6" f/10 refractor build.

[argonaut]

Before going on I would like to say that I have no business connections with any of the products I have used for the build. I would also like to apologise for all the grammatical errors which there will be many of. I don't mind people taking the mickey,but no flaming please:)

The lens and cell is from Istar Optical and it is a 6" f/10 Achromatic air spaced doublet.

The dimensions:

Full OD 192mm

Front of cell 168mm

Rear of cell 168mm

Width of cell 53mm

Weight 5lb 9oz

The first thing I did was to draw out a full sized light cone. Doing it this way I could find the required length and width of tube I would need. Of course it also allows you to draw in the focuser and diagonal.I found this was far easier than doing the maths.

With the engineering infrastructure going down the tubes fast (no pun intended) it was difficult to find what I needed. I decided on a aluminium tube which I purchased from www.aluminiumwarehouse.co.uk These people did a 6" OD tube with a 3.17mm wall,and the shortest length available was 2.5m which is just over 98".I'll describe later how you fit a 6" aperture into a 5 3/4" tube. When I received the tube I unwrapped it to find it was in a very clean condition so I decided to polish it with Solvol Autosol(all you bikers wiil know this) and a nylon pan scrub. To get the finish I wanted I had to then start rubbing it with a cloth. This took me around 3 weeks.

The next ting to buy was aluminium for the countercell and the focuser holder. The aluminium for the focuser holder was bought from the aluminium warehouse again,this piece was a 6"diameter x 4"length bar. The aluminium for the countercell was bought from www.metalfast.co.uk in Stockport,this piece was 8"diameter x 4"length but was a 1" wall tube.

I'll stop for now it's late,well early really.

PS. If you have any questions so far,please ask.

[Doc]

Thanks for taking the time and effort to do this. It's something I would love to do in time.

This drawing of your light cone is this a full size drawing and does the light cone come start at the edges of your lens and go down to a point at your focuser. How do you determine the length of the tube.

[dweller25]

Great project and a stunning looking scope, thanks for posting the write up.

[AndyH]

Looking forward to this thread

This is something that I've considered doing for the past few years.

One thing I've always wondered about is, where the two lines meet, this point of focus. Is that at you're eye, behind a focuser and eyepiece, or just the focuser. Does the eyepiece then bend the light further?

Hope I've phrased that correctly.

Cheers,

Andy.

[Capricorn]

Was there any reason why you chose the F/10 lens and not the F/8 offering?

Also what does the multi coating look like. I know it is yellow and I have the thought that it could appear awful. Just don't fancy yellow. Also not that familiar with seeing many lens with a yellow multi coating. Would feel happier with a good deep green.

Found an aluminium supplier that lists tube diameters up to 8 inch dia with 1/4 walls, so was thinking of using items from them.

For AndyH the focus of the image plane of the objective needs to be at the focus of the EP (focal length) if you put a collimated beam back into the EP. Much easier to draw. Overall length is therefore Objevtive focal length+EP focal length.

[argonaut]

The tube length is worked out as Capricorn suggested. You must allow for your focuser holder and the focuser. I just spread the drawing out I'd made on the floor and placed the required components on the drawing. You can also work out the baffle system using the light cone.

I chose the f/10 version because I thought the CA would be improved over the f/8. The coatings on mine are Green and are beautiful. I know on Istar Opticals beta website say they are Yellow

I can actually buy tube upto 20",it's the wall thickness that puts me off. I went for the 6" with 10SWG walls. I can manage the weight.

[argonaut]

I'll carry on a bit now.

After receiving the blanks for the countercell and focuser holder I drew out what I needed. The first was the countercell. With the countercell design I had to make sure that the lens and cell would be outside of the tube,due to the fact that the tube ID was narrower than the aperture. I worked out using my light cone drawing that I need the rear of the lens to be 38mm outside the tube. I also wanted the cell to fit into a turned recess in the countercell to allow for the push pull collimation.

I did try my mates Boxford lathe. No chance. I did know of a 'jobbing shop' in the village,so off I went. I would have liked to have turned it myself but health and safety proceedure would'nt allow this. It turned out pretty good really. See first pic. I had Mark turn out the ID to be 2mm above the 152.5mm of the tube. The reason for this is that I wanted the countercell and lens cell to cool quicker. I have made a slip ring 20mm wide that slides upto and away from the countercell when needed. I lined the slip ring with scopestuff flocking. Stops the tube from geeting scratched.

The countercell is held on the main tube with 6 x 4mm stainless button head cap screws,equally spaced of course.

Now the focuser holder was made to fit into the main tube with a 20mm shoulder. The Skywatcher focuser was 96mm if I remember rightly. I had a bore of 100mm to allow me to collimate it if needed. Another 3 x 4mm button heads hold the focuser in place and 3 x 4mm grubscrews take care of the collimation. See second pic.

I also polished the components. The reason I polished everything is a personal belief that polished aluminium cools quicker than painted.

Going to have another break.

[argonaut]

Hi Mick,

Sorry I missed your question.

I measured the light cone from the rear of the lens at max aperture,which is 150mm and not 6".The measured focal length is 1504mm. To be honest I found getting my head around where the focus point would be was a little difficult. I read through many ATM forums before it sank in,but if I said I fully understand it I would be telling lies.

[argonaut]

Dweller25,

Thank you for the kind comments. I think we only live about 10 miles from each other. If you want to come and have a look and a brew it wouldn't be a problem.

[Doc]

Is the tube painted inside matt black?

[SpookyKatt]

I'm loving this thread Argonaut, that scope looks lovely and

I can't wait to hear how it performs.

