HOTECH Advanced CT Laser Collimator

[billhinge]

In case anyone is interested I just ordered one of these from astroshop.eu, expect a 2 week wait

http://www.hotechusa.com/category-s/23.htm

I have a C11 and I have not been quite happy with my collimation using a star so I decided I wanted something that was 100% reproducible and not reliant on the weather, plus my Bob's knobs always seem a bit tight in the wrong place when doing it in the dark

This is my best attempt using star based collimation but the difficulty is that the atmosphere never seems steady enough to get it spot on.

Possibly a bit expensive and a bit OTT for those blessed with better seeing but hopefully will give me some consistently good collimation. I'll let you know how I get on when it arrives...

[ampleamp]

How did you get on with this re accuracy, repeatability etc etc, and more importantly, have you seen a difference in view/images?

thanks

[MarksCPC]

I would also be interested to know how you get on with this

[billhinge]

I plan to give it a whirl at the weekend, I had some business trips and other things to sort out. Should have a free weekend this coming

I'll let you know asap, thanks for asking

[ampleamp]

Great, thanks. I know that they aren't cheap, but many people will have eyepieces more expensive (and cameras certainly are) so makes sense to get the best from your scope.

[MarkSix]

Please post a review in the equipment review section, I'm very interested in this device

[billhinge]

Just to let you know, I haven't forgotten, got promoted recently and old & new job was keeping me busy, I did have a go using the hotech last weekend but there was a bit of a learning curve.

I did think I had successfully achieved collimation but wasn't sure due to my interpretation of the user manual which seems to leave a few questions unanswered

( see user manual http://www.hotechusa.com/v/vspfiles/assets/images/ACT%20Laser%20Collimator%20Manual%20V8.pdf )

I emailed David at Hotech and got a reply within a few hours answering my questions and it seems I was actually successful in my first attempt

My collimation was completely out before I started since I had removed the corrector to clean it and then flock the tube. In the process I also messed with the secondary screws for good measure

I'm going to have another go this weekend armed with my new answers and I'm sure it will be simpler next time around

Actual hardware is good quality and I suspect once you know what a 'good result' looks like and how simple the set up actually is (the manual seems to make setup look longer than it perhaps needs to be) then next time will be easier

[billhinge]

I managed to get my C11 calibrated this afternoon, in all including setting up C11 on mount and Hotech the 3rd time around took about an hour for a badly out of alignment secondary

I couldn't use a solid floor as recommended this time as it was being used so I did the collimation in the hall on a wooden floor (not recommended as the laser is very susceptible to vibration and you have to wait between each tweak). Now that I have a good collimation I can refine it quicker when I do this again on a solid floor.

To get the collimation in the ball park I first looked down the tube from a distance to get the mirror reflections as concentric as possible but as I found out later this trick doesn't always work

When I started the process I had great difficulty getting the main reflections of the 3 collimating lasers to appear on the Hotech plate.

Apparently this can happen if the telescope is badly out of focus so I turned the focuser on the back of the OTA

As David from Hotech advised:

Q4: Ideal collimation distance:

a). Three collimation dots far apart:

- If the dots are far apart outside the target, please adjust the telescope focus to bring all three closer. When the three dots is on the same track (4.5) as the laser exit dots, it means the reflector mirror is positioned on the focal point. This is the ideal collimation distance.

. Co-alignment distance:

- The co-alignment distance is when you use the crosshair projection and the diffuser technique to line up both the collimator and the telescope in the first stage (like aiming & tracking a star during star collimation).

- The farther away from the scope the higher the accuracy you can achieve but the co-alignment became too difficult to aim square and the collimation became too sensitive.

- The ideal co-alignment distance is a little shorter than one tube length in front of the scope where the crosshair or the diffuser donut on the collimator target is about the size of ring 1 or 2

Once the dots were visible on the plate the rest was a simple but delicate matter of turning the secondary screws to get the dots aligned correctly

David says that accuracy to within +/- half a ring is acceptable, first time I spent ages getting them on the same ring, this time I got them better than half a ring but not quite perfect

One aspect that may cause some confusion is that the reflected dots are not perfect sharp dots but extended dots with diffraction rings especially if you do the test in the dark - this is what confused me the first time I did the collimation .

David explained,

Question: Are Large diffraction rings around the dots in the dark, normal?

Answer: The image has been expanded by the secondary mirror twice (in & out of the optical train) so you are referencing the far-field image of a laser dot. As long as you use the center of the dot as the collimation reference on the track will be fine.

Next time I will use repeat to collimation on a hard surface to get it critically accurate - this was essentially a practise run for the purpose of writing this record

Apology for photo but I was trying to reach across the optical path to take it and it was causing some floor vibration - you really need a solid floor

Other pic shows all the lasers with the Hotech sitting on a Berlebach Report 8023 Ash wood tripod (from FLO's clearance page) - you obviously need an adjustable tripod to hold the Hotech and the Report 8023 fits the bill nicely

Once I get the collimation perfect I'll give it a proper star test when weather permits.

