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ISTAR 5" Refractor completed!


DaveGibbons
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Well it's taken longer than I thought but it's complete My own build F8 achromat refractor is assembled. Based round an f8 127mm Istar objective. Finally did the last machining on the all important counter cell last night and apart from one or two small jobs such as a small amount of flocking all the hard work is done.

I must at this point thank my good friend Graham and his little lathe in a garden shed - the countercell is the single most important component in this project and we both spent many hours turning this work of art from a solid piece of 7"x 6" aluminium billet into the missing link for the whole project.

I love achromat refractors and currently own an excellent 100mm f10 Tal that has first rate optics. I wanted to build something with a bit more aperture and a wider field of view that I could use on open clusters and starfields, along with low power binoviewing whilst still able to perform well on moon and planets all in a manageable package. Yes I know the Physics of an f8 achromat means that a degree of false colour will be evident on bright planets and moons limb etc, but having owned many achro's(and apo's) it is something that has never bothered me and I think people just get hung up on it. I am purely a visual user andjust love the contrast and sharpness that a high quality refractor gives.

So running through the scope we started with a meade ar5 refractor tube, cut this down on the lathe ensuring ends perfectly square and also checking tube run out in the process, very little evident approx 0.3 mm. I had drawn a full size ray trace drawing of the light cone so we could ensure internal baffles not intruding into the light cone, also making sure I would end up with enough in focus to allow the use of my William Optics binoviewers without needing to resort to additional image dimming barlow in order to achieve focus.

Focusser end is well looked after with a dual rate rotatable moonlite unit I originally bought for the AR5 (broken objective) . Moonlite's are superb!

Objective end is an Istar f8 127mm achromat objective mounted in cnc push pull cell. Istar is a relatively new company offering high quality achromat objectives a good deal cheaper than the likes of D&G and to a degree untested and indeed I have never seen a review of my particular lense the 5" f8 but hey someone has to go first and as they say the price is right! The build quality of lense and cell are superb but the proof of the pudding is in the eating and until I star test and have a number of viewing sessions under my belt I will reserve judgement on that one. If it gives star test on par with the Tal that's good enough for me.

The countercell is probably a little heavier than needs be and could have been reduced in weight with further turning and tapering but it's such a nice job as is, accurate to a thou of an inch and fits both tube and cell beautifully with countersunk stainless cap head screws.

The push pull style cell allows collimation of the beautiful fully multi coated objective although all the way along we have tried to achieve mechanical and geometric perfection with focusser, tube, countercell and objective in order to get the best optical alignment.

It was my intention to strip the meade paint job off and fully polish up the alloy tube but to be honest just could not be bothered although I may still do this in the future.

With the addition of tube rings , dovetail and fully loaded with binoviewer and eyepieces it now weighs in at 21 lb . This is still fine on my cg5 mount for visual use.

I am still sorting out the dew shield and have just bunged on the old ar5 short one for now.

Here are a few pics, I posted the comparison one of the 5" tube against the 4" f10 tal just so you can see how much shorter the tube is .

cheers

Dave

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Edited by DaveGibbons
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Superb job on that Dave, I was looking at the Istar site the other week and was pondering something along these lines but using a SW as the doner

Looking forward to your 1st light report it'd be nice to hear how the Istar optics performs.

Superb machining on that countercell

Philj

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Dave that looks a great scope and gives hope to many who now fancy the idea of making something just that little bit different and to their own requirements. If it was me, I would strip the paint off in order not to give Meade too much credit (though I suppose they did help with the tube :hello2:) Two question please. Firstly, what ideas have you got regarding a dew shield? Lastly, how did you get on with Istar as a company, what are their delivery/production times like etc.

Again congratulations on a great job!

James

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Two question please. Firstly, what ideas have you got regarding a dew shield? Lastly, how did you get on with Istar as a company, what are their delivery/production times like etc.

Again congratulations on a great job!

James

James, I think I may go down the commercial route and purchase a Astrozap flexible dew shield. I will probably purchase one for a 10" sct as they can be trimmed in diameter and you then have the extra length need at least 2.5 x opjective diameter to keep the dew at bay. If I can find some suitable white flexible plastic poster style material(I once used a world map) flock the inside and that would look good.

I initially enquired about the objective with Ales the owner of Istar, he is a great guy and replied from the States promptly to several e-mails and is very customer focused. He pointed me in the direction of Peak 2 Valley the U.K. distributor for Istar products and I have nothing but praise for John Timmins at p2v. They sent me the engineering drawing for the objective when I asked and again great communication and very friendly chap.

My order took about 5 weeks but christmas was in the middle of that and rather than individually getting the lense shipped I waited for it to tie in with a scope order p2v had as it was cheaper for me that way.

I have just come in from real first light and the optics are the best I've ever had in a refractor they are superb ! More on that when I post a real report(long one) after a few more sessions.

Cheers,

Dave

Edited by DaveGibbons
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Dave many thanks for your kind response. I don't think I am ever likely to buy a 'commercial' scope again as there is so much flexibility in constructing your own. The problem was always the objective lens cell and great as D&G are, the waiting time and the availability of production runs was going to make the whole experience of construction a lot more of a hassle. I know a lot of the time it comes down to budget but when I looked on the Istar site, my eyes (and I'm sure yours too) do start to wonder across the different menu choices, especially when you see what's available be it achromatic, aprochromatic and then 'SUPER' aprochromatic - what's next?

Definitely looking forward to reading your first light report and hope that you might be blazing a hot trail for the rest of us here Dave!

Clear skies and thanks for your reply

James

Edited by JamesM
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Fantastic looking instrument Dave - great piece of work :hello2:

I look forward to the reports !.

I've wondered a few times about replacing the objective of my Meade AR6 with an Istar replacement and see if I can get one that matches precisely with my Chromacor - that would be an awesome scope I reckon but I'd probably need a replacement counter cell as well I guess :hello2:

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