20" f/4 Dob build

[angusb1]

Looking good Steve. Most interesting thread for me on SGL at the moment, can you post what the UTA weighs when you know please?

[swamp thing]

Looking good Steve. Most interesting thread for me on SGL at the moment, can you post what the UTA weighs when you know please?

Thanks very much. That is high praise indeed.

Glad it is of some interest to you. I most certainly will post any info you need buddy.

I'm hoping it will inspire others to have a bash at ATM it's immensely satisfying.

Give me a little reminder after the focuser is fitted and I'll let you know.

Regards Steve

[angusb1]

I'm halfway through the Kriege and Berry book at the moment and planning my first dob so watching your progress is great. The whole process looks like a lot of fun.

[swamp thing]

It is great fun.

What aperture are you thinking of?

The only thing I would recommend (if going for a 20") is a thicker primary mirror than the one I'm using. This would enable you to use a more straight forward 18 point cell. This would save a lot of messing about at that stage of the build.

Regards Steve

[angusb1]

I picked up a 6" f8 mirror set off ABS a couple of months ago so plan to do that one first as an airline portable "selfcontained" dob. I'm saving for a big mirror set as well, so I hope I can get the principles sorted out on the 6" then make a big one afterwards. I did plan to build the 6" set into a string telescope and made a model in google sketchup but I'm definately going to be changing the dimensions of the mirror box since reading the Kriege and Berry book.

[swamp thing]

Still waiting on parts for the big lump at the moment but I did get this yesterday:

Picked it up for £25 from Telescope house (as I was passing by and accidentally veered in there;))

Now I know it says, D=114 F=1000 f/8.8 but it gets there via a Barlow lens in the base of the focuser.

So I figured chop this out and I've got the little F500 f/4.4 RFT superfinder I'm after for the mirror box on the Dob.

I've now done it and it focuses okay on distant objects just waiting to try it at night. If its okay it'll get a birthday strip Dow and paint, as there's no way that is going anywhere near my Dob looking like it does now.

[michael.h.f.wilkinson]

Still waiting on parts for the big lump at the moment but I did get this yesterday:

Picked it up for £25 from Telescope house (as I was passing by and accidentally veered in there;))

Now I know it says, D=114 F=1000 f/8.8 but it gets there via a Barlow lens in the base of the focuser.

So I figured chop this out and I've got the little F500 f/4.4 RFT superfinder I'm after for the mirror box on the Dob.

I've now done it and it focuses okay on distant objects just waiting to try it at night. If its okay it'll get a birthday strip Dow and paint, as there's no way that is going anywhere near my Dob looking like it does now.

I tried something like that, but found the max true FOV of 3 deg a bit restrictive. Would be better on a bigger scope, no doubt

[swamp thing]

that looks great.

Three degrees restrictive.

I couldn't think how I could get it much bigger. How did you do it?

Trouble is I don't want a fast frac on the upper cage as they're heavy and adding weight to the upper cage isn't a good idea on a large Dob. Every pound on the top means many pounds on the bottom. This is going to be a mobile scope after all.

Therefore by putting one on the mirror box it really needs to be a newt otherwise the eyepiece will be on the floor.

If you have a better idea I'd love to hear it as its a bit of a brain teaser.

Cheers

[swamp thing]

I guess it will be more in scale with a larger scope

[michael.h.f.wilkinson]

I didn't get more than about 2.8 deg out of it (28mm Kelner from an old pair of bins), and it never got much use as a finder. I now have a 70 mm RACI finder made from a light achromat. My donor scope in the above case lacked the barlow, so it was a native 4.5 f=500mm. The only way to get a larger FOV is with 2" EPs which requires a much larger secondary. You could mount a RACI finder at the rear, I suppose.

[swamp thing]

According to the TV eyepiece calc I can get just scrape 3.1 deg with a 32mm plossl.

I have a 32mm plossl somewher I think.

I think what I'll do is try it hand held I can manage that easy enough with this little thing and give it a whirl.

After all I will have a Telrad to get it near.

I also have a 16mm Nagler that I never use that might be nice in it for some higher power narrowing of the field.