One observation I made though is that looking down the tube the images do not appear perfectly concentric, it does say in the manual

"DO NOT use the center of the secondary mirror assembly as a crosshair centering reference since the secondary mirror might not be perfectly centered on the corrector plate. Referencing the center of the secondary as the pointing axis does not mean the primary is also on the axis. You can use it as a quick gross aiming, but not for final aiming adjustment. "

[KenB2]

Thanks for the write up - I too was wondering if this system was worth the money - it does sound like it might produce pretty reproducible results, once you get the hang of it...

Ken B.

[billhinge]

Thanks, for the record I did the callibration again on a solid floor more accurately this time and after looking down the tube there was a more concentric set of reflections at the finish plus a nice symmetric set of 6 laser dot reflections on the main mirror which I assume must be good

it does get easier to do with practise and usefully it can be done in 2 tube lengths of space provided you have a hard surface (does specify this in the instructions)

[alan potts]

i have to say the star test looks OK to me. I wanted one of the colimation lasers but the cost put me off a bit however I may still get one. I messed about and managed to get good colimation with my SC scope and smaller RC scope, the Mac/Newt i have never touched and it seems fine. My biggest gripe is that they always seem to make it difficult to do this job, why should anyone paying over 4000 in the case of the SC then have to spend on something to improve an allen key screw. Surely something else could be fitted in the first place to make it easier and it would be a selling point until it was copied.

Alan

[rodrigol]

I purchased one these Hotech CT units for my SCT and thanks to that and help from Hotech (David) I detected several defects on my CPC800: One was focuser centering related and was resolved using schimms. My scope has a Fastar secondary and so I could deal with this issue using the CT reflection mirror cross-hairs. The second issue was secondary/corrector centering. My CPC800 had no schims on the corrector. Testing it became obvious it could do with schimming to help place it/push it in one direction. Experimenting with 2 schimms resulted in full co-alignment of the optic train. Collimation of the secondary followed and a subsequent star test revealed a slight offset. This was corrected with a slight touch on one of the secondary collimation screws. This took 3 minutes.

I then pointed the scope at jupiter. I've never managed to clearly see GRS, 6 belts and Europa's transit's end. I was finally rewarded with these and realising the potential I expect of my CPC investment. I still need to fine tune a procedure to reproduce these results (or better) and must admit it requires patience and perceveranse during the co-alignment phase.

[DrRobin]

Hi,

I believe my secondary is not central with the main mirror after the holder came loose and have tried centering the secondary/corrector plate by sighting and using the main mirror, but I am not sure it is in the right place.  Any chance you could expand on how you centered yours using the Hotech Advanced CT as I have one of those as well?

Robin

[rodrigol]

hi Robin,

I'm not an 'expert' but followed the procedure outlined in the Hotech CT YouTube video 2.  I see you have an Celestron 1100 HD EDGE so you are in luck and wont have to do more than what is outlined in the video! Basically loosen the corrector holder and adjust the position of the corrector/secondary using the adjustment screws built into the corrector cell.

For other SCTs you may need to remove the corrector plate holder and look carefully around the edges of the glass for cork, cardboard or paper schimms. These may be 'worn out' or flattened by humidity. Locate the position of each schimm carefully if you want to examine these. It should be fairly easy to diagnose if the schimms need replacing: Place the SCT fully vertically. Using the secondary mirror holder try to move in all directions to see if there is play between the corrector and it's cell holder. If there is, you will need to replace the schimms. 

[rodrigol]

I think it is also worth noting that not having a very good co-alignment may lead you to think you have alignment issues in the optic train. I found this is really easy to misdiagnose when the setup is not optimal. I went through this and managed to convince myself I had a visual back/focuser problem and seconday-corrector plate misaligned! If you are noticing these things check, and check again that your co-alignment between the primary and the Hotech as really good. Another thing to be really careful with is If your SCT suffers from mirror flop. You need to co-align making sure you are only focusing in one direction. If you have an HD EDGE you are lucky as can lock the mirror in place. For other SCTs a focuser (e.g. Crayford) may be the way to go to prevent the effects of mirror flop caused by using the primary focuser knob. 

[DrRobin]

Thanks for all the help and suggestions. I took the corrector retaining ring off last night and noticed for the first time the four corrector adjusting screws, I never realised it had them. They all appeared to be tight, with the corrector a fair way over to the left, but reading on another board, this seems the norm for these scopes, so I don't think it has moved since it was set up.

The corrector has a single thick black alignment mark which is currently in the 3 o'clock position, again, I think this is the standard place so I don't think the corrector has moved or rotated since it was aligned.

