Stalled ... (again)

heh ... 10 months later and the cardboard tube is still standing in the corner, next to the (unfinished) model railroad ...  :/

Couple of problems, that just kept me from progressing ...

1) The tube isn't quite big enough ... I REALLY would like 1" airspace between the incoming light and the mirror edge to minimize thermals ... and the tube is only 7" inside ... not enough.

2) Thick cardboard is heavy.

3) I am concerned with the "stability" of the material ... this tube is designed to be low cost as its only purpose was to serve as a disposable support to hold a paper product.

In the meantime, I've reconsidered my original plan.  I will stick with the hexagonal tube, but will use plywood to create support "rings" and use a thin masonite "skin" to make the tube.  The advantage of the masonite is it will 1) provide a finished surface on the outside; 2) a rough surface on the inside (to help mitigate grazing reflections); and 3) it is dimensionally stable from temperature and humidity.

I have worked up an inverted primary mirror cell, that will be more compact (important for keeping the overall tube length down); while allowing for easy removal of the mirror (by removing the three collimation nuts from the bolts).

Let's see how long it takes me to make progress or change my mind ....

A New Beginning ...

*cough*

Has it really been nearly 18 months?  Where did the time go?  :(

I built the saw horses ... and then went on to construct a model railroad.  :p

It has been raining a lot here in Sri Lanka, very seldom do I get a chance to see the stars.  I did purchase a nice set of binoculars to view with; however, they have sat, pretty much unused ...  :(

The prospect of building a wooden tube has resulted in "Analysis Paralysis" ... I just could not bring myself to cut wood without a clear path forward for ensuring the resulting structure would be serviceable.  That changed the other day.

There is a digital printing shop in a small shopping center that I frequent.  They print large banners and such (I had them print a backdrop for my model railroad).  The material comes on rolls, supported by a cardboard tube.  I had seen many of these tubes discarded outside the shop, waiting for collection by the clean-up crews; but they had all been only 4' in length, and the internal diameter was not much more than the 6" diameter of my mirror; so I never gave it much more thought.

The other day I saw a much larger tube laying there ... over 10' long and with a 7" inside diameter ... ureka!  Problem solved.  I spoke with the shopping center "watchman" (as it was after hours and the shop was closed); but he was new and didn't want to take the responsibility for letting me walk off with a large cardboard tube.  So, the next morning, bright and early, I went to the shop.  The tube was still there and it had not rained that night.  The shop was just opening, so I ducked under the partially raised security fence to speak with the folks inside.  Seems the watchman had mentioned that I had been there that night asking about the tube and the shop keep was more than happy to let me take it.  He even requested my telephone number to call me as they became available.  :)

So, with a tube in hand, it seems I can finally get moving on this project again.  :)

Next: Design Update

More wood ...

lol ... one of the reasons I was so interested in having my woodworking friend construct the wooden assemblies of the telescope is because I HAVE NO PLACE TO WORK WOOD!  :p

So, it was off to the Internet to search for plans for building a pair of sawhorses.  :)

I came across this interesting set by Ken ... exactly what I was looking for.  Simple enough to build, low enough to use a knee to help hold the work, and 'stackable' ... a very nice feature for storing them.  I will buy a door to use as a flat working surface.

Today, after church, stopped by the wood shop and placed my order for some 1x6's and 2x4's, which will be available tomorrow evening.  Note that I am using  2x4's vice the 2x6's for the top as 2x6's were not readily available.  Wood in Sri Lanka is mainly rough finished, so the 2x4's are an actual 2" by 4" in size, which should provide plenty of surface area.

Feels good to finally be moving forward with this.  :)

Change in plans ...

OK ... it has been a bit frustrating these past two weeks; and finally today, I've listened to what my 'gut' has been telling me for some time now ....

Some good news:

- My Moonlite focuser arrived about ten days ago (about two weeks sooner than anticipated).  It was delivered to my door, so I avoided paying 'duty' on it ... big surprise there!  It is absolutely beautiful and the operation is silky smooth.

