Telescope bit and bobs

Welcome to Telescope Bits and Bobs.

Here I will post pictures of my telescopes, the modifications I make to them over time, and sources of inspiration from other people that are wonderful ideas.  A little place for my ATM and DIY.

~ x - X - x ~

(29th July, 2016)

My Sketch Pad Rig

Hi all,

Thought I'd show the sketch pad rig I use and made for myself.

It came from a desperate need to have something made quickly for an upcoming dark sky trip, and that holding the sketch pad in my hands was not the way to go when up a ladder.  Yet it was the very ladder that I used that proved the source of inspiration and design.

The Ladder

Taking after my mum and dad, I am not the tallest fellow in the world.  And even though I built my 17.5" Karee dob to have the zenith height of the eyepiece as low as possible to the ground, I still need a single step up to reach the eyepiece at zenith.

Ok, I now need a ladder - but which one?  I had to find one that satisfied a tight set of parameters:
*  Stable & sure footed
*  Easy of a novice to negotiate in the dark and allow them to hold onto so that they didn't first reach for the scope, especially little kids

Might seem not a broad set of criteria, but most ladders are not suitable.  Either too tall, the rungs too narrow, set too steep, even too many steps or too few.  Three steps would be idea.  I had an old step ladder at home that satisfied the second point of my needs, and had just three, but in the dark it was too small to be stable.  Then a chance visit to the local hardware store, and BINGO!  There was the perfect step ladder!  Three steps that were deep and wide, tall frame that allowed for easy gripping, and has a collapsible try at the top that I can use to hold the odd bit of gear.  Problem solved!

The sketch pad rig

With the step ladder issue solved, a desperate need to knock up a rig to hold my sketch pad became apparent.  The tall frame of the step ladder held the clue to how to design it.  All I had to do was find or design a hook for the clip board I was to use to hook over the ladder's frame.  Another trip to the hardware store and I found the perfect broad hook!  How lucky is that!!!

Another difficulty popped up when I thought about the sketch pads I use and could use.  I had A4 pads, which the clip board I had was designed for, and of course it could also hold smaller pads.  But, it couldn't take a larger pad, say of A3 size.  So the clip board was discarded, and a piece of perspex (Plexiglass) that was scrap from another project was the perfect size for A3 paper, and a little larger.  It was also thick enough to allow the hook to be screwed into it without having the fixing screws cause interference.  If I had to make this rig again, I would use plywood instead of perspex, as the perspex is very reflective and difficult to paint black.  I only used perspex in this instance as it was the material I had at hand at the time as I didn't have any plywood.

The "clip" that holds the paper securely I made myself from a timber off-cut, a couple of machine screws and a couple of wingnuts.  This mechanism allows me to not only accommodate any thickness of paper pad, but I can also regulate the squeezing pressure it applies so I can use either a single sheet of paper, or a really heavy and bulky A3 pad with equal easy and security - something that an off the shelf clipboard just doesn't allow.

Then the eternal problem that is the bane of amateur astronomers reared its ugly head - DEW.  A heating element for the sketch pad was out of the question as the thickness alone of the sketch pads can vary, would take ages to heat the paper pad, and would just complicate the entire set up, which I wanted to keep as simple as possible.  So, after some thinking about how dew forms, I came up with a simple solution.  Dew forms by falling.  So all I needed to do was design a shelter for the paper.  And the local council elections gave me the solution for the material to use - black Coreflute® sheet.  As I am right handed, the left side of the shelter could come straight down the side of the sketch rig.   Overhanging element I extended out further than the edge of the rig so that I wasn't hindered by the shelter, and the overhang also provided a little more protection from dew than if I just trimmed off the Coreflute® flush with the sketch rig.

The final design allows me to hook the rig not only onto my step ladder, but also just about any chair should I need the rig to be set down lower than the ladder will allow for.

There was just one element left to allow for - lighting.  That proved very easy to deal with.  Firstly, the lighting should not come from the direction that one is looking from.  Having the light come from where you are standing, square to the paper, you will get the glare from the reflection of the light off the paper come straight back at you, reducing contrast, washing out faint details, and hindering your precious night vision.  Having the incident light come from the side, the reflection off the page is sent off to the other side of the rig, and with very little coming up into one's eyes.

At first I used a bulldog clip to clamp the red light torch I first used onto the side of the rig's shelter.  Today I use a dual light gooseneck lamp with red LEDs that has its own clip that I mount into a notch I cut into the shelter.  I can then move the gooseneck lights freely at will to any part and direction onto the page.

So there you have it.  Inspiration, good fortune, stuff that's lying about the place, and a council election all serving to create the sketch rig that I use.


