Focusing new DOB?

[notoriousnick (Nicholas)]

Hi All,

While waiting for rain (and fog ) to clear after getting my new SW 10" collapsible DOB, I've been having a go at collimating it, using the old 35mm film canister method. This worked well enough, except when trying to focus on an object about 30 meters away, I had to pull the 25mm Plossl out of the holder by about 6 inches to achieve focus!

So I surf the net and lo-and-behold find a discussion on IIS suggesting that the primary mirror is too high up in the tube. Indeed, I could have parked my car in the gap. So I cranked it back down all the way and collimated it again.

Now, the object (a tree) 30m away can be brought into focus when the Plossl is pulled only just clear of the focus tube, instead of six inches away.

The further away the object the more 'in' the Plossl can be. However, given fog, rain, etc. I can't easily find anything far away to focus on to see if the Plossl will focus OK while still inside the focus tube.

So is it likely (from my description) that the ep will focus ok on distant objects without having to take it out of the focus tube? Or does it look like I may have to play around with the secondary mirror as well?

Cheers,
Nicholas

[DavidU (Dave)]

It should be fine Nic. Try to use a power pole or roof top etc but 100 meters away or more.

[notoriousnick (Nicholas)]

Originally Posted by DavidU

It should be fine Nic. Try to use a power pole or roof top etc but 100 meters away or more.

Thanks, Dave!

I had a feeling 30m might be way to close to test out the focus. Your comment will now help keep me settled until the weather clears a bit

I'm surrounded by trees here in the Adelaide Hills so finding distant objects to focus on is difficult. I could do it but would have to go outside the house - probably not a good idea when its raining.

I do have reasonable views of large sections of the sky though, but this is of no use while there's 100% cloud cover

Maybe I can wax and polish the OTA while I'm waiting

Cheers,
Nicholas

[DavidU (Dave)]

Nic, get one of those round shiny silver xmas tree decorations and put it in the sun as far from the scope as possible.
I have done this with the scope pointing out an open door (while raining).
The silver ball thingy makes a great artificial star so you can do star testing and collimation during the day time.

[DavidU (Dave)]

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

[notoriousnick (Nicholas)]

Originally Posted by DavidU

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

Thanks Dave for the link. That is some interesting data presented there, making a compelling case for keeping one's telescope collimated to the highest degree.

At the moment I only have 10mm and 25mm Plossls, so it would be difficult to do the star test in optimal fashion (lack of magnification). I will get more EPs in time (and Barlow), which should fix that problem.

I like the Christmas Bauble idea! Should be easy to find a suitable one at this time of year - thanks for that.

Incidentally, I noticed that a lot of the low cloud had gone away, so I pointed my scope through the open double-doors, through the gaps in the trees, across the valley at least a couple of kms away. SUCCESS!

Sharp focus is achieved within the focuser's range. So I'm happy...

Cheers
Nicholas