Friday, 30 July 2010

20" project

As mentioned in my previous post, I have the opportunity to buy a 20" Dob from someone on the mainland. He transferred the optics to a permanently-mounted equatorial scope and now has the empty scope for sale. All being well, I am going to buy this from him and get a new mirror to fit it and the cost of the entire project should be less than ordering a brand-new, scratch-built custom scope. The original mirror was an f/3.7, so I am either going to have to get a 20" f/3.7 mirror made, which will be slightly pricier than a standard f/4, or get an f/4 and alter the scope by adding longer truss poles, new secondary holder, changing the position of the attachment blocks, and so on, but by the time I've done all that, I may as well spend the extra couple of hundred quid on an f/3.7.
I'll need a Paracorr as the coma will be bad with such a fast mirror, but I can live with that, it'll be worth it to have large aperture! But, we're getting ahead of ourselves - the one item of bad news is that I have to save up first. It'll take me about three months to save for the scope and then at least 8 to save for a mirror so this won't be up and running for the best part of a year yet, unless I get lucky in the wallet department. exciting! I've wanted a large scope ever since I became a deep sky observer the best part of 20 years ago, so this will be an ambition come true.

I'm planning to spend the weekend putting the finishing touches to the shed. Then I need to put the castors on my 12" scope's base so I can roll it in and out then we're in business!

There's no sign of the weather clearing up. It's typical unsettled English summer weather. I hope it clears up in August, so I can catch the Sagittarius and Ophiuchus summer goodies before they vanish into the twilight.

Oh, and I got my monitor back on Monday. Only it's not my monitor, it's a replacement, which is good as it's a brand-new machine and unlikely to go wrong - I hope. At least PC World eventually got it sorted for me after first trying to fob me off with this 'You have a contract with the manufacturer, not us' crap - they need to read the Sale of Goods Act, especially this bit. Stuff less than a year old should not break, but it does occasionally and the retailer has a responsibility, under the Act, to refund, repair or replace the item and not give the customer the flick with some stupid excuse about 'extended warranties'.
The sketches, I am pleased to say, look fine.


  1. Wow, a Twenty inch Dob.

    I can only imagine what the views will be like through that monster.

    I also want a light bucket.

    I have set myself a goal, beginning next year, to have money saved in four years, to buy an 18" as a 50th birthday present to myself. I've already seen the 18" telescope that I want, I just hope David Lukehurst is still around.

    I commented on UKAI yesterday about the lack of clear skies, and said jokingly "it was the words hose pipe ban"that's brought all this rain.

    I have an observing week-end planned from the 5th to the 8th August so I'm hoping for clear skies.

    Paul A Brierley
    Observing Co-ordinator, Macclesfield Astronomical Society.

  2. Hi Paul

    I was going to go for a David Lukehurst 18" but I was told about this one, and buying an already-made scope and getting some optics to go in it will be cheaper. However, as I have to save, it means I'm not going to have a mirror until at least May next year which is annoying. I just wish this stuff wasn't so costly - one of these mirrors is worth more than my car (a six year old Citroen C3), which is amazing when you think of it. But then I'll have a precision optical instrument which will give me excellent views of the deep sky - and I hope it'll behave better than my car does!

    Good luck with the weekend you have planned; it's frustrating, this waiting game. It's typical, it's hot and clear in June when the nights are useless for deep sky anyway but as soon as the dark nights start to come back, so do the clouds.



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