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Portabowl project

GazK

By GazK
in DIY Astronomer

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GazK

GazK

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OK, so this is my virtually completed portabowl 4.5" f7.3 newtonian ball scope. It's built more-or-less to Jay Scheurle's original plan (available here) - and it's highly ingenious.

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The scope is based around an Ikea stainless steel salad bowl with a barbell weight to give it "bottom". Other key components are a broomstick, two CDs, a few pieces of wood, various screws and a piece of 3mm aluminium bar. The whole thing including optics has come in for only £78. At that price, my 6 year-old can use it without me fretting about damage, and also it's just the right height for her.

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I made some modifications to the plans, due to three factors:

- I couldn't get hold of a cost-effective 4.5" f8 mirror set in the UK, so I used the optics out of an old TAL1M 4.5" f7.3 instead - thanks to Ed (NGC 1502) in Essex for giving me the OTA at a very reasonable price

- Having got the complete TAL OTA I thought I might as well make maximum re-use of the components - so the focuser and secondary holder have both been put to use

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- I wanted to improve on the coffee-can style base, so I got hold of a conical wooden salad bowl and mounted three teflon pads on the lip instead. It works a treat!

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I also had some issues with metrication, given that the plans are all based on American sizes - e.g no 10lb weight, so 5kg will have to do - but nothing serious.

The only things left to do are:

- fit a red dot finder - if anyone has one they don't need, please get in touch. I have also found that at low altitudes the scope is a little light at the bottom, but for that I just add my magnetic weight stack as necessary.

- add some shrouding around the primary to block stray light

All collimated - first light tonight!

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GazK

GazK

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Thanks Steve. Here is the next Brian Cox at the eyepiece:

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hopefully this will inspire others to have a go - there's really nothing here that a competent DIYer couldn't tackle.

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gliderpilot

gliderpilot

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Great project, Garry. I remember you describing this when we were observing on friday night.

Few qs for you:

How much counter weight is in the bottom? I assume this is to keep the CoG inside the bowl?

Secondly does the supporting strut pass through the light path?

Cracking project and the nipper will enjoy using it.

Mark

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GazK

GazK

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Great project, Garry. I remember you describing this when we were observing on friday night.

Few qs for you:

How much counter weight is in the bottom? I assume this is to keep the CoG inside the bowl?

Secondly does the supporting strut pass through the light path?

Cracking project and the nipper will enjoy using it.

Mark

Thanks Mark - I'll bring it along to sixpenny handley and you can try it out!

Yes, the CoG is in the bowl - the barbell weight is 5kg and most of that weight is below the fulcrum, which is a line drawn across the top of the bowl. So essentially the weight is a large mass, short arm lever which counteracts the low-mass, long arm lever of the secondary/focuser. The genius of the design is in keeping the top end mass low by using the minimum of materials. Even so I'm topping the counterweight up with an extra 1kg for larger EPs at lower altitudes (the Baader costs more than the scope but gives the lovely 1st moon view above).

It's a real pity that this constraint becomes more and more of a problem as scope size goes up, otherwise I think all us dob users would be packing 10" ball scopes - it is pretty much the ideal mount for people who don't need RA tracking. No zenith dob-hole with this scope!

The struts are clear of the light path - they start out at the base quite wide apart, and by the time they reach the diameter of the primary they have moved away from the mirror circumference on their way to the main pole - hope that makes sense. Like I said, genius.

One thing this project has really brought home is how lost you are without a finder - it was taking me 30 seconds to find the moon last night. I haven't a hope in hell of seeing anything else until I get one!

One last important point: my daughter says that this scope is called "penelope" - because she likes long names. Who am I to argue?

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GazK

GazK

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Yes Steve - the guy who designed the scope used a low-cost gunsight red laser dot finder call a "daisy sight" - but unfortunately they don't seem to be available this side of the pond. I'm hoping to find someone who has upgraded the stock red dot on their scope to a telrad or something.

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GazK

GazK

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I've got a daisy red dot finder from when I was in the US with work. Yours for a few beers when we next meet?

Mark

That'd be grand Mark, thanks very much - any chance I can get hold of it before Sixpenny Handley? I'd like to have a working scope before then, I think it'll attract a bit of interest!

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GazK

GazK

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So I finally completed the portabowl. After a delay when other thing got in the way, and my unfortunate frying of the first red dot finder - sorry Mark - I have now added another RDF, and wired it to be powered by AAs instead of the sorry watch battery it comes with.

Here is the complete scope:

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And the RDF head:

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and the battery pack at the base, with stick-on balance weights to offset a big eyepiece:

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Hmm... need a new project now... *scratches head*

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simon955

simon955

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Wow nice job, my wee astronomer has to be lifted or stood on a step ladder to see through the eyepiece. Can a 6 year-old steer it around the sky? My little astronomer is 8 and I'm sure she would love it if I made something low to the ground for her.

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GazK

GazK

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A 6 year old would have no problem steering it. Whether a 6 year old would be able to *find* anything is a different question of course. But then practise makes perfect...

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GazK

GazK

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I got mine out of a TAL-1 OTA which was going spare in someone's shed. There must be plenty of 4.5" f7 or 8 OTA's out there gathering dust - just put out an appeal.

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Keiran

Keiran

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Thats a pretty amazing little scope.... Now time to go large ! :p

How big could you go without making it too large, that it would no longer be portable... 10"?

Great craftmanship in the bowl :)

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