- Last Saturday (01 Feb), the eyepieces and Telrad finder arrived.  Duty was charged only for the set of four 1.25" eyepieces (the 2" 38 mm and finder were 'ignored' as they were listed on a separate invoice) ... 2,000 Rs. (~$18), so not too bad, considering ...

So ... all the parts that were moving from the USA have arrived.  :)

But ...

I have NOTHING to use them with.  I was hoping to get a friend from church who is quite skilled in woodworking and has the proper tools to build the wooden parts for my 'project'; however, his schedule seems to be very busy and in three weeks time, all that he has accomplished is to create some 'drawings' (for my approval), which I still have not seen.  Last Sunday he was tied up with church matters, but agreed to get together with me on Tuesday.  Tuesday came and 'something came up' on his end, so he rescheduled for Thursday.  On Thursday, it was 'something else', try again tomorrow.  On Friday, I had a scheduling problem, so deferred to Saturday (today).  Today, couldn't meet again as he was busy with cabinets, but he would see me tomorrow after church.

After that conversation, I confirmed that nothing had been purchased (nothing had) and told him that I'd simply run out of time (not to mention patience) and that I'd build it myself.  So, Salani (younger daughter) and I headed to town to see what we could come up with.  Salani had the good sense to ask a 'three-wheel' (rickshaw taxi) driver if he knew where to get plywood; he did, and we went there.  I got a 3' x 6' sheet of 12mm (1/2") cabinet grade plywood to make the tube.  I also ordered a 4' x 8' sheet of 18 mm (3/4") cabinet grade plywood for the cradle and base (normally not stocked but available in full sheets).  It will be here Wednesday.  Purchased a Skilsaw (circular saw) to make straight cuts (already own a jig saw and drill).  Looked at routers ... was told for the little bit of work I required one for, I could rent.

Next up, need to buy some lumber to construct some sawhorses to use as a work bench.  :p

So, I have regained control of my own destiny.  The finished product will not be as nice as what my friend could have produced; but at least it will get finished.

Update ...

I have passed along my design for a six-sided tube to my wood-working friend from church.  It will be 5" on a side, made from 9 mm (3/8") panels with longitudinal supports on the corners for strength.  It will have a minimum of 8" internal diameter which will provide for the 1" minimum clearance around my mirror.

Preliminary design indicates a tube length of 67" (see the "Newt on the Web" data sheets for details); so I will ask for 69-72" long and trim to fit after verifying the placement of the mirrors.  Easier to make it shorter than the other way around ...

My friend will first construct a much shorter (couple of inches long) 'mock-up' as a proof of concept of the design.  He thought it would be ready by Sunday; no real rush as until the materials I ordered from the States are in hand, the tube, primary cell and secondary holder are pretty much the only parts that can be built for now ... everything else (including the final length of the tube) depend upon the final placement of the hardware, including the focuser and eyepieces.

Just for giggles, I've been playing around with "SketchUp" a 3D drawing tool that helps put things into perspective.  Below you can see a 69" tube with a 6" mirror ...

Status Update ... sort of

I ordered some eyepieces and a Telrad finder today; estimated delivery by mid-February.  So, with the Moonlight focuser from the other day, everything that I think I need to purchase is now moving toward Sri Lanka.  :)

As I've been pondering my options for a tube, I've pretty much convinced myself that I'd prefer the look and feel of a hand crafted wooden tube.  I traced out a full sized sketch of a six-sided tube with 8" inside face-to-face and I like its proportions.  Will speak with my woodworking friend tomorrow and see what he thinks.  A nice dark stained wooden tube would really be sharp.

"Paint it Black" ... I bought some very flat black paint today to paint the inside of the tube and all the mounting hardware for the mirrors.  It is important to minimize or eliminate any stray light which reduces the contrast of the image.  Basically, the only light you want reaching your eye, is light that has been reflected (and focused) by the primary mirror.

