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350 newt project


paul11
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Hi all I have recently finished a 200mm F 5.6 C/F Newt and for a first telescope I am pleased with it but now I would like to make a bigger one. I would like to make a 350mm F5 and put it in a 400mm C/F tube, I want to use it to image the planets. Now in theory would this out perform my Meade 250mm SCT for imaging planets? Thanks,,,Paul.

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Hmm pretty tough and probably too close to call...

The longer focal length will always be a key for contrast and definition so the meade does have the edge...over light grasp.

Said that - if the optical set is very good 1/8th then it would be pretty hard to tell if not possibly better than the Meade..

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Close call....the longer FL of the meade may edge it as you're not worried about light grasp with planets, and generally atmospheric conditions will prevent the 14 inch from getting up to its full resolving power.

DSO's though....different story.

The 14 will be really good, and a perfect FL for many galaxies.

Cheers

Rob

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I think it will be a great project, and having seen your other scope, I'd be interested in having a bash myself too, as a big lightbucket is something I don't have, and a 14 inch F5 or so scope is perfect for most of may favourite objects ;)

Lets face it....if you don't want to keep it, you'll have no trouble shifting it....just give me first refusal ;)

Cheers

Rob

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Sounds like a great project Paul.I'm in the process of doing the same thing. Grinding my own 14" F5 mirror, although I am still in the planning stages at the moment. I don't intend to put it in a solid tube though,as I don't have the storage space for an enormous looking missile launcher lol. Instead I am basing my design on the Obsesssion style truss dob, which will also have a home built GOTO drive system, and Homemade DSC. Well thats the plan anyway. lol

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Sounds like a great project Paul.I'm in the process of doing the same thing. Grinding my own 14" F5 mirror, although I am still in the planning stages at the moment. I don't intend to put it in a solid tube though,as I don't have the storage space for an enormous looking missile launcher lol. Instead I am basing my design on the Obsesssion style truss dob, which will also have a home built GOTO drive system, and Homemade DSC. Well thats the plan anyway. lol

Yours also sounds like a great project,have you hogged it out yet and if so how long did it take you and what grit did you use and how thick is your mirror blank,,Paul

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Hi Chris I will get a 350 blank from Galvoptics and grind my own mirror and I will get a secondary and spider from ProtoStar

When I can shift the load of guitar commissions I have (wood dust free workshop), I am going to grind a 12" mirror.

Are you hand grinding, or do you have a motorised setup? How long did it take you to grind the last one?

Edited by Beulah
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Hi Paul,

Sorry about the delay in replying.

Yes, it is my first blank, and I know most will advise me to use a smaller blank, but I have a lifetime's experience in hand crafts, some heavy duty (wood), bags of patience, and chats with other mirror grinders have been encouraging, providing I am careful. :eek:

Edited by Beulah
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Hi Paul

Sorry the delay in replying. Been away for a few day.

I havn't even started anything yet. Im still in the design stages. Ive neva ground a mirror b4, and i know they suggest that you should start off small, but I intend to take my time with it. No matter how long it takes. Its been my dream since childhood to have a big scope, so ive finally decided to bite the bullet and get it started this yr. On the plus side, I have a very good misses who is very understanding. She thinks its a great idea in fact. I no my ex wife would neva of agreed. lol

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Paul, I see that you have ordered two 360mm blanks, are you thinking of using one as a tool? I mention this as it would be a lot quicker to grind the mirror face up with a smaller tool in order to put the curve in. Some folks use a barbell weight for this hogging process. For a 14 inch mirror a 5 or 6 inch tool would work well, use 80 grit abrasive or courser. The tool is worked centre thru centre inorder to put the curve in. When the curve is just over the required depth make a full sized cement tool by fixing a damn round the mirror blank about 1 inch high. Pour in a mixtute of pure cement and water and level off the top. When this has fully dried remove it from the blank and cover it's surface with cermamic tiles (about 1.5 to 2 inches square should be OK), use epoxy adhesive to stick them down. You now have a grinding tool. You could make two cement tools, the other to be used for the pitch lap, alternatively you could use a thick plywood base for the pitch lap. Just a few thoughts, hope they are of some help. Great to see people thinking of grinding and polishing their own optics.

John

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Yes, that's it. Don't make the smallest 'hogging' tool, buy a barbell weight. You don't need contact between the entire weight and the glass as long as the outer edge of the weight is in contact with the glass. Then you have your full sized grinding tool, cement with ceramic tiles stuck on it. Your final 'tool' is for the lap to be poured onto, this can be cement or plywood if you prefer. If it is plywood it needs to be 1.5 inches thick for a 14 inch mirror.

If you need further clarification please ask.

John

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Hi Paul

Sorry the delay in replying. Been away for a few day.

I havn't even started anything yet. Im still in the design stages. Ive neva ground a mirror b4, and i know they suggest that you should start off small, but I intend to take my time with it. No matter how long it takes. Its been my dream since childhood to have a big scope, so ive finally decided to bite the bullet and get it started this yr. On the plus side, I have a very good misses who is very understanding. She thinks its a great idea in fact. I no my ex wife would neva of agreed. lol

Like you I also wanted a big telescope (14”) and the only way I was going to get one was to make it. I made an 8” F5.4 for my first mirror, I didn’t want to I wanted to make a 14” but I was advised every person I talked to on the Internet not to. There was a very good reason for this they knew a lot more about making mirrors than I did,,,,Paul.

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Can I ask how much the blanks tend to go for? On the site you mentioned the 6" 'kit' is 65 quid I think. They dont list the 14" blanks - I assume you can order blanks separate to the kits by phone?

John

Hi John my blanks were £66 here is a link to the site all the prices are there just click on dob mirrors telescope kits, build your own. newtonian , dobsonian

Edited by paul11
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Lots of people have successfully made their own mirrors, its not easy, but then nothing really worth doing is! Making a large fast ( ~F4-ish) mirror for low power wide field work is certainly doable. For planetary work an 8 inch F10 is relatively easy to figure and will give superb images, but that's a long scope! The real challenge comes when you need high power viewing from a relatively fast system. Be realistic about what is acheivable and why you are doing it. There are many resources out there to help with mirror making, take a look at this list.....

Stellafane Links Page

Have a look on youtube, some stuff there on mirror making. I have been making mirrors for over 30 years and would be more than happy to help anyone with their projects. Mirror making is a lot easier when you have someone to turn to when the going gets tough, I know I've been there! The thrill of veiwing the heavens with your own optics is will have made all of the hard work making the mirror worth while.

John

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