again and cast an image on the collimator surface(2nd pass) that amplifies the error and gets a very high degree of precision. 

 

Collimating the 8RC should not be that difficult. If you are able to 1) mark 4 points around the circumference of the tube front precisely equidistant both front and back of the tube and co-align collimator with tube and 2) get the primary aligned precisely to the tube as I described earlier, you are nearly there. It helps at this step to close the secondary with a shower cap or something like that to reduce additional reflections. 

 

The truss scopes might be a bit easier to collimate as you can see where the rays are going. But I used to have a 8" RC that I collimated very precisely using this approach. Dont worry about the fact that the 8RC doesnt have independent focuser tilt adjustment. That is not necessary if your scope itself is in good alignment.

 

I suggest you dont give up on the RC. It took me numerous attempts at collimation and a lot of time before I finally succeeded in achieving repeatable results: I bought numerous devices (several glatter laser collimators, tublug, artificial star) and researched the heck out of the topic. I found the hotech the only one that I could trust in the end.

 

 

Thanks Ram., It is good to know that you have been able to collimate it with the Hotech ACT.   It is just annoying to deal with collimation problem during  clear nights which are way harder to come by lately. I will re do the collimation procedure and update. 

[rainycityastro]

 

If you are getting great results with a Tak Collimation scope, then congrats! Your optical axes are closely aligned to your mechanical axes.This is the way a scope ought to be when it ships out of a factory and reaches the customer.

The Tak collimation scope helps align the secondary so that it is orthogonal to the axis of the focuser tube. It assumes a few things here:
1. That the dot was accurately placed on the center of the secondary.
2. Primary did not suffer from any miscollimation (decentering or axial tilt) after shipping.

If you were to have a small accident (scope got shocked due to rough travel etc.), it is possible that the Tak collimation scope will lead to errors.

The only way that I know of aligning all optical elements in a scope without assumptions on mechanical alignment is through the use of a mechanism similar to what the Hotech provides. This is why many manufacturers today are actually using Hotech to do the initial alignment in the shop. They then use a sharpie to mark the center of the secondary (RCOS was doing something like this) and make other alignment marks to make it possible to put the scope back without a complicated star test.

I disagree with you on the Tak scope. It absolutely can be used for primary missalignment just a Cheshire. The goal is to get the secondary centered and then the primary reflection centered in the eyepiece. The glatter holographic laser approach also works well.

I'm also confused on your TEC ADL. That scope is a cdk just like the planewave with a spherical secondary. On planewave, Tak Mewlon, and Orion U.K. versions they don't even let you move the primary because it is unnecessary.

 

The gold standard for collimation is a star test: essentially parallel rays of light coming into a telescope. You adjust the optical axes of primary and secondary so as to be coincident.

The hotech simulates this process and takes it one step further by making it double pass so as to magnify the error. 

BTW Planewave uses the Hotech collimator to do their collimation in shop prior to shipping. So does TEC. Quite possibly several the other manufacturers do as well. 

 

Also, If you happen to have basic misalignment of your mechanical axes (center spotting of secondary is not accurate or not present), a tak scope will not help you here. Same with the glatter collimator. 

 

I think you have a misunderstanding as to the need for adjusting the primary for spherical secondary scopes. The axes of cassegrain type scopes need not just be parallel but coincident. This applies regardless of whether it is an RC or a CDK.  If your primary is way out of alignment, there is no way out but to adjust it. Several scopes make the primary completely non adjustable after permanently aligning the primary to the mechanical axis. This is a reasonable design and TEC did use this approach in their ADL prior to my scope. They changed the design in addition to making the primary cell extremely robust when they built my scope.

[FredOS]

Hello, bringing back this thread to life as it seems the best one on the Hotech ACT. I was wondering once the collimator and scope are aligned what is the best sequence for a Honders-Riccardo (RH200):
- primarily is adjustable
- secondary cannot be removed and in fact shouldn’t be moved
- tip tilt plate on the back considering these are fast (F3) scopes

I have an opportunity to test the ACT in a few weeks but wasn’t sure how to use to adjust primary and tip tilt plate. Thank you

[wvreeven]

I just wanted to add my thanks for this topic. I have been struggling with the collimation of my 8" TS RC and very much with using the Hotech ACT laser collimator. This topic has given me new hope that it can be done and I think I have gotten as close to perfect collimation as I have ever been. There still is room for some improvement and I think I can achieve that with even better alignment of the telescope with the lasers. This RC has a fixed focuser so there is no way to adjust that. Still, I think I am very close to perfection now and I can't wait to try this out!!!

 

 

 

 

Clear skies, Wouter

Edited by wvreeven, 19 June 2020 - 01:57 PM.

[Object_FS]

I followed the advice from another topic (and from Hotech manual too) and I have sent an e-mail to Hotech in order to get the detailed explanation to know how to collimate my TS-Optics 10" RC, but I never heard from them.

 

Complete noob here, this is the first time that I will collimate a scope, so I'm looking for detailed explanation for the procedure. I got this advanced laser collimator kit from Hotech because I heard very good things from it and I just have all the time in the word to spend around this. Besides that, I want the scope collimated as perfect as possible (I know, I know... the first times I'll collimate, they will be faaaaar from perfect).

 

Is there any other option like sending a private message to David from Hotech? Does he has an account here on Cloudynights?

 

Thanks

Edited by Object_FS, 10 January 2021 - 01:38 PM.