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Daytime collimation of a Cassegrain scope/RC

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#1 rainycityastro

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Posted 20 August 2015 - 12:57 AM

I've recently had the need to collimate my Cassegrain/RC style astrograph scope. While there are literally hundreds of posts describing ways to collimate the RC scope, no one has so far posted anything about using the Hotech Advanced CT Collimator to accomplish this process. I hope this write up is useful to those that wish to use this collimator.

 

Why the Hotech ACT collimator?

 

Basically the ACT collimator simulates a distant star beam by using three parallel lasers. Further it accomplishes a double pass collimation: the parallel beams of light go through the scope and converge at the eyepiece where a reflective mirror sends these beams back to the source. Any optical misalignments are magnified twice. Even the smallest misalignments are very easy to see and correct.

 

Unlike a star test, it is possible to diagnose your optical train component by component and determine what is out of whack (displacement or tilt) and by how much. 

 

I managed to get my TEC ADL300 scope badly out of collimation when it rolled around uncontrolled in the back of my car (thankfully in its case so no damage). After getting my scope thoroughly banged around, my dark sky trip last weekend turned out to be a nightmare. The scope was so terribly decollimated that nothing would come to focus.

After struggling to collimate for over 2 hours on an incredibly beautiful, dark, clear night, the best I accomplished was 5 arc sec FWHM stars. All the while I was scared of dropping my allen key on to the primary mirror while fiddling with the secondary.

 

David Ho of Hotech was very nice to speak with me on the phone for over 1 hour to walk me through the process of collimating this scope. Once I figured out the basic steps, it was reasonably straight forward.

 

As recommended by David for my scope, the basic steps I used were the following.

 

1. Align the scope truss with the collimator
2. Adjust the primary so that the cross hairs from collimator reflect back to collimator
3. Adjust secondary so that the 3 beam reflections are perfectly equidistant on the collimator face.

 

Depending on your scope a slightly different mechanism may be preferred.

For instance, if it is possible for you to remove the secondary, it will be better to align in the sequence primary, focuser and secondary bypassing the tube alignment altogether. I unfortunately could not do that. I believe this method can be used on the GSO/Astrotech RCs and you can get excellent alignment as a result.


Edited by rainycityastro, 20 August 2015 - 01:10 AM.

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#2 rainycityastro

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Posted 20 August 2015 - 01:01 AM

Aligning Scope with collimator

 

Locate 4 points perfectly at right angles to each other on front and back of scope. They would be the reference points. Be very precise and deliberate at this step. Use a metallic Sharpie pen or something similar to mark these spots on the scope. I initially marked the spot using a post-it note and after confirming it was accurate, I marked the scope with the sharpie. The picture below shows the markings and how it looks when the collimator is perfectly aligned with the truss tube of the scope.

 

20685748042_5cdedcdeb4.jpg


Edited by rainycityastro, 20 August 2015 - 01:12 AM.

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#3 rainycityastro

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Posted 20 August 2015 - 01:02 AM

20669405386_b9d8829b84.jpg

 

The cross hair lasers would be aligned with these points.

It is worth mentioning that this step usually trips a lot of people up. This is because there aren’t as many degrees of motion as necessary in a typical tripod and telescope mount set up.

 

You need three degrees of freedom for translation (X, Y,Z) and three axes of freedom for rotation (X, Y, Z). These axes may be shared between scope and collimator so you dont need them all on a single device.

 

I set up the telescope with counterweights parallel to ground (AP mount Park Position 1). 
I set it up to only operate the mount dec axis so as to provide rotation around X axis.

 

I also used a tripod with a geared center column (Z axis movement) and a macro rail that allowed (X axis movement).

 

The distance from collimator to scope can be fixed at around 1.5 scope lengths for maximum accuracy of the collimation procedure.

 

The collimator has movements to allow rotation along Y and Z axes. So between the scope and collimator there are enough precision controls to achieve accurate co-alignment very quickly.

 

At this step, about 80% of the work is done. This step should typically not take more than 20 mins once you figure out how the axes move relative to each other. 


Edited by rainycityastro, 20 August 2015 - 01:15 AM.

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#4 rainycityastro

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Posted 20 August 2015 - 01:04 AM

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.

 

20072498874_517353a538.jpg


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#5 rainycityastro

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Posted 20 August 2015 - 01:05 AM

Adjusting the secondary/focuser tilt

 

Adjust the secondary to get the 3 lasers to focus symmetrically on back of scope.
Now focus the scope so that the 3 lasers converge to a single spot. If the focuser has no tilt and focuser is mechanically at the center of the optical tube, the 3 lasers will be dead on in the center.

If there is any tilt in the focuser, it will be mercilessly revealed at this step. If your focuser allows it, now is the time to adjust the tilt so that the beams are centered on the partially aluminized reflector.

