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Collimation woes with HoTech SCA laser collimator


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I've recently bought HoTech's SCA laser collimator after reading lots of very positive reviews. It seemed like a good match to my 12" dob as it meant that, in theory, I should be able to collimate it accurately and quickly all by myself. Unfortunately I think something is going awry.

After I have finished my collimation routine the laser is in the middle of the donut on the primary, and the reflected laser is exactly in the middle of the grid on the side of the HoTech.

Just to go over the steps quickly (in case I've missed something), here is my routine:

1/ Insert 2" adapter into focuser, insert HoTech into this.

2/ Tighten HoTech until I am unable to rotate the compression ring any longer (I assume this means the SCA has done its thing and centered the laser in my focuser)

3/ Switch laser on and look at the donut on the primary to see how far out the laser is

4/ Adjust the 3 allen key bolts until the laser is dead centre on the donut (note: I have noticed that the 3 bolts are all relatively loose to achieve this)

5/ Go to the end of the scope, and look at the laser grid on the side of the HoTech.

6/ Adjust the primary mirror (using the 3 knobs) until the reflected dot is bang in the middle of the targeting grid. In fact the dot 'disappears' in the middle of the grid - I can only see the crosshair surrounding it.

7/ Return to the secondary mirror and make any adjustments to re-centre the laser in the donut if necessary.

8/ Turn the HoTech off and remove the device.

However, when I do star tests and pull the star out of focus instead of seeing concentric circles I end up with a flat edge across the image. If you look at the letter 'C' and draw a line between the two 'ends' it will look similar to what I'm trying to describe.

I then decided to peer down the focuser, expecting to be able to see the 3 primary mirror clips. However, I could only see one, and directly opposite the clip I could see was the same black line cutting off the rest of the circle. I've edited a picture from Astro Baby's excellent article on collimation (I hope she doesn't mind - really should have asked first) and attached it below.

I'm pretty sure this is not right. My thoughts are I've either done something wrong, the HoTech is not completely accurate or I have a more serious problem with the mirror itself.

After spending a lot of time researching collimation I have noticed there are a few things I do NOT do, namely:

1/ Use the central screw on the secondary to move it up and down

2/ Use the locking screws on the primary

3/ Use a system of 'loosen one, tighten the opposite' when adjusting the secondary - I tend to move to each one in turn and try turning it CW, then CCW and see which direction moves the laser closer to the centre of the donut. I get each one as close as I can then move onto the next screw.

Could one/all of the above be the problem? If not, can anyone help me find a solution?

Thanks everyone.

post-16962-133877434273_thumb.jpg

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Your routine is identical to mine so it's not something you have done wrong.

If you are looking down your focuser and can only see one mirror clip then this tells me that your secondary mirror is not centered correctly.

What you will have to do is loosen of all three secondary screws and the centre screw only a little bit and turn the whole secondary until you can see all three mirror clips and then retighten the centre screw and recollimate.

Have you got a collicap to look down the focuser with?

Edited by Doc
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I would say you need to make your self a collicap....as your secondary mirror is not in the correct position? I made mine out of the blank that covers the focuser when not in use, or you can use a 35mm film canister. just find the centre of the blank/canister, drill a small hole in it and drop into the focuser and look through.

That should get you to be able to align the secondary.

I use both a chesire AND laser collimator. Laser is good as it's quick and easy but I use the chesire for making sure the secondary is properly aligned etc.

Hope this in some way helps, I'm sure some one will be along to give you a more sceintific answer ;)

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Thank you both for the suggestions. I do not have a collimation cap, so that will be my first port of call. I love the idea of using the blank - I actually have 2 (one for the 1.25" adapter and one for the 2" adapter) so I'll use one of them as a colli cap and the other as my dust cap.

Once this is on the focuser, and I've loosened the centre screw and the 3 adjustment screws, is it then a matter of looking through the hole in the colli cap and rotating the secondary until all three clips are visible?

From this point should I commence my routine or is there anything else I need to be aware of? I understood from reading some articles on collimation that before I adjust the 3 screws on the secondary I should first ensure the secondary is in the right place by loosening the centre screw and moving it up and down the tube. Is this necessary or is rotating the secondary enough?

Mick, do you tend to collimate every session? I am doing this currently but have read that actually the secondary should hold its alignment reasonably well for some time. I just wondered if I am fiddling with it unnecessarily.

Thanks again for the help!

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Thats correct make a little hole in the centre of the cap maybe 2mm. Insert this into the focuser annd look through it. Rotate the secondary mirror until all three mirror clips are visible and the secondary mirror looks as central as you can get it, then tighten centre screw.

Insert Hotech annd collimate as normal.

If you have it bang on your secondary will look circular not eliptical.

Mine being a 16" truss tube dob I find I have to collimate everytime I use it. After a while it becomes second nature to do.

If you feel brave on a rainy night just keep mucking your collimation up and learn how to put it right again, thats how I learnt.

Edited by Doc
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Thanks Mick.

Actually that's almost exactly what happened - I took the scope out but after about 10 minutes the sky clouded over. I wasn't happy with the views I had of Mars during those 10 minutes so decided to investigate, which prompted a lot of messing around after I realised only one primary clip was visible. Thing is I wasn't really sure what to change so tried all sorts, my thinking being that if I turned a screw the wrong way I could always turn it back again!

