Monday, 10 April 2017

Tricking up rigidity on a DIY wedge

A few years ago I salvaged an old set of SCT forks that was about to be trashed.  The RA motor still worked, and its handset still controlled the RA rate.  I realised that this old fork mount would be perfect as a video astronomy rig.  So I turned my DIY hand to making a wedge to suit.

For my purposes, my immediate design appeared to be fine.  With sketching being my primary niche in astro, and at the time using another SCT, I never really paid much attention to the consistent error in alignment that the mount had.  No matter how accurately I set the wedge to my latitude, and no matter how much care I took with my rustic alignment, there was a persistent misalignment that saw the object drift out from the FOV.

I recently bought a brand new SCT OTA that I coupled to these old forks.  When I coupled the scope to the wedge, I noticed something shocking!  Something that I should have been aware of and stopped from occurring when I built the wedge – FLEX!!!

Despite the 30mm thickness of the platform that the mount sits on, long and unsupported cantilevered plywood platform yields to the large mass of the likewise cantilevered mount!  A doubly cantilevered system.  And now add the heavy mass of the new OTA, and the amount of flex in the platform increased.

This explained the consistent misalignment of the video rig!

But how to fix this?


A pair of aluminium angle brackets placed along the length of the mounting platform.

The mechanical properties offered by the brackets acts much like an RSJ bar (I bar).  And with using just four screws to fix the brackets, the bracket/platform arrangement acts just like a laminated system, increasing the rigidity of the system.

Over the weekend I tested the pimped wedge.  When I coupled the mount to the wedge I cannot say I noticed any flex in the platform.  With the magnification on the scope at 250X, there was only the most minor misalignment noticeable that despite my rough and ready alignment of the wedge kept Jupiter in the FOV for a couple of hours. 

Flex problem 95% solved!  Any further improvement to the remaining flex will mean rebuilding the whole wedge - something I have not totally discounted as this pine plywood can be improved upon that will further improve rigidity, the new design would reduce the length of the unsupported mounting platform, and the wedge bracing would also be more centred over the tripod.  All of these are weak points in the current wedge.

Now for some clear skies for a sketch or two!


Thursday, 6 April 2017

My $1 scope case (well, sort of $1...)

Hi all,

With the sky continuing to be terrible for sketching, it has afforded me time to consider other projects I’ve had in mind for some time.

Well, I have my Frankenstein SCT, but I don’t have anything to store it in out of harm’s way other than a plastic bag – and that just won’t do.  My old orange tube C8 came in a trunk, and something that size would be perfect for this “new” instrument.

Purchasing something ready-made though was not ideal as really for a few reasons:  either be too large or too small, not support the scope as needed to minimise undue stains on clutches and the OTA, and adapting a ready-made case may be either difficult or implausible due to the materials it is made from.

So the only option is DIY.

The other day I went to my local hardware store to look for some plywood for this project when I noticed a few long sheets of MDF that were being collected for disposal.  Their apparent size quickly drew my attention, and with a tape measure I realised that just one of these sheets would be perfect for my project!  So, $1 later, I carted off a brilliant find for me.

Ok, the $1 cost of the case is just of the MDF.  But this was the only additional expense I had in making the case as every other component I already had at home.  The true cost of the case with the other components considered was closer to $60.  Other than the MDF, all the other timber elements were scavenged off-cuts from home, and the foam rubber was also off-cuts.

Knocking up the box was easy.  What concerned me was making it too easy for little creepy-crawlies to sneak in through the gap between the lid and base, so from a plywood off-cut I fashioned a lip that the lid sits around, and provides a better seal.

Ok, now I have a very drab case made out of MDF large enough to hold the scope.  So it won’t be enough just to make a scope case, but the MDF needs to be pimped too.

The real work now commences to fabricate the necessary supporting elements.  That was the one problem with the case of my old C8 – the case was just that, an empty box, and the fork and OTA were pretty much left on their own inside the foam filled case.

First thing though was to varnish the entire case inside and out to protect it from shedding its uncoated fibres.  The varnishing is also the first part in the pimping process to seal the surface.  This allows for the decorative processes to happen.

The OTA is supported as is the fork mount, to minimise undue stain on the clamps and mounting brackets, and to allow for the OTA and fork mount to be supported should the clamps come loose.  The distribution of the supports and the OTA also allows for plenty of storage space for bits and pieces.  Down the track I can install compartments to hold accessories.

The decoration of the external MDF surface was done taking inspiration from some how-to videos on Youtube on how to achieve a faux-timber appearance on MDF.  The decorating was done before the aluminium trim was fitted.

The screws used to fix the trim were selected according to the faces that the case is to sit on, namely the bottom face, and the back face with the hinge (the case is stored standing on this hinge face).  Those faces not taking the weight of the case have pan-head screws that sit proud above the trim.  Those two faces which are used to sit the case on, the screws are countersunk below the trim.

Even the handles have special consideration.  As MDF is not the hardest material, special measures need be taken to reinforce the areas around the fixing screws.  The top pair of screws bite into the hardwood plywood lip.  The bottom pair of screws are machine screws that back onto a pair of massive washers to distribute the load of the case over a very large area, and thereby avoid the risk of screws pulling out.

I am very happy with the way this case has turned out.  A couple of friends have commented that they couldn’t believe that this was MDF!  There are plenty of hollows which can be used to store accessories, such as a diagonal and finder.  I can add compartments to the case as my experience with the case grows.  Velcro straps could also be used to hold some of the accessories.

If you’ve been considering scope case options, I hope this project of mine has given you some ideas on what can be done.  My project is certainly not the be all and end all with scope cases.  It is just my take with the materials I had at hand, the ideas of what I wanted, and the inspiration that came to me.