(25th October, 2015)

New use for old telescope fork mount

Hi all,

Many people who purchase a fork mounted Schmidt Cassegrain Telescope remove the optical tube from the fork and couple it to a german equatorial mount.  However, this then leaves a vacant set of forks.

So, what to do with a set of forks, and maybe even the excellent electronics that may have come with it, and maybe also the equatorial wedge and tripod all associated with the original SCT purchase?

I’ve combined this with my passion for outreach!

One thing that I love doing is sharing my love of astro in outreach evenings.   I’ve conducted outreach nights on my own as part of larger groups.  From pre-schoolers to Uni students, from little kids to members of the third age.  I just love sharing my passion.

I am also aware that light pollution is an ever increasing problem, and that in most instances, novice eyes combined with a washed out city sky, most people just will not be able to view things like galaxies, or bright nebulae.  This is where an electronic eye in the form of an astronomy video camera, can be a great asset to have up one’s sleeve in the big smoke.  Using an astro video camera is the only form of imaging that I do, and only as an addition to a direct visual experience.

Over the years, I have collected various telescopes.  Some for specific use, and others with a particular ATM project.  One such collection was an old set of Meade forks that once did carry an SCT.  The mount was destined for the tip, and the good mate who owned them offered it to me before it was ditched.  This fork mount came with the drive and a hand set to control the rate of the RA drive, and fortunately for me, can with some hardware to allow for encoders to be installed onto them.  And I had a couple of telescope candidates to couple to it.

Joining the two tynes together requires careful measuring and cutting of the joining board.  Too tight or too loose, and the bearings are forced into a binding.    Once the joining board is complete, the various OTAs can be coupled to the newly recommissioned forks!

The wedge for this Meade mount I had to make, which was easy to do with plywood.  To reduce any possible flex from the cantilevered telescope and mount, the 15mm ply was doubled.  The plywood wedge I can also vary its latitude position in a similar way to commercially available wedges.  I then had my kids decorate it!

I now use this fork mount now with a humble 114mm f/4.4 reflector.  Surprisingly, this little telescope is sensational with an astronomical video camera.  Its short focal length allows for a very wide true field of view, and the 114mm aperture is surprisingly powerful under light polluted skies!

At outreach events, I double team this set up with a fork mounted 8” SCT.  Folks can then both view through a telescope, and with some explaining, come to understand the limitations of both an urban sky and our human eyes, and then what an electronic ‘eye’ can offer us with even just a small aperture by comparison.

Recently I have added a set of optical encoders to the recommissioned forks.  These encoders I have coupled to a Nexus devise from Astro Devices.  The Nexus allows for a wireless connection and use my smart phone loaded with Skysafari to navigate to objects in the sky.

This complete recommissioned fork mount, modest scope, Nexus, astro camera & Skysafari setup goes a long way to show just what can be accomplished with modest equipment, and a new life for equipment fist thought to be redundant.

If you have a disused set of forks, or know of a set, I hope this article serves to breathe new life into them.


(20th April, 2015)

Article:  Cooling of Newtonian Opitcs - an insight

Cooling of astronomical telescope optics is a field that is misunderstood in amateur astronomy circles.  Many of the ideas being used at first sound feasible, but they ignore the very thermal properties of the materials that are involved.  This can result in dissatisfaction with a new instrument we have bought, or incorrectly blaming a manufacturer for problems that are of our own making.  Here I will mainly discuss the cooling of Newtonian optics in relation to the different ways they are presented (solid tube or open ‘truss’), and the different requirements of visual and photographic, but only token mention of other scope designs.  This revised edition of this article also includes suggestions and corrections offered by fellow Ice In Space members.  Their suggestions made me aware of the visual bias I had unwittingly put into the first article.  For this I am grateful!

Glass and how we use it
First thing to mention is glass.  Whatever material the substrate is, plate, borosilicate, etc, all are poor conductors of heat.  This means that they will take time to cool down.  However, as oils ain’t oils, glass ain’t glass.  Borosilicate glass types are the preferred material for astronomical instruments as they have a much smaller coefficient of expansion compared to plate glass.  This means that they expand less for the same increase in temperature.  The benefit for astronomical instuments, particularly for photography, the mirror is most dimensionally stable during the course of the night.  Yet as all glass types are poor conductors of heat, how they respond to cooling, natural or forced, needs to be understood and work with it, not against it.

In the original version of this article, I mentioned that the mirror doesn’t need fan cooling.  This is not incorrect, but it was misunderstood by some people, though poorly phrased by myself.  If the instrument is used for photographic it is advantageous to quickly have the primary mirror reach equilibrium with the ambient temperature to maximize productivity.  Fans ARE effective for this, but their application is what is most important here.  Incorrect application can have deleterious results.

