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Cant Collimate 10" Dob.

hippie

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hippie

hippie

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I fitted a new Moonlight focuser to my 10" Dob skywatcher Now i cant collimate my scope? Help please?,

First, I centred the secondary mirror to the focuser placing a piece of paper to stop seeing the primary mirror, 

Step two, I set the secondary mirror to centre in the prime mirror using a Cheshire eyepiece,

Step three, I set the primary mirror to  line up using a Baader mark III laser eyepiece.

Checking the laser beam down the scope after lining every thing up the laser beam is set a long way off the centre spot of the primary  mirror, (1" to 2" out of centre spot alignment )

When i then set the laser to the centre spot on the prime mirror this throws the secondary mirror way of centre to the primary mirror.

I then try to reset the secondary mirror to the prime mirror and then secondary mirror goes out of centre alignment in the focuser ?.

I then try to reset every thing over using the laser collminater and get the same results   ?. 

 I have watched differant videos & read how to collimate your scope over & over following the instructions and still cant get the scope collimated ,

Any help much appreciated with my problem. 

Ian.

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jambouk

jambouk

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Ian

I'm new to newtonians (pretty new to astronomy equipment altogether really).

The two times I have collimated my newtonian, I had to cheat, but the resultant collimation wasn't far off when I tested it on some out of focus stars.

I did as you have done, but then I cheated. With the laser in the focuser, shining off the secondary and down onto the primary (it was way off centre), I tweaked the secondary knobs to get it to hit the primary in the middle. Then I re-checked the secondary was roughly in the right position still when looking through the focus tube and did that process a few times. Eventually the secondary looked to be bang in the middle of the focus tube when looking through a pin hole, and it looked very round, and the laser hit the middle of the primary. When I went out at night I just adjusted the primary a bit to get the out of focus stars to have nice concentric rings around them. So, for visual, it's good, I've not tried to image yet so it might be awful!

Do you have any local astro-mates who are good at this sort of thing and willing to help?

Good luck.

James

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ronin

ronin

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What f number ?

When they fall into the "fast" definition what you see appears to change subtly, just wondering if this is what you are getting.

Think Astro-Baby had a good description of this on her site.

Usually f/5 is "normal" and f/4.5 is where things are fast and start to look different.

Even if yours is f/5 it may be worth checking.

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wookie1965

wookie1965

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I have a laser and it was out of alignment i have now fixed that i have just re collimated my scope and tried to use the laser and it shows that my primary is out by at least 1cm yet my Cheshire says its spot on so i am going with the Cheshire.

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swamp thing

swamp thing

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Have you checked that your new focuser is square to the optical path? It might require shimming.

To do this remove the secondary and place a piece or bar or dowel in the spider that goes down the scope far enough to be seen through the focuser. Place your Cheshire in the focuser, rotate it so one of the cross hairs runs down the length of the bar. Ignore the other cross hair for now, we only want to check the one running down the bar. It should be in the dead centre of the bar. If not you need to adjust the focuser base plate so it does.

Next remove the bar, place a combination square on the top of the tube and slide the rule part down the inside of the tube until it can be seen touching the other line of the cross haris through the Cheshire. Tighten the square. Put the square on the opposite side of the optical tube, sight down the Cheshire, again the line should still be on the end of the square. If not adjust the focuser base plate the other way until it does.

That should get your focuser spot on square.

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jambouk

jambouk

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Yes, I hadn't thought about the focuser being off on a tangent.

I do think it is very difficult though to get the secondary mirror bang in the right place by just looking through a pin hole. Dion describes a method using a webcam and a reticle on the webcam on a laptop and lining the secondary up that way.

James

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Scooot

Scooot

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Yes, I hadn't thought about the focuser being off on a tangent.

I do think it is very difficult though to get the secondary mirror bang in the right place by just looking through a pin hole. Dion describes a method using a webcam and a reticle on the webcam on a laptop and lining the secondary up that way.

James

Try putting the cheshire in and looking with the primary blocked out. Ignore the crosshairs and try to encircle the secondary with the cheshire.

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Jason D

Jason D

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Checking the laser beam down the scope after lining every thing up the laser beam is set a long way off the centre spot of the primary  mirror, (1" to 2" out of centre spot alignment )

When i then set the laser to the centre spot on the prime mirror this throws the secondary mirror way of centre to the primary mirror.

Read the last post on the following page:

http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Board/reflectors/Nu...

First paragraph starts with:

"In addition to the collimation cap that comes with many mass produced reflectors, many beginners purchase a laser collimator and follow the proper steps to find out they can't see all the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can't reconcile between the collimation cap and the laser collimator."

Jason

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hippie

hippie

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I found the problem with my collimation was with the secondary mirror adjustment screws  digging into the secondary mirror mount throwing collimation of ,

I fixed this by finding a large Steel washer and cutting it down to size to match the secondary mirror mount and using that for the adjustment screws to adjust against.

Now I have my scope collimated again using my Cheshire, So need to do a star test for final tweaks if we ever get to see those elusive  stars again soon  :smiley: .

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bambuko

bambuko

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Have you checked that your new focuser is square to the optical path? It might require shimming...

I thought most of the focusers can be adjusted square in a manner similar to collimation using adjustment screws, without any need for shims?

That would be grub screws next to black screws securing focuser body to it's base?

Or have I got it wrong?

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bambuko

bambuko

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I found the problem with my collimation was with the secondary mirror adjustment screws  digging into the secondary mirror mount throwing collimation of ,

I fixed this by finding a large Steel washer and cutting it down to size to match the secondary mirror mount and using that for the adjustment screws to adjust against.

Now I have my scope collimated again using my Cheshire, So need to do a star test for final tweaks if we ever get to see those elusive  stars again soon  :smiley: .

and that's my version of the same - made adjusting secondary very easy

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wookie1965

wookie1965

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and that's my version of the same - made adjusting secondary very easy

banbuko that is what mine looked like i have a sharpening stone so i rubbed it down to get the indents out and made the base flat then used the two plastic washers, i did use a metal washer but Moonshane will tell tell you we were having trouble with the collimation took the washer out put plastic ones in and its brilliant now Jupiter two nights ago was fantastic i could see four bands of colour.

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bambuko

bambuko

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.... Moonshane will tell tell you we were having trouble with the collimation took the washer out put plastic ones in and its brilliant now Jupiter two nights ago was fantastic i could see four bands of colour.

What troubles did you have?

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swamp thing

swamp thing

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I thought most of the focusers can be adjusted square in a manner similar to collimation using adjustment screws, without any need for shims? That would be grub screws next to black screws securing focuser body to it's base? Or have I got it wrong?

Mine has adjustment screws yes, but I'd no idea if the OP's does or not. Not all of my focusers have had adjustment screws. Obviously if one has them, use them. ;)
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bambuko

bambuko

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kept slipping i think it was my washer as it was just a tad bigger than the holder

It's not the size - if you look at my washer it has depressions for each of the three screws (that helps to locate and prevents washer from sliding out)

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