DMK 21 focus

[Astroblagger]

was out today trying to get a mosaic, they wont stitch togeather! but managed this today, my dmk 21 wont come to focus without a barlow which is a real pain as you need to build a large mosaic which doesnt seem to want to go togeather?

how much more inward focus would i need to get it in focus without a barlow? Im already using a shorty 1.25" tube for it, need to bring the sensor closer to focus point, i was thinking can i remove the eyepiece holder and machine afew mm off the top? to bring the sensor down the tube??

panel 27.tif

[DrRobin]

Hi,

Some PSTs archive focus with a DMK21 with a short nosepiece others don't. Mine was one of the ones which won't. I had a second eyepiece holder made that was 5mm shorter and this allowed my DMK21 to achieve focus without a Barlow.

Others have taken a saw or milling machine to the top of the existing eyepiece holder to achieve focus, but if you do this remove the blocking filter first, otherwise it might get damaged.

Which Barlow do you use? I used to use a screw in Barlow on a very short 1.25 adapter to give less than 1.5x magnification, that would come to focus in my PST, but not reduce the picture size too much.

The Barlow I used is here

http://www.scopesnskies.com/prod/astro-engineering/barlow/astrophotography/AC555.html

I wrote a thread about modifying the nosepiece here

I hope this helps.

Robin

[Astroblagger]

I used a skywatcher x2 Barlow with the lens part removed, and as you say it reduced it a little, I see the eyepiece holder completely unscrews from the body, I have access to a lathe, but the eyepiece holder top part itself looks glued to the body, how many mm would I need to remove?

[Merlin66]

The top part does separate from the bottom part....

Does your PST have a filter element at the bottom of the eyepiece holder and another smaller (red) filter at the eyepiece end?

[Astroblagger]

Yes it has a filter at the bottom and small red one inside, iv tried unscrewing it its seems solid, you can see a glue type substance where they join?

[Astroblagger]

Or can you get a reducer that would bring it to focus to get the whole disc on the sensor?

[Merlin66]

If you're currently using the short shoulderless nosepiece in the DMK and still not achieving focus then a barlow lens (or shorten the eyepiece tube) is the way to go.

A reducer will probably not work until you can achieve at least "normal" focus....

[Astroblagger]

I can turn the eyepiece holder down on a lathe, will just have to ensure all filters are well covered, any idea how many mm to take off?? Has anyone done it?

[DrRobin]

The eyepiece holder is in two parts, the top holds the [red] blocking filter and the lower part a energy reduction filter. They two parts do separate but when they were assembled Coronado used a locking compound like Loctite and some of them take some removing.

I bought a boa wrench (oil filer wrench) to get min apart, but it still wouldn't unscrew, so wrapped the top part in a rubber tape and use a pipe wrench to separate them. It took a fair amount of force, but once the seal was broken it unscrewed by hand. The red blocking filter is just a press fit into the upper eyepiece holder, but again thanks to Loctite it is a very tight fit. I pushed mine out with a socket that was big enough to fit over th metal and a vice. Again it was very tight until it started to move. I was quite surprised by how tight everything was although I do know that these vary from PST to PST.

I have worked on engines in the past and some of the bearings are quite tight so the amount of force I had to use wasn't for the faint hearted.

If you do get it all apart, I would take about 3-5mm off the top, you might need to drill a new hole for the eyepiece lock screw and then it should achieve focus in both a camera or an eyepiece. There are a couple of posts on here from people who have done this so you might want to search for them first.

Of course any of this is going o invalidate the warranty, but more importantly don't remove or discard any of the filters as it might make the PST dangerous.

Regards

Robin

[Astroblagger]

Thanks for that robin, ok I've managed to seperately them and they both unscrew, can't get the small push in filter out, seems pretty tight, does this filter just push back in? Or do I need to glue it in place? Are there any alignment issues when it goes back together?

[Merlin66]

IMHO if you're going to machine a couple of mm of the end, just cover the exposed filter with a piece of masking tape and take care.....

[DrRobin]

Mine was very tight, but it is just a push fit. It didn't need glueing in, it was tight enough as it was.

You could leave it in place and just tape over it as Merlin suggests. When you are finished, just give it a blow out with a blower brush before removing the tape.

Robin