TBH maybe I ought to just sell the Nagler and buy something more useable.

Decisions, decisions never been good at em.

[michael.h.f.wilkinson]

A 32 mm gives a 7.3mm exit pupil. A 22mm Erfle like I use in my finder or 24mm SWA would be better.

[swamp thing]

You are right Michael. If it was a regular scope I would try and get the exit pupil right but it's prime job will be as a finder where FOV is obviously more important at the moment.

So I think my old 32 plossl will do for the time being. Perhaps once the main scope is finished I'll look at treating the little fella to a new eyepiece. In the meantime it should be happy with the new paint job it'll be getting.

[michael.h.f.wilkinson]

True, but you do not get the full benefit of the aperture. Furthermore, the secondary may show in the image (more of a problem in daytime than at night). The Plossl gets 3.2 deg at an field stop of 28 mm I worked out. The Prostar 22mm Erfle with illuminated cross-hair I use gives 3.08 deg, so just marginally less, at an exit pupil of 5mm, so fainter stars show up better. I ordered mine from Australia. Even with import duties, it was slightly cheaper than the similar 20mm from TS, and it gives 20% more surface area in the sky.

I would check the secondary mirror diameter. In mine it was a mere 28mm, which means I was using only 95mm of the main mirror. You need at least a 40mm, and currently I use a 50mm (in its incarnation as a 4.5" mini-Dob for the kids), which is what you need to eliminate vignetting over at least the centre 25 mm of the field stop.

[Peter Drew]

I think the Barlow is part of the correcting system for what is probably a spherical primary. If so, operating at f4.4 it's not going to give very good star images. Hope I'm wrong.

[michael.h.f.wilkinson]

I think Peter is right. However, TS sells a 4.5" f=500 parabolic mirror for 55 euro. Should the image be too poor even for use as a finder, you can replace the mirror without breaking the bank.

[swamp thing]

I think the Barlow is part of the correcting system for what is probably a spherical primary. If so, operating at f4.4 it's not going to give very good star images. Hope I'm wrong.

I suspected as much too, but for £25 I thought I'd give it a bash.

Still haven't had a chance to try it yet on the sky altough it does focus nicely on distant objects without it.

If it proves a waste of time, I will refit the Barlow then...... I'm sure I can find a youngster that would want an early Xmas present.

[nytecam]

Hi Steve - your 'new' little Meade scope looks alot like my Jone-Bird reflector featured at

Mine doesn't have a Barlow but a J-B corrector for the spherical primary mirror - I'd be interested how it performs BEFORE you remove the Barlow/corrector

[swamp thing]

Hi Steve - your 'new' little Meade scope looks alot like my Jone-Bird reflector featured at

Mine doesn't have a Barlow but a J-B corrector for the spherical primary mirror - I'd be interested how it performs BEFORE you remove the Barlow/corrector

Oops! Sorry buddy. Done it already

I will post a first light on it.

What I can do is see how it performs without it, if it's a disaster I can refit (I think) and give it another bash.

Either way I'll keep you posted, okay?

Gives me something to play with whilst waiting for parts.

[Lifeisgood0414]

wow... impressive...

keep working hard~~

[swamp thing]

wow... impressive...

keep working hard~~

Thanks. Will do.

If these bloomin parts ever get here.

[fwm891]

Hi Steve, Great 'little' finder. I'd be worried about the amount of weight you're putting at the top-end and how much extra weight you'll have to add to the bottom to balance it out ?

[swamp thing]

Hi Francis

The little scope will be going on top of the mirror box (if it works). Don't worry it's not going near the UTA, that will be Telrad only.

That's why I wanted a newt as the eyepiece is at the right end for being down there, a RFT frac would put me laying on the floor to look through.

Only reason I took the pic of it near the UTA is I haven't started on the mirror box yet.

[Moonshane]

hi mate, just me being pedantic but some button (allen) head or cap head stainless bolts would look nicer than the hex heads you have?

[swamp thing]

Good point Shane, I agree.

Will sort that. Not got much else to do on it right now.