What did come loose is the secondary holder, which I tightened, but I have never really been sure it was back in the right place or correctly orientated. I noticed last night that there was three thinner black marks in the 2 o'clock position on the edge of the corrector and assumed these were alignment marks for the secondary, but I am not sure how the secondary aligns on these. It looks like the centre mark is the notch or one of the adjuster screws and the other two adjuster screws seem to line up with the other two marks when the secondary is rotated in to what I believe is the correct position. I am going to check it again tonight, then as it is cloudy star and Jupiter test it on another night.

From what I have read horizontal/vertical and rotation of the corrector and rotation of the secondary are the most important with horizontal /vertical position of the secondary less so as small movements are compensated for by tilt of the secondary. There is a small amount of horizontal/vertical movement of the secondary holder in the corrector plate, so once the rotation is correctly set I can try moving it in to the optimum position.

I will let you know how I get on.

Robin

[rodrigol]

My SCT did not have any of these markings. I was lucky because I took picture of my corrector, and thus the position of the secondary irt the front. My secondary holder also went loose at one point and I use the pictures to ensure the cell and the secondary we're tightened in the same position they were when I took the pictures. Later on, I found that for Fastar secondary holders, the label is normally horizontal irt to the scope. There is a marker dent in the secondary holder you just take into account when slightly -carefully- tightening the holder in place. Too tight and you may break the glass of the corrector. Do a star test if the weather is good.

I really liked your web site and gallery. I envy your location and look forwards to your observatory build pictures.

[DrRobin]

Hi,

I checked it again, with the notch in the secondary pointing at the middle of the three marks the 'Fastar' label is just off level, pointing down to the right slightly. The three marks look evenly spaced so I assume that the notch in the secondary holder is meant to point at the centre mark, I might ask Celestron, the scope is only just outside warranty, so I should expect some help. If I point the notch at the most anticlockwise mark the Fastar label is very close to been level, with one adjuster screw pointing almost exactly straight up, but I am not sure that the position of the label or the screws is a good indication.

I remeasured the distance between the secondary holder to the edge of the OTA again, once the secondary was rotated, it looks like it is a little close to been central. I couldn't quite work out from what you said if it has a locating notch in the corrector plate, it it does, perhaps it has now jumped back in to the right place. I tightened the secondary holder when it came loose, it will still rotate/move with a little pressure, but is not so tight it will crack the secondary. Am ultra careful now when I remove the secondary and insert a Hyperstar, I don't tighten any more than just run up the bezel to make sure it doesn't move again.

I now know that even if the optics are rotated or even out of line it is still possible to get good diffraction rings on a star, but relatively poor resolving power, for example imaging Jupiter under high power.

I have got really slow at updating my website, but pictures of my obs are in this thread

http://stargazerslounge.com/topic/196833-an-imaging-obs-in-northumberland/?hl=+northumberland%20+obs

I got fed up with lugging an EQ6 and C11 out only to put it away early when the cloud came across, hence the small imaging obs. It wasn't the cheapest of solutions, but it does fit in the space available and so far seems to be doing what I want, it is also quick, once the scope is collimated and aligned (mount permanently powered, so normally ready to go), I can be up and imaging in around 10-15 minutes and pack away in about 5 minutes, it makes even short clear spells usable.

Robin

[rodrigol]

Hi DrRobin,

Thanks for the link to your obs work. That looks really nice and I just have to confess to having an envy attack just now ;-)

Sorry if I got you confused about the locator notch. What I meant was a notch in the secondary holder as depicted in the following post: http://stargazerslounge.com/topic/68351-c8-and-fastar-compatibility/. If you are almost certain of the correct orientation of the secondary holder irt to the scope you should proceed to check if the secondary is centred using the Hotech CT. From your scope's picture in your obs post it seems the fastar label is almost horizontal. For me, getting the secondary center aligned with the optic path was a revelation in resolving power when viewing jupiter.

I'm still lugging equipment  in and out as weather permits (done my back a few times) and would love to have a more permanent solution.

R:)

[DrRobin]

Hi,

I thought I would pass on a few comments about how I got on.

My Optical Bench

Okay, since my mount is now in an obs and I really wanted to be able to check the collimation inside and sitting down I decided to build a very quick optical bench.  Essentially both the scope and the Hotech are mounted on a piece of thick timber (4x2"), with the scope mounted on a thinner bit of timber (scope arm) that rotates around a coach bolt at one end.  I can therefore move the scope in the x direction by moving the scope arm.  To move the scope in the y direction I simple slide a wedge under the scope arm.  Note, I can only tilt up in this design.

The Hotech is mounted on the micro adjuster and sits above the normal optical path.  The scope movement is a bit crude, I had intended to use some screw threads to move and lock the scopes position but thought I would give it go without first.  It works okay, just need to nudge the scope arm to make small adjustments.  I used a spare mounting saddle to attach the scope to the scope arm.