As a further check move the focuser back and forth. The beams will focus and defocus but always stay centered on the axis.

There is a slight bit of iteration (secondary adjustment and focuser tilt adjustment) required at this step. If your scope is mechanically excellent and you have accurately aligned the collimator with mechanical axis of scope, then there will be no tilt adjustment necessary. I didn’t need any adjustment at this stage.

 

20508939229_2db4f3eae6.jpg


Edited by rainycityastro, 20 August 2015 - 01:16 AM.

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#6 rainycityastro

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Posted 20 August 2015 - 01:07 AM

Adjusting the secondary

 

Aligning secondary with collimator involves moving the secondary screws until the 3 reflected laser dots align on the collimator face along the same circle.

 

See the picture below to see how it looks after the alignment is complete

 

20072498874_517353a538.jpg


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#7 rainycityastro

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Posted 20 August 2015 - 01:08 AM

Star testing

 

At this stage you can be quite sure that your scope is in good collimation. It is however still necessary to validate your work by doing a star test. My star test involved accurate focusing using a Bahtinov mask on a central star. I then measured the FWHMEccentricity using the Moffat4 function in Pixinsight. I used several 5 min subs to remove the effects of tracking and seeing.

The best subs were under 2.1 arc secs FWHM with Eccentricity under 0.30. This is about equal to the best results I have seen here and is I believe seeing limited. I did not touch up any collimation as a result of the star test.

I further checked whether there was any pattern to the tilt based on the eccentricity contour maps by checking these on numerous subs. There was no pattern that could be discerned and were completely random.



#8 deefree49

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Posted 20 August 2015 - 08:08 AM

Thank you for those excellent and very complete instructions! Of course one needs to acquire the laser collimation device but someone inclined to that level of accuracy should have one anyway.



#9 rainycityastro

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Posted 20 August 2015 - 04:41 PM

Hi DeeFree,

 

At this point I have spent a fortune on laser collimators/attachments and none of them really work properly for an RC type scope.They all rely on different parts being aligned properly with each other.

Since they dont have an external "correct" point of reference (like a star or parallel beams of light) they are all somewhat flawed.

 

The only collimator that has worked has been the hotech as it remedies the above deficiency of other collimators (laser or otherwise)

 

Of course it is quite costly. But for what you get, it is well worth the price IMO.



#10 Richard Francis

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Posted 22 August 2015 - 04:08 PM

Thanks for the pointer. In fact I've had one of these HoTech collimators for years:  I bought it for a Meade LX200 but, probably due to insufficient care on my side, I found the collimation was worse afterwards and I had to revert to a star test again. I never used it after that.

 

Now however I have an RC scope and I'm finding collimation a bit of a trial. The technique I've been using is a combination of a Howie Glatter concentric-ring laser and a Takahashi collimation microscope. It starts with the secondary and finishes with the primary.

 

So today I got out my HoTech collimator and after hours setting everything up nice and square, I could align the primary to the scope mechanical axis. Unfortunately I failed at the next step: the use of the 3 parallel lasers. My scope is a bit bigger than yours (360 mm diameter) and the 3 lasers nicely illuminate the back of the secondary ... they never make it through to the primary so that's the end of the road.

 

A pity. It looked like it could have solved my problems.

 

cheers,

Richard

 



#11 rainycityastro

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Posted 22 August 2015 - 07:14 PM

That's unfortunate. But I believe hotech makes collimators for larger scopes as well as custom orders. It will be worth calling them to find out more. 



#12 neilbuc

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Posted 11 September 2015 - 11:58 AM

Glad to see someone had posted of the pain that is using the Hotech ACT with a RC. The thing that has been so frustrating is the lack of decent instructions when collimating a two mirror system. The manual clearly stated to contact Hotech for instructions on collimating a RC. Why should that even have to be done? Just put it in the manual in the first place. It doesn't matter if its complicated. I have sent two emails regarding this asking for help and here NOTHING back since.

 

Since getting my 2nd scope (GSO 8"RC) with a Moonlite CSL in March I have yet to get it properly collimated. I have tried about every tool there is and although its OK it isn't great. Software reports can vary but it is often 23% off with also a tilt in there too.

 

So I was delighted when I seen this post, well done, very helpful. I have tried to take the methods out lined here along with the best of the other ones out there and hopefully after a star test (when clear) it will look better.

 

One thing I would love to know is how to get the primary co-aligned to the Hotech? That seems to be the first and key step. I have tried accurately measuring where the laser crosshairs hit the tube, but that didn't really lead to accurate collimation.

 

So today I took some of Jared ideas on another thread hear, plus this, and got what visually during the day looks good. 