Kathleen

[argonaut]

I'll explain the tube cutting and finishing.

I had worked out the tube to be 44" long but I bottled it and decided to cut it 46".

I had a large drawing pad and taped two sheets together and wrapped these around the tube with an overlap. This gave a square edge to cut. Using a hacksaw with an old blade-don't use a new blade it clogs and picks up- I scored the tube all the way round. I just let the hacksaws weight cut the tube. It takes time and patience but it is accurate to within half a millimeter. I then cleaned the end up with 80 grit wet and dry glued to a piece of 4 x 1 wood.

I decided that Scopestuff flocking would be a bummer to do so I gave the inside of the tube 3 coats of Halfords Grey primer and 4 coats of their Matt Black. I did wash the tube beforehand with washing up liquid and water. It's certainly not the mattest of Blacks but I was going to put baffles in anyway.

I drilled and tapped for the focuser which I could fit. At the countercell end I wrapped masking tape do I could push fit the cell on to test my eyepieces out. I didn't have enough in focus,the tube was to long. I did have to cut those extra 2" off, damn.

I then drilled and tapped the cell end to attach it to the tube. All my eyepieces then came to focus. In the long run I might have taken the tube down to 43" to give me a little more room for binoviewing.

[argonaut]

I'm loving this thread Argonaut, that scope looks lovely and

I can't wait to hear how it performs.

Kathleen

Thanks Kathleen,

I just wanted to show that it can be done and there is always plenty of help available. I must say though that I was an engineer for many years and that grounding gave me a the advantage.

Clear Skies.

[jnp]

I posted by own 90mm refractor build, but I did it very cheaply in case it didnt work. Yours looks truly excellent.

I had worked out the tube to be 44" long but I bottled it and decided to cut it 46"

You must allow for your focuser holder and the focuser.

Oh yes, I remember these issues well. Getting the tube length right is compounded if you want to hook up a DSLR, barlows, focal reducers, (and in my case support 2 objective lenses). I ended up attaching the focuser to a slightly smaller tube that can slide in the other (and can be locked). A cop out!

What made you choose the iStar lens and how do you know how it would compare with say MOUNTED TELESCOPE OBJECTIVE 127MM DIAMETER, 1200MM FL - Surplus Shed

This looks like an superb build. Love to see the results.

regards

John

[dweller25]

Dweller25,

Thank you for the kind comments. I think we only live about 10 miles from each other. If you want to come and have a look and a brew it wouldn't be a problem.

You have PM .....

[hunterknox]

Having seen those pictures I can't believe there aren't more polished metal scopes around. That thing's gorgeous! Well done, sir.

[George]

Excellent post Argonaut and well done on the finished scope....looks beautiful

[argonaut]

Thanks for the very kind comments people,much appreciated.

John(jnp),

The surpluss shed lens is excellent. I know quite a few people who have built very nice refractors with this lens.

I chose the 6"f/10 because I wanted more aperture. A touch of that terrible disease,aperture fever. Fatal with a dodgy back:)

I chatted to Ales at Istar for a while before ordering it. He is so enthusiastic about his new business and very helpful.

I got the first one which came with a report. I know some people think that reports arn't worth the paper they're printed on. I have no views either way.

[argonaut]

The tube length is 58" including a 2" diagonal. The weight with tube rings and diagonal came in at 30lbs.

I chose the Giro 3 mount and counterweights. The tripod and extension is the Losmandy G11,which I bought used. So all up weight for the Giro 3 to carry runs out at about 55lbs.

If anyone is interested in a first light report,just let me know.

Thanks for reading.

[AndyH]

I take it, the tripod is as solid as it looks? What a beast !!

Cheers,

Andy.

[argonaut]

Yes, it is Andy. The weight of the tripod is around 35lbs and then another 5lbs for the extension and then another 4lbs for the adaptor I made.

It's a 3 trip carry out with the help of my long suffering wife.

[AndyH]

She must love you

Mine would tell me where I could stick the tripod

Andy.

[argonaut]

38 years this year. I got a good one first time round

[Doc]

This is a great thread.

Can you tell us about the baffles inside the scope.

[argonaut]

Hi Mick,

I knew this one was coming

With baffles I was on a loser straight from the start with having a narrow tube. They do say that the narrower the tube,the more baffles you need.

Anyway, I wanted a 5 baffle tube but just couldn't fit 5 in. Well,not really,I could fit 5 in,but not where they would make a difference. I ended up with 3. I made these from .7mm Aluminium sheet. I was going to mount these on three equi spaced pieces of dowel. What I needed to do was cut the largest first and place it in the tube and look through the focuser to see if I could see the outer edge of the lens. This would tell me if there was any masking of the aperture and to see if the focuser vignetted any of the image. I made the dowel a tight fit so that I could move it if needed.

I then cut the second baffle which I placed around 9" behind the front baffle and went through the same procedure about masking and vignetting. The 3rd and last baffle was done in the same way,checking all the time through the focuser.

There are diagrams on the internet somewhere. I just used my own eyes. When everything looked OK I slipped out the baffle system from the OTA and gave it a couple of coats of undercoat and matt Black. The contrast difference between a matt Black tube without baffles and one with baffles was stunning(my first superlative ) I just wish I could have added a further baffle nearer the objective but the narrow tube held me back. Some people also leave gaps in their baffles where they touch the OTA. This I believe helps with tube currants. I didn't do this. I suppose my scope is still a work in progress.

I also need to find a better matt Black. I wouldn't like to try covering the inside of the tube with flocking. Mind you,as far as I know Televue don't use baffles,it's some type of gritty matt Black paper.