Collimating secondary on skywatcher 250px dob

[Victor Boesen]

I have recently bought the skywatcher 250px dob, and I wanted to collimate it yesterday before I had my first light. I tried adjusting the three screws but they were very tight. Should I loosen something before adjusting them, or am I just to "kind"?

[Charic]

Maybe a little to kind!

If you turn the centre screw on the secondary mirror anti-clockwise (from memory) that will move the mirror towards the primary, taking the tension off the three adjustment screws. I would hold the mirror assembly without touch the mirrors face whilst making any adjustment, as a precaution, also keeping the scope horizontal/level, so that nothing falls onto the primary mirror. The secondary mirror should be the first mirror to adjust.

Once you have access to the screws and they rotate freely, unscrew them a little, to allow further adjustment of the secondary, ensuring that the mirror is concentric  to the inside of the focuser tube assembly (follow one of the regular guides ). The centre screw on the primary will adjust the position of the mirror. You will also rotate the mirror by hand  to ensure that you see a  circular mirror that is concentric within the  focuser tube, as the mirror is elliptical by design. Once your happy that the mirror is circular and concentric, seen as  equal spacing around it and the wall of the focuser tube, its now time to nip up the three screws which will help keep the secondary in position. Further adjustment will/may be required, but at least now, the screws are accessible to operate. 

[Tommohawk]

46 minutes ago, Victor Boesen said:

I have recently bought the skywatcher 250px dob, and I wanted to collimate it yesterday before I had my first light. I tried adjusting the three screws but they were very tight. Should I loosen something before adjusting them, or am I just to "kind"?

Do you mean the screws on the secondary or the primary?

[Victor Boesen]

7 minutes ago, Tommohawk said:

Do you mean the screws on the secondary or the primary?

Secondary

[Tommohawk]

Ahhh - that would be why you said "collimating secondary" in the title then!!

Sorry, the "trying to be helpful" response kicked in before the "engage brain" response.

Charic has it right I think. Good luck!

 

[Victor Boesen]

Thank you for the advice. I just tried it again and this time I could collimate it perfectly (I think)!

[JOC]

Having experienced similar problems I can confirm that loosening the middle screw of the secondary will help to release the tension on the other three.  What I did find was it took a little while to them all nipped up again  without mucking up the way I had them aligned.  So don't worry if it seems to be a bit fiddly.

[Charic]

5 hours ago, Victor Boesen said:

Thank you for the advice. I just tried it again and this time I could collimate it perfectly (I think)!

Find yourself a bright Star, I use Polaris, which is not the brightest, but its remote and easy to spot, but most importantly, its almost stationary, giving you plenty of time to check the collimation.

Using a high powered eyepiece, say your 6mm, focus on the Star to see the Star for what it is, then de-focus the Star? When you de-focus you should see a pattern similar to this one. Notice how everything is central, concentric. Achieve this and your scope is perfectly collimated. If the image is not centred, it will centre as you adjust the primary mirror adjusters, with the lock knots slacked off. When similar to the image, just nip the lock nuts up, be kind if you wish! here's the image......

[JOC]

In Astrobabys collimation guide she does a 200P.  With the star test circles above, she says they have to be perfect concentric perfect circles, but points out in the 200P that the circles though the correct shape could be slightly off centre.  This is worth noting as it was the case with mine.

[John]

1 hour ago, JOC said:

In Astrobabys collimation guide she does a 200P.  With the star test circles above, she says they have to be perfect concentric perfect circles, but points out in the 200P that the circles though the correct shape could be slightly off centre.  This is worth noting as it was the case with mine.

The diffraction rings around the star during the star test should be as concentric as possible.

The offset that is mentioned refers to the relfection of the secondary mirror when you are collimating the scope. In newtonians faster than around F/6 the secondary mirror is offset slightly from the centre of the optical axis and this offset shows when you use a cheshire eyepiece to collimate the scope.

 

[Racey]

10 minutes ago, John said:

when you use a cheshite eyepiece to collimate the scope.

I presume you're sending a subliminal message to use a Laser there @John ?

[John]

7 minutes ago, Racey said:

I presume you're sending a subliminal message to use a Laser there @John ?

Whoops !!!

Typing too fast there - I'll see if I can edit it.

Edit: Yep - done !

[JOC]

FWIW I initially bought a laser thinking it would be easier.  I failed with this for several reasons.  In the finish I got a cheap Cheshire and with this and Astrobabys guide I finally succeeded.  John is correct in his description of the offset.  I didn't get it quite correct, but what he describes is worth noting.  I now use the Cheshire for the collimation and just put the laser in at the end for the satisfaction of seeing a spot-on return.  There seems to mefar more advantages with the Cheshire that the laser can't offer.