 

When it is clear next I will be tried out the GoldFocus with Collimation mask I have to really dial it in. I spent hours with this last night but adjustments failed to make much difference as I don't think the primary was close to start with.

 

Jeff at Gold Astro was helpful and sent the complicated but concise instructions as soon as I asked for it. Round of applause for Jeff there.

 

There is now way to eliminate fully astigmatism in a RC system, but at least it can by symmetric and by design there is no spherical aberration or third order coma. Provided it is collimated properly....which is why we are all reading this!!

 

Clear Skies 

 

Neil



#13 neilbuc

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Posted 11 September 2015 - 12:09 PM

 

 

Depending on your scope a slightly different mechanism may be preferred.

For instance, if it is possible for you to remove the secondary, it will be better to align in the sequence primary, focuser and secondary bypassing the tube alignment altogether. I unfortunately could not do that. I believe this method can be used on the GSO/Astrotech RCs and you can get excellent alignment as a result.

 

See the part about removing the secondary so as not to have align the tube (not sure how I can do that with the GSO) any chance you can elaborate what you mean my that?

 

I tried this and messed it up somewhere. I had the Farpoint Collimated Laser in the focuser a checked to see it it was exiting dead center of the secondary central hole, which it was. However when I think about it isn't that just like checking the laser is on the secondary mark?

 

Thanks.

 

Neil



#14 rainycityastro

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Posted 11 September 2015 - 02:18 PM

 

 

 

Depending on your scope a slightly different mechanism may be preferred.

For instance, if it is possible for you to remove the secondary, it will be better to align in the sequence primary, focuser and secondary bypassing the tube alignment altogether. I unfortunately could not do that. I believe this method can be used on the GSO/Astrotech RCs and you can get excellent alignment as a result.

 

See the part about removing the secondary so as not to have align the tube (not sure how I can do that with the GSO) any chance you can elaborate what you mean my that?

 

I tried this and messed it up somewhere. I had the Farpoint Collimated Laser in the focuser a checked to see it it was exiting dead center of the secondary central hole, which it was. However when I think about it isn't that just like checking the laser is on the secondary mark?

 

Thanks.

 

Neil

 

Neil, If you are using the hotech, there is no need to use any other lasers or gadgets :)

 

I own the AT8RC, same as the GSO 8" RC. The entire secondary assembly comes off easily. Once you do this, the alignment process is exactly what David has on his youtube video. Align the primary, the focuser axis and then the secondary.  The cross laser allows you to get the primary perfectly orthogonal to the beam for primary alignment, similarly use the cross laser to align the focuser axis.

 

The secondary is aligned using the 3 return laser dots. For confirmation, ensure that the 3 laser dots converge to a point on the partially aluminized mirror at eyepiece position.

 

If the farpoint laser is exiting dead center in the secondary central hole, it means that the focuser is axially aligned with the secondary holder, not necessarily optically with the secondary (your secondary may be slightly misaligned with respect to the secondary central hole) . The hotech is the only device on the market that doesnt merely use mechanical identification marks to determine alignment and uses actual optics to do so.

 

Hope this helps.



#15 rainycityastro

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Posted 11 September 2015 - 02:36 PM

 

 

 

One thing I would love to know is how to get the primary co-aligned to the Hotech? That seems to be the first and key step. I have tried accurately measuring where the laser crosshairs hit the tube, but that didn't really lead to accurate collimation.

 

 

Hi Neil,

You dont need to measure anything for primary co-alignment. You simply need to ensure that the return cross hair beam from primary is symmetric about the center. This is easily said but requires movement on several axis (3 rotational and 3 translational) to get it exactly perfect. 

With SCTs, there is no primary adjustment. However primary mirror for RC can be out of alignment. So it is best not to try for primary co-alignment with the hotech. It is better to use the sequence below:

1. co-align tube with hotech

2. Align primary with hotech just using primary tilt adjustments. If this step doesnt work, primary is shifted inside tube. You may need to take the primary out and reseat it again. In 99% of the cases, you dont need to do this.

3. Align focuser with hotech just using focuser tilt adjustments. 

4. Finally align secondary using return dots.

 

BTW, The hotech is extremely sensitive to small errors and can measure tilts at the focus plane that are less than 0.1 degrees! You will get quite excellent results: well within diffraction limits even if you aren't absolutely spot-on.


Edited by rainycityastro, 11 September 2015 - 03:10 PM.


#16 neilbuc

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Posted 11 September 2015 - 04:40 PM

Thanks for the reply. I found the other thread too so will be trying this out for sure. I knew I wasn't doing something right but was struggling to find out the correct method. You said Davids YouTube video. Do you just mean the one were there C11 is collimator. Or is there one on a RC?

Clear skies.

Neil

#17 rainycityastro

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Posted 11 September 2015 - 05:10 PM

The C11 instructions will work great if you can remove the secondary. The only step I would use is to get the optical tube in reasonable alignment first. 

