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150mm F5 refractor refitted with Istar glass

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#1 John Jarosz

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Posted 24 September 2011 - 09:32 AM

I began reading S&T (as a teen) in the mid 60's and was always interested in the ATM columns. So the Jaegers ads were something I knew pretty well. The one item that really intrigued me was the 6" F5 refractor objective. The price ($180 glass only or $195 mounted in a cell) were out of my reach, but the idea of a short refractor stayed with me. The same ad had 6" F8 mirrors for $50. I started observing with a 6" F13 reflector, so maybe that's why I kept coming back to short tube instruments. The other two scopes I've built are a 6" F4 reflector and my Dall relay scope is really short and doesn't even have a tube.

Fast forward 30+ years. There was a cheapo 6" F5 objective on ebay that was badly scratched during cleaning for cheap. I decided to follow up on the short refractor just for fun. I figured if I liked it I would get a better objective some day. I made a non-adjustable cell and tailpiece, and had a short section of tube. To keep things simple I used a Kineoptics helical crayford focuser and a Lumicon 2" diagonal, because I could easily remove one of the the tubes from it and replace it with the focuser.. I read over and over how that short of a refractor would have a lot of CA, so I got the Baader semi-APO filter that is permanently mounted between the objective and the diagonal. Over the years there have been lots of articles both in S&T and online about very short focus 6" refractors - so I kept hearing about it with different points of view. Some refractor people really don't like short focus because of the CA. Others (usually reflector people like me) don't seem to mind at all.

The picture of the original construction is below. I originally left it unpainted and basically unfinished. Due to the cleaning scratches the resulting views were really soft and low contrast, but I really didn't mind the CA using the Baader filter. So I felt I proved out the design. I started waiting for a reasonable objective to come along.

I looked at the new Jaegers site in 2008 that listed the 6" F5 lens set, but they wanted $1750 for the glass. No real description or qualifications so I wouldn't take that chance. The scope sat until Istar came along. They listed a 6"F5. I waited while someone else here (Mr. Bill) at CN took a chance. While he had some glitches, things worked out well for him so I decided to give Istar a try. The price certainly was right.

I received the lens last week. The workmanship is very good with nice polish and coating on the lens. The push/pull cell is well done. I had to make a tube end that mated to the cell and adjust the tube length to compensate for the cell. My 1936 9" lathe had just barely enough torque to turn the parts. Almost every detail was worked out on a drawing before cutting metal. I can post a PDF of the drawing if anyone is interested. The dew shield is a stainless steel cannister with a cool rubber cap that seals off the lens from the outside. Handles are from an aluminum pot. There are 2 internal baffles. Without an eyepiece the scope weighs 17 pounds.

The Kineoptics focuser has 1.25 inches of travel. I use Vixen LVW eyepieces plus a 31mm Baader Hyperion. There's about a 3/4" difference in focal plane between the Vixen and Baader eyepieces so there's no problem there. The diagonal makes the view upright but leaves it reversed left to right. For this type of scope, the 2" diagonal and 2" focuser does chop off part of the useful light cone. If I feel I need to correct that the tailpiece is large enough so I could attach a larger mirror diagonal (but I'd have to scratch build it).

The skies have not been amenable for star testing lately. The terrestrial views are quite good but I need to get some star work in. I'll report on that when the clouds clear.

I'll put up some photos of the new construction in subsequent posts.

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#2 John Jarosz

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Posted 24 September 2011 - 09:33 AM

Here's the finished scope on my G11

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#3 John Jarosz

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Posted 24 September 2011 - 09:34 AM

Lens & cell

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#4 John Jarosz

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Posted 24 September 2011 - 09:36 AM

The assembly is aligned by laser. You can see the dot close to the center of the aperture.

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#5 Mike I. Jones

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Posted 24 September 2011 - 09:37 AM

Beautiful, well thought out scope, JJ! Can't wait to hear your observing reports. You and I must be about the same age (and degree of deterioration!).
Mike

#6 John Jarosz

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Posted 24 September 2011 - 09:38 AM

Here's a detail of the cell. The contact point has been designed to be more compliant that solid metal so there's less contact stress on the glass.

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#7 John Jarosz

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Posted 24 September 2011 - 09:40 AM

Rear view

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#8 John Jarosz

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Posted 24 September 2011 - 09:42 AM

Here's the diagonal/focuser with the Baader filter

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#9 Mirzam

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Posted 24 September 2011 - 11:00 AM

Hi John,
I have a 5" f/5 homebuilt scope with a Jaegers lens. I have never been bothered by the CA. You can certainly see the CA if looking at the moon or Venus, (or Vega etc..) but most of the time the scope is used for wide field deep sky viewing. I use magnifications up to 63x with a 10mm Ethos eyepiece, which gives remarkably sharp focus across the FOV.

Have you tried the scope without the filter?

I also built the diagonal mirror into the tube to avoid vignetting.

Beautiful looking scope you have there!