The width of the scope arm and mating surface isn't really wide enough, so if you are not careful, the whole scope wobbles when you touch it, but it seems to go back to the neutral position, so just make an adjustment, wait 5 seconds for the scope to settle and check the lasers.

Testing

One thing that struck me, what happens if the secondary mirror is not central on the holder?  The Hotech relies on it been central as it uses the shadow of the holder to determine if it was central or not.  First stage to measure with a vernier the distance between the secondary holder and secondary mirror, mine was on;y 0.1mm out worst case, so I assumed that the mirror was central in the holder.

I first centered my focuser with the secondary removed and the Hotech.  Not only do I get the cross hairs returned, I also get a nice set of concentric rings, I think from the focus tube.  Anyway all looks good here.

Next I checked the secondary to be centered.  This is actually quite hard.  I actually photographed the Hotech face plate on my phone, then used a ruler to check it was centered on my phone screen.  You look for the outside of the cross hairs to tell if the Hotech and scope is centered and you use the inside of the cross hairs for the secondary.  Mine was actually a little bit out in both the horizontal and vertical directions.  I found it easier to take two photos one for the horizontal and one for the vertical plans since you can't get the camera square on.

Now putting on the three spots and adjust the secondary screws.  I tend to repeat all of the other adjustments if something doesn't centre.  So if I get three nice dots on the target, but the spot on the reflector is not central, I go back to the start and re-check everything.  It is amazing how far out something can be and for you not to notice.  Ah the benefit of doing it inside on the desk in the study.

Notice the three returned spots and the diffused diffraction pattern.  If the all line up on the same circle and the one in the middle is returned to the centre line, then you are there.  Notice a small offset to the right on mine, turns out the spot on the reflector was slightly out as well, so I went back to step 1 and repeated the whole alignment.

When it is very close to been perfect, or is actually perfect, I have noticed a small well defined series of rings on the target.  I think these come from reflection between the secondary and the target mirror.  I just can't quite get all three to line up, perhaps they never do, but certainly the appearance of these well defined rings seems to indicate good collimation.

As a final test, I removed the target mirror and replaced with a laser collimator.  I took out the secondary and saw my laser dot appear to be central with the Hotech.  I then put the secondary back in, the returned laser dot (diffused by the secondary) was almost exactly in the centre, certainly within a margin of error caused by the 5x magnification of the secondary.

Star Testing

Okay, no star testing last night, it was snowing, but next time I get chance I am going to follow Thierry Legault's collimation guide, it explains a lot about the pitfalls of star testing.

http://legault.perso.sfr.fr/collim.html

One other thing occurred to me, if your secondary is not exactly centered with the rest of the optical train then when you align on a star, you will incorrectly align the secondary to compensate for the position error.  Star testing without an airy disc will not tell you something is wrong and star testing with an airy disc will tell you something is wrong, but not what is wrong.

The proof will be in the views of Jupiter, but it looks like the weather has other ideas.

Robin

You can't even set your secondary with a ruler as there is no guarantee that the primary or even the focus tube is central. 

[billhinge]

I'll keep an eye on this, something tells me it maybe a while before some decent weather arrives so I may take your lead on this and follow your guide

[DrRobin]

I will. Let you know if I get a reply from David Ho about the fine diffraction rings. I will also let you know if all this effort improves my scope.

Robin

[DrRobin]

Hi,

Just thought I would share a photo of the corrector plate of my C11 Edge.  There is a thick black mark with the serial number etched on the front in the 3 o'clock position and then these three marks at about 2:30 position.  I am guessing these are alignment marks for the secondary, possibly they represent the three secondary adjuster screws?

Anyone know?

If I align the secondary screws up with these marks then when the shutter is closed to cover the adjuster screws, the Fastar label is about 23 degrees off level.  David Hinds told me that it is supposed to be level when the secondary is correctly rotated, but then again according to Celestron the EdgeHD series don't have alignment marks?

Robin

[rodrigol]

DrRobin you seem to have made some excellent progress and I really like your optical bench. When you get confirmation from David Ho about the concentric rings on the 3 return lasers as an indication of good collimation please let us know. Could you please let me know what type of focuser you have on your C11? Thanks!

[pyrasanth]

Looks like a  nice way to avoid faffing around with real stars & the weather! but a question for you.

I'm confused by the version 8 instructions posted in this thread which are completely different from the video instructions shown on the FLO website. The video tells you to remove the secondary mirror but the version 8 instructions use a shadow technique to align the secondary mirror's optical path- which method is correct? and I'm assuming that the video instructions can only be applicable to faststar telescopes as other scopes can't remove the secondary mirror from the corrector plate. Which method or instructions did you use for your telescope?