I purchased a set of four eyepieces of 20/15/9/6 mm focal lengths.  All have good eye-relief and present a 66 degree apparent field of view.  They are often recommended as being very good for their price; i.e., "bang for the buck".  :)

The 20 mm will yield a magnification of ~80X, and will show a patch of sky ~0.8 degrees in diameter (the full moon is about 0.5 degrees in diameter, so it would be half again as large).  For a 6" telescope, this works out to about the maximum magnification for resolving details ... as we increase the magnification, the image gets larger, however no new details will be noticed.

The 15 mm provides a magnification of ~110X, with a field of view of ~0.6 degrees of sky (slightly larger than the full moon).  I don't anticipate using this eyepiece that much, wishing it were a 12 mm instead ... but that is the way it was packaged.

The 9 mm will provide a magnification of ~180X, which is about the maximum magnification one can use on a 6" scope under normal conditions.  I expect this eyepiece will be used the most.

Finally, the 6 mm will provide a magnification of ~270X, which is about the maximum magnification one can use under very steady 'seeing' conditions with a 6" scope.  Although the nights are rare when one can use that much magnification; it is nice to have the means to take advantage of excellent seeing when one gets the opportunity.  :)

The Telrad finder is a 1X finder; i.e., what you see through the finder is the same size as it normally appears ... just like looking through a window ... which is essentially what you are doing.  The main difference is the Telrad projects a set of circles on that window that can be used to help point the telescope at the target ... or at least get it close enough that you can see the target with a low power eyepiece (i.e., the 20 mm in my case).  These finders are simple to use and most everyone who has ever used one recommends it.

Working up the Design ...

... Crunching the Numbers.

A Newtonian telescope is a simple design consisting of:
1) a paraboloidal primary mirror, which focuses the incoming light to form an image;
2) a flat secondary mirror, which reflects the converging light beam to the side, where it can be observed without the observer's head blocking the incoming light; and
3) an eyepiece used to examine the image formed by the primary mirror.
The eyepiece is normally placed in a focuser, which allows small changes in the separation between the primary mirror and the eyepiece, to properly align the eyepiece with the image formed by the primary mirror.

The mirror Kevin F. sent me is a 6" f/10.6.  This simply means that the mirror is 6 inches in diameter (which for a Newtonian design will also be the aperture) and that the focus is 10.6 diameters away from the mirror, or 63.6 inches.

The secondary 'flat' mirror is an elliptical shape and 1" wide along its smallest dimension (minor axis).  Since it will be tilted at 45 degrees to reflect the light cone outside the tube, the elliptical shape will present a circular shape to both the incoming light beam and the eyepiece (held in the focuser) ... this minimizes the amount of light blocked by the secondary mirror.

We now have enough information to properly design the Optical Tube Assembly (OTA) and be confident that it will work after construction.  There is a very nice web-based program that will help us design our Newtonian telescope called 'Newt for the Web' and you can find it here:
http://stellafane.org/tm/newt-web/newt-web.html

Under the Specifications tab I entered the following data:

The information for the "Optics" section comes from my mirrors.  The information for the "Tube" section is based on my 'ideal' tube; it may be adjusted later if necessary to account for small changes in diameter or thickness.  The information for the "Focuser" section is based upon the data given for the Moonlight focuser from the company web site.

Moving to the "Ray Trace" tab, we can see what our design will look like:

I turned the 'baffles' option off, as there is no real advantage to baffling the tube of a Newtonian (unlike a refractor); and honestly, including tube baffles in a Newtonian may prove detrimental to the performance as the baffles will force the warm air rising from the mirror as the night time temperatures drop into the incoming light path ... remember those 'thermals' we are trying to avoid?

The "Performance" tab has some interesting information:

The main things here are that the 1" secondary mirror will produce a 17% central obstruction to the incoming light path.  We desire this to be small, the goal being <20%; so we are good here.  Also note that the fully illuminated field (100%) is 0.467"; close enough to the 0.5" that is often referenced as adequate for visual use.

And finally, the Dimensions" tab:

This tab is where we find the critical measurements for construction of the OTA.  I've already determined the diameter of the tube (which was input in the "Specifications" page); and now I can see that I'll need a tube at least 67" long.  Again, we will ignore the baffles data as tube baffles will not be used.  :)