JimC

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#10 John Jarosz

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Posted 24 September 2011 - 11:18 AM

Jim:
Cool. It looks like a Newtonian. You could confuse people at star parties. :grin:

Whenever I've checked out the refractor forum it always seems like there is a discussion about tiny amounts of CA. It doesn't bother me either. I have it on the list to check out the view without the filter.

As I remember Jaegers (back in the day) used to charge about half the cost of the 6" F5 for the 5" F5. But I wanted to go whole hog.

I have made a conscious effort to avoid looking thru an Ethos. That way I don't know what I'm missing. :wron: :wron:

John

#11 Mark Harry

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Posted 24 September 2011 - 11:27 AM

Nice looking scope, John! Looks like a keeper. Ingenious idea with the low-stress pressure point lens cell.
Mark

#12 Mirzam

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Posted 24 September 2011 - 11:33 AM

Another thing to add to your list would be to try an OIII filter on the North American Neb using the 6" f/5. (Under a good dark sky of course).

JimC

#13 Wes James

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Posted 24 September 2011 - 11:44 AM

John-
I like this scope a lot. A LOT. I like your use of the helical focuser on the diagonal- I've not seen that- and like the idea- also your handles on the scope! All in all, a very interesting scope! A very interesting example of evolution! The ring against the objective, fwiw- is exactly like the retaining ring washer on my Zeiss Telementor 63mm objective/cell. A very effective way of not putting excessive pressure on the glass.
What kind of a tripod are you mounting your G-11 on??

#14 mikey cee

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Posted 24 September 2011 - 11:51 AM

Wow John a real comet sweeper! Nice workmanship to boot! Where did you get the idea for those nice stout handles? :question: :roflmao:Mike

#15 John Jarosz

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Posted 24 September 2011 - 12:30 PM

OK, now it can be told. I STOLE the idea for the handles from mikey cee. :grin:

That cell detail is part of Istar's design, but I thought it worth mentioning.

The tripod I built a long time ago but I forget there's a whole new crew of people that haven't seen it. The tips are ash, the main body is mahogany. Nothing is adjustable and the legs are rigidly attached under the center column to absorb the overturning moment when the top of the mount gets pushed on. It's really a stiff tripod. The golf cart wheels have been added recently. With the handle mounted in the dovetail I can wheel it around like a hand truck. The G11 is really nice but it is REALLY heavy.

Thanks for the compliments.

john

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#16 neo

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Posted 24 September 2011 - 01:50 PM

Absolutely beautiful scope :waytogo:
I love that greenish coating from the pic with the lens, it's trully mesmerizing :)
How much does the Baader filter help?

#17 mikey cee

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Posted 24 September 2011 - 02:09 PM

John I too have those slots in the spacer. I don't know but I believe this ring just sits there and only the retaining ring above it is threaded. My feet are over the spacers. Yours may have worked their way off center. :question: I don't really know I'm just speculatin'. My Tasco 20TE has a similar setup. :grin: Mike

#18 seryddwr

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Posted 24 September 2011 - 08:05 PM

John, where did you get those handles? I have a 6" f/4.8, I think, that could use them.

#19 John Jarosz

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Posted 24 September 2011 - 09:50 PM

Well.........

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#20 mikey cee

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Posted 24 September 2011 - 10:31 PM

John.....My wife bless her heart put my $80 stock pot to good use in the back yard. It has flowers planted in it. Like "they" say......"nothing in nature goes to waste". :lol: :lol: Mike

#21 seryddwr

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Posted 24 September 2011 - 11:20 PM

Oh HO! That's a clever idea. Hmmm, where can I find a cheap stock pot... :thinking:

#22 Wes James

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Posted 25 September 2011 - 08:42 AM

John-
I'm interested in more details on your diagonal/focuser... can you offer more details or photos on how you made that? What did it start out life as?? Obviously not a stockpot! ;) Did it start out as a conventional diagonal that you wanted to mount the helical focuser to? Thanks!
Wes

#23 John Jarosz

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Posted 25 September 2011 - 09:01 AM

Wes,

The Lumicon 2" Diagonal is somewhat unique in that the tube that the eyepiece is placed in is simply bolted on to the mirror box. So you can remove it and replace it with whatever you like. I just made a block that accepts the Kineoptics focuser and made the block so it would bolt onto the Lumicon diagonal. It's really pretty simple.

I don't think there's anything I've bought that I haven't modified in some way.

It is a good diagonal but I did find mine used so it was cheaper. :wron:

#24 Wes James

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Posted 25 September 2011 - 11:23 AM

Ah, ha... makes a lot of sense, now knowing that! I'd never seen a diagonal made that way.
So what'd you do with the rest of the stockpot?? *grin*
Wes

#25 John Jarosz

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Posted 25 September 2011 - 11:30 AM

Where can you get a pair of handles like that for $16. :grin:

I love using things for purposes that they were not originally intended for. I sort of take it like a challenge.

While the dewshield on this scope is a SS cannister, I'll say that there are some new aluminum stockpots that have very thin walls that also would be good for dewshields. They might even come with handles. :whistle:

Just don't take them from the kitchen, or you'll need to sleep with one eye open. :smirk:






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