I think I'll have another go at home tonight with the colli cap. If it seems okay I'll deliberately mess the alignment up then start again, and repeat until I've got the hang of it. Ironically I thought I already had it sussed, and was so pleased with the HoTech because it made the process quick and painless. Just goes to show clever tech is useless unless the operator knows what information is going in and what should be coming out!

What benefit would a cheshire collimator bring to the party? Does it perform the same job as the colli cap but with greater accuracy or is it used for a different stage of the collimation process?

One other question, do you unlock the primary locking screws before adjusting that, then tighten them once you've finished? I've never actually used them!

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I bet Mars was rubbish because the scope hadn't cooled enough and you started fiddling and moved the secondary.

A cheshire is basically a sight tube with crosshairs, you just move your secondary until your 3 mirror clips are visible and then move your primary until the donut is ontop of the crosshair.

Locking nuts I never use always leave them loose I find they tend to move the primary abit.

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Adrian - when the laser puts a dot on the secondary before going to the primary - is that dot in the centre of the secondary? If it is it should show that the secondary is parallel to the focuser. Once that is achieved and you adjust the 3 bolts to place the dot on the donnut the returning laser dot should be in the same position on the secondary. If you have 2 laser dots on the secondary that is your problem.

I don't use my Hotech everytime I have made a collimation cap with tin foil on the inside to see the hole better.

Hope this helps if you understand my waffle.

Mark

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Mick, I think Mars was disappointing because of the poor conditions. I'm not sure how to describe it, but whereas the site I was at usually has a limiting magnitude of around 5.5 on this occasion I struggled to see the 3 stars that make up Orion's belt! At this point there were no clouds in the sky, I can only assume it was mist or something similar. Mars actually gave some detail (a dark scar slightly northwest of its equator) but I couldn't bring it into any sort of focus. As a rule of thumb, though, how long should I allow for the scope to cool? I usually leave between 30 and 40 minutes depending on the difference between the car and ambient temperature. I have also taken to driving to the site with all the windows open!

Mark, yes, that does make sense. I've not tried to see if the laser is centered on the secondary (both immediately after it leaves the HoTech and on its return journey) but would I do this by using a small mirror to look at the underside of the primary?

If so, at which point would I pop this step into my routine? Would I check and adjust (if necessary) at the same time I perform the check advised by Mick? (Rotate the secondary until all 3 primary mirror clips are visible and the secondary is a circle in the centre of the image through the colli cap). Thanks for the tip!

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Adrian. The first action that you must take is to centre the secondary mirror under the focuser so that you can see the whole mirror. Tilt the OTA to about 45 degrees and loosen the 3 allen bolts (not too much) then adjust the central screw until you have the 3 primary mirror clips visible in your focuser. I suggest the 45 degree angle because you don't want the secondary to drop on the primary. This is when a collimation cap is convenient. Once this has been achieved it is unlikely to change that much.

Now insert the Hotech and see where it strikes the primary - adjust to the donnut. Then return to the Hotech and adjust the 3 primary mirror bolts to bring the laser into the central hole.

Hopefully you will now be collimated. As Mick as stated do it a few times until you feel confident. At some stage you might consider some 'Bobs Knobs' rather than trying to adjust the 3 secondary bolts with an allen key - especially in the dark. They cost about £15 from the States.

Mark

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If it seems okay I'll deliberately mess the alignment up then start again, and repeat until I've got the hang of it.

Best way Adrian - Collimation is not tough once you realise that the perfect set up is in the adjustment - somewhere.

Just to be sure, I'd recommend the following...

1) Screw down all 3 primary screws so that the tension on the springs is at "max". Then unscrew each one turn. when you actually get around to adjusting the primary, only adjust 2 screws.

2) Regarding the secondary if you want a "rough cut" way of ensuring your mirror is aligned to the focuser, then screw the focuser all the way IN. Measure the distance between the very top edge of the focuser draw tube and the top edge of the mirror. It should be the same as the measurement between the bottom edge of the focuser and the bottom edge of the mirror. Make any final adjustments with the collicap

3) Ensure your centre dot in the primary is actually in the middle. There are plenty of ways to do this (look them up on the web...if u get stuck)

Apart from that, just type "collimation" in the search on SGL, look at the myriad responses, get out your screwdriver and have a practise ! You can't do any harm (watchout for falling screwdrivers though !)

Good luck

Steve

Edited by albedo0.39
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Well, this is embarrassing. I decided to have a play with the scope last night under some decent lighting. Before I did anything I thought I would take a picture of the offending black line that was blocking part of the primary. I peered through the focuser, and reached up the side of the tube so that I could rest my finger on the lip of the OTA, directly above where the mysterious black line was. I then looked down the tube at the spot my finger was pointing to and proceeded to kick myself. Hard. Anyone like to guess what I saw?

My shroud had become loose at the bottom and part of it had snuck in the gap between the top and bottom sections of the tube. As a result, when the scope was extended part of the shroud was left inside the tube, and obviously that part completely obscured the section of the primary it was above. Doh!

I didn't have time to make any adjustments after that as something came up, but needless to say I felt like a bit of a wally. Then again, I learnt several new things about the art of collimation which will no doubt be of great use. I am actually looking forward to getting the HoTech out now!

Oh, and while I was drilling the hole for my collimation cap I thought I'd also drill a small hole through my screwdriver's handle. I threaded a piece of lace through the hole and now I can keep it wrapped around my wrist when adjusting the secondary - no more nightmares about dropping the screwdriver onto the primary!

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