 

Not to add additional expense to an already expensive collimator but using a macro rail for lateral movement and a geared tripod center column will speed up collimation and improve precision dramatically. 


Edited by rainycityastro, 11 September 2015 - 05:10 PM.


#18 neilbuc

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Posted 11 September 2015 - 06:49 PM

The C11 instructions will work great if you can remove the secondary. The only step I would use is to get the optical tube in reasonable alignment first.

Not to add additional expense to an already expensive collimator but using a macro rail for lateral movement and a geared tripod center column will speed up collimation and improve precision dramatically.



#19 neilbuc

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Posted 11 September 2015 - 06:57 PM

I think I have it setup of my tripod correctly so that I can move in x, y and z axis between the az-eq6 gt and the horizon tripod and collimator head unit. It's actually pretty good I think after following the instructions here. I found the thread were someone had posted what was sent from David at hotech for a 10" Rc. The only issues I have is that the secondary cannot pass through the spider viens as it's too big.

One thing. So on doing a quick star test to check. If there was any on axis coma? Would I adjust the secondary only? I ask as it have tried this and it doesn't seem to make much difference to centering the defocussed donut. Sorry for all the questions but it is good to talk to someone who has the same gear.

All the best.

Neil

#20 rainycityastro

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Posted 12 September 2015 - 11:23 AM

If you have on axis coma, you need to adjust the primary. 

If your out of focus image shows the donut is not symmetric, then adjust the secondary. 

 

Hope this helps


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#21 neilbuc

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Posted 13 September 2015 - 06:36 AM

So here is a unprocessed 180s ISO800 on NGC 869 from last night. Pretty bad seeing to be fair. What do you think?

 

I have deffos messed up the Mirror Spacing after removing the secondary as astrometry.net reports back a FL of 1586mm. (1624mm) is what it should be. Is this too big a difference? It effects back focus. In fact it moved in so much I could not get an eyepiece to achieve focus.

 

I was thinking that now I have the Primary aligned maybe I can just adjust the lock ring on the secondary to move it back (or forwards don't know which) to see if I can get it to closer to 1624mm. I guess that is why they say not to touch the central screw.

 

All the best.

 

Neil

 

get.jpg



#22 neilbuc

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Posted 13 September 2015 - 07:45 AM

If anyone else happens to have a perfectly collimated 8" RC (Any brand)  I would love to hear what your spacing is. You will see from my pic mine is 38mm which translates to a FL of 1586mm, which should mean that I have some aberration going on (I think). It has certainly brought the focal plane too far in to be useful with a Moonlite 2.5CSL.

 

Clear Skies.

 

Neil

 

p.s. I have a nice chap in the UK going to measure his too as he has his scope pretty well collimated. Will keep you updated as people may want to remove the secondary so it would be nice to know how it should go back in. Why this info isn't available online I don't know.

Attached Thumbnails

  • GSO8RC Secondary Spacing for 1586mm.JPG


#23 flolic

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Posted 14 September 2015 - 06:31 AM

Neil, you can find drawing of RC8 system on the bottom of this page:

http://astro-foren.d...étien-203-1624/

 

Distances are between optical elements, so not too easy to check.

 

I can measure secondary spacing on my scope, but it was disassembled too many times and (still) not in a perfect collimation.



#24 neilbuc

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Posted 14 September 2015 - 07:01 AM

Neil, you can find drawing of RC8 system on the bottom of this page:
http://astro-foren.d...étien-203-1624/

Distances are between optical elements, so not too easy to check.

I can measure secondary spacing on my scope, but it was disassembled too many times and (still) not in a perfect collimation.


Cheers for that. Like you said tricky to get the optical distance measured.

Clear skies.

Neil

#25 glend

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Posted 15 September 2015 - 02:22 AM

Honestly I would not fiddle with the lock screw on the secondary, there are enough ways to screw up a RC already why add another one.  I seem to recall a couple of instructions on RC collimation that specifcally said don't touch that screw ring. I believe 90% of RC collimation problems arise from people trying to adjust the primary and then getting into the constant loop of correction because there is no solid aligned reference point.  Perhaps there are too many old newt people buying RCs and thinking that collimating them is the same and then they mess up the primary.  The best suggestion I ever saw was to remove the internal baffle tube so that you can see the entire vane and ring layout from the rear, this makes lining up the primary easy. Obviously to remove the internal baffle tube you have to remove the secondary assembly first (mark it so you know exactly how to reinstall it), and then remove the baffle tube, reinstall the secondary and go collimating from there. Once it's all aligned reverse the process and reinstall the baffle tube and secondary and do a star test to fine tune. Worked for me.


Edited by glend, 15 September 2015 - 02:23 AM.



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