Discovery 12.5" PDHQ

I picked up my new 12.5" PDHQ Saturday She's a biggun', but very pretty. It's a split tube and that seems to work very well. It comes apart and goes back together easily and holds collimation pretty well after doing so. Just a tweak or two of the primary adjuster screws was all that was necessary.

All of my eyepieces (31mm T5 through 7mm T6) reach focus except my 12mm T4. That one requires about an inch more outward focuser travel (back focus?) than is available. The 31mm focuses the farthest inward, about an inch from all the way racked in. I could move the mirror to try to make both ends meet (so to speak) but I'd have to position it within a tolerance of plus or minus .10" to get both to work. Maybe a 11mm T6 instead of the 12mm T4?

One funny thing though...late in the session I noticed a bit of astigmatism. It wasn't evident early on when I was collimating and star testing, but showed up a couple of hours later. I don't think it's my eyes, the direction of the astigmatic artifacts stayed in the same position no matter where I rotated my head (or the eyepiece).

The primary mirror seems to be mounted in a stress-free manner (sitting on 3 pads of silicone RTV) but the secondary mirror has an overly generous amount of foam behind it. It seems to be exerting a noticeable amount of pressure, perhaps more than is necessary to hold it in place securely. I lightened the load a bit and will observe again tomorrow night to see if that helps. I've attached a pic of the dark blue beastie for your viewing pleasure...

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Congrats Ed!
Enjoy it!


Ivan

Very nice scope. Your thoughts on the secondary could be the trick, let us know how it works out.

Ed Moran said:

All of my eyepieces (31mm T5 through 7mm T6) reach focus except my 12mm T4. That one requires about an inch more outward focuser travel (back focus?) than is available. The 31mm focuses the farthest inward, about an inch from all the way racked in. I could move the mirror to try to make both ends meet (so to speak) but I'd have to position it within a tolerance of plus or minus .10" to get both to work. Maybe a 11mm T6 instead of the 12mm T4?

The nice thing here is your 31 comes into focus. The only time you really need to move your mirror is when you have inward issues. If the 12mm is close to getting focus, I suggest getting the 2" barrel adapter for it. That should allow you to slide the 12mm out a bit more than with the 1 1/4" barrel. I've done this in the past and it works great. The adapters are like $10 or something small like that.

Mike.

The Discovery has a nice look to it, are you going "trick it out" like Hal P did to his? Maybe a 7" finder and platinum highlights?!

Enjoy your new scope, I hope to pickup my 14" May 7 from the builder *fingers crossed!*

Have you tried the 12mm using the 1.25" and not the 2" barrel? This will give you a bit more travel.

Thanks everyone, for the comments and suggestions!

The 2" barrel extension from TV for the 12mm T4 is something I'm considering. I see that the Moonlite focuser people sell a focuser tube extension that might work also. I haven't, as Floyd suggested, tried to use 1.25" part of the 12mm but that might work too, so I'll try that tonight when I see if I've cleared up the astigmatism. I'd like to keep the 12mm if I can.

As for modifications, well, isn't that one of the reasons to own a dob like this one ? I just took delivery of a few square yards of ProtoStar flocked paper today and I'll probably add U-shaped panel to the front of the rocker box to stiffen it up a bit. To be fair to Discovery, it doesn't really need it despite the high side panels. The rocker box is very stiff considering how light it is (lighter than the heavy particle board box for my Orion XT8!).

A set of wheelbarrow handles and wheels are in the works, too. That will make setting up at home much easier.

Ed

Reducing the pressure on the secondary mirror seemed to do the trick. The perceived astigmatism at high magnifications was eliminated. Yet I still see "something" at low power. I now wonder if what I perceive as astigmatism is something else? For instance, at high power during collimation (350x and 450x) there is no trace of astigmatism at all. Nice round defocused star images on either side of focus, with no flopping oval shape evident.

At low power however, with my 31mm T5, I detect what looks like astigmatic stars. Is this coma instead?

At any rate, Saturn looked great at 350x, with more detail and contrast than ever before. Much better seeing than Saturday night. I even tried 450x (7mm T6 w/Barlow) and it was almost do-able. A bit hard to track though...

The 12mm WILL focus when the 1.25" barrel is used so I guess I'll keep it.

Thanks,

Ed