Collimating 250PX???

[sn00p]

Ive been reading through AstroBaby's guide and wonder if the 250PX should be offset or not??? and if so by how much?

oh and wish me luck this collimating mallarcky sounds a right chore :-(

[Merlin66]

Based on previous comments from others, Skywatcher place the secondary mirror in an off-set position on the holder. This being the case there should be no need to apply a further off-set. Just get all the edges concentric ie focuser, secondary, main mirror and reflection.

Ken

[John]

Ive been reading through AstroBaby's guide and wonder if the 250PX should be offset or not??? and if so by how much?

oh and wish me luck this collimating mallarcky sounds a right chore :-(

Collimation is something that is an issue with all types of scopes but if does become even more critical in fast scopes - the 250PX is a fast scope at F/4.7. I have an F/4.8 and check and fine tune the collimation each time I observe - it just becomes part of the routine.

At least the secondary offset is one less thing to worry about with that scope though because,as Ken, says it's already taken care fo by the manufacturer.

[NGC 1502]

Hi, I owned a 250PX until last year, only sold it because I had to lug it up & down stairs.

I can confirm that the secondary offset is taken care of during manufacture. If you

look down the tube, from about 3 ft away & keeping your eye central to the secondary, you can see the offset. Same on my S.W. 200 F6, but not as obvious.

Regards, Ed.

[sn00p]

Thanks for the replies... So im pressuming the spider vanes should also be central??? only mine arent.... im pressuming the mirror is offset where it connects to the vains and the vains should therefore be central???

[astromerlin]

Hi, I have an SW250, I've been through the collimating learning curve, even for me it was fairly head-scratching at times. Set the spider central and then follow AstroBaby's tutorial it was a great help to me amongst other tutorials found on the web.

If you dont have one, get a proper cheshire collimation tool, I could not have done it without one. I used a cap with a hole drilled central in the EP first to get the secondary mirror central and then to see the mirror clips are all showing equally and then place the cheshire in the focuser and do the rest.

The first time (as mine was right out) is the hardest and took several days to get right but after it just takes a few minutes to give it a tweak.

[sn00p]

Thanks... and yes i have cheshire... just need to get a cap. And 2 days 8-( geesh

[astromerlin]

2 days? It was more like two weeks and a very large post on SGL but that's just me.

I will say though that once it is spot on the views are astounding compared to what it was before and you will notice that when focusing on the subject, the focus point will seem to snap in sharply as you turn the focus control.

You can make a colli cap if you have an old film canister and drill a hole in the center or like me you have the original dust cap that fits in the focuser and drill a small (1-1.8mm) hole in that.

[Michael Rapley]

I've just got a 250 dob, the colimation is well out. Iv'e tried following guides but think i'm actually making it worse

Going to beg the help of my club members i think.

[astromerlin]

I've just got a 250 dob, the colimation is well out. Iv'e tried following guides but think i'm actually making it worse

Going to beg the help of my club members i think.

Yep, that's what I did but it's just part of the learning curve. If you have help then alls fine but if like me you live miles from any help then all you have is the patience and help from forum members.

Here's a few tips;

1, Make sure you lay the OTA horizontal, maybe on a table or in the mount. You wont want screwdrivers or allenkeys hitting the primary mirror.

2, Measure the distance on each spider vane to check if they are equal, this will make the secondary holder central in the OTA.

3, Place a piece of paper between the secondary and primary so that it obscures the image of the primary mirror when you look into the focuser, a piece of A4 will just about stand up in a 250mm OTA.

With that done it is now easy to check by looking into the focuser (a cap with a hole is best here) to see if the secondary mirror looks like a perfect circle in the center.

With some good tips from forum members I used a digital camera to take an image down the open focuser (a camera with a zoom feature is good here) make sure you centralise the lens of the camera to the focuser opening and then using software create 4 equal round dots and placed N,E,W and south on your image to see if the edges of the secondary are equal from the sides of the focuser tube. You can get better accuracy like this if like me you've only just started out.

4,With that done and the piece of paper removed, use the colli-cap (DIY-film canister with hole in the center) and adjust the 3 secondary holder screws (minute adjustments here, takes ages) to get the image you see of the primary mirror clips equally placed at the edges of you view. At this point the best advice is to ignore everything else you see when looking down the focuser, all you need to do is to look at the mirror clips.

These tips helped me a lot and once you are at this stage of the process the rest is simple (As per some great tutorials). the other bit of help was; when you get to the stage of adjusting the primary screws, first tip the OTA so that it is vertical and then angle it at about 45 degrees or at your normal viewing position. This settles the primary in it's seating and then adjust the primary using a cheshire tool.