Solid tube instrument and all photo
 We know the fundamental that hot air rises.  In a long tube system, heat only has one way to go – straight past the secondary mirror and on occasions the focuser too.  The metal tube will cool very fast due to its large surface area, but the primary will be dissipating heat for a longer period.  When using high magnification, these captive convection currents can become visible, and very disruptive for photography.

Two things conspire against us here - the length of the tube and the bulk and consequential residual heat of the primary.  The only way to change this is to have an efficient way to create a heat exchange.  The easiest is to have the rear of the scope as freely open as possible to have cool air be drawn in to replace the warm.  However, this is a slow process.

Now, the quintessential rear fan…
The other way to create the heat exchange is to force air.  This is done by the use of fans.  But the way they are employed is the key.  Most mass production instruments make use of a very open, almost skeletal, mirror cell assembly.  This is fine for a long slow cool, but a poor option for the use of fans.  Most of these instruments that have a fan attached to the cell have the fan blowing directly onto the back of the mirror.  Two problems here:
1, the draft of the fan is focused on one spot.  This creates a constant cold spot on the primary creating a strain within the glass due to a forced temperature differential with the warmer extremities of the mirror.  Plate glass in particular is more susceptible to deformation as a result, but even for borosilicate glass this is not ideal when a better solution is possible.
2, the air within the tube is not being exchanged as the draft from the fan is bounced back off the mirror straight out the open cell structure – this is the path of least resistance for the draft and it will not go around the edge of the mirror and up the tube.

Best practice
The best way to force a mirror to cool, with the least way of causing a constant temperature differential within it, is to draw the air over the largest possible surface area.
Note:  forcing a mirror to cool does induce a temperature differential, but this is a temporary one if the following system is employed.

In situations of a solid tube, and for all photo applications including with an solid tube or open, the mirror cell needs to be very open, but the rear opening of the scope immediately behind the cell needs to be closed, and the fans that are attached to the openings of this closed end need to blow OUT of the scope, not into the tube and hence the back of the primary mirror.  This is contrary to the initial thought of exchanging heat out of a scope.  However, several things happen in our favour:  with air being drawn out the back of the instrument, it makes it impossible for standing convection currents to form in the tube and the ‘boundary layer’ that forms in front of a warm mirror is disrupted; the largest possible surface area of the mirror is being subjected to the draft that is created instead of a single spot; and the draft of air helps in the prevention of dew formation.  This last point is very significant for instruments that have an open tube structure. 

Open tube or truss scopes
Many of us today make use of open tube or truss scopes.  These instruments can give us access to large and portable instruments.  But the cooling requirements of these, particularly for visual use, are a whole less burdensome.  The first thing in favour of open structures is there is no closed tube to allow a standing convection current to develop – warm air vents straight out through the open structure of the scope.  The typically very open structure of the rear cell allows for air to very easily and quickly move around the primary mirror to evenly cool the mirror.  And lastly, a slowly cooling mirror is less problematic for visual use as we constantly switch between eyepieces, and so alter focus, any change in the position of the focal point is a whole less an inconvinence.  The one instance when this can be a problem is if high magnification is used from the very start of a session and the eyepiece is not swapped – then, maybe a tiny and most likely imperceptible  change in star size might be noticed by the most acute of eyes as the mirror cools over a couple of hours.

Fans on the cell of such open instruments are a futile exercise.  Cold spots will be created which on very large mirrors can create noticeable distortions in out of focus stars if these fans are employed constantly during the course of the night.  These fans will also not blow air up the tube structure for the same reason as above – the path of least resistance is straight back out the open cell.  Any chance of dew control from this arrangement is also lost.

Photo application
With open tube instruments, the primary mirror of these can be very exposed, on occasions the mirror completely exposed.  In these situations if the primary mirror is to be force cooled, the ‘best practice’ solution described above is still the best arrangement.  This will require some type of dew shield or cuff to be placed around the primary mirror, and the back of the mirror cell closed off in order to create the most favourable movement of air around the primary.
Cooling of mirrors is important for imaging to increase productivity.  But it needs to be a controlled cool.  If you have a closed tube scope, reflector, refractor, SCT, Mak, etc, letting your scope cool to the ambient temperature is advantageous as I mentioned above if you are viewing under high magnification.  If you seek to employ fans, there are best practice methods that will be most effective.  If your instrument is totally closed, like a Mak or SCT, unless the instrument comes with factory installed fans/cooling system, there is little that can be done to accelerate cooling.  If your scope is an open tube/truss one, a cooling period is not necessary for visual use.

If you would like your scope to cool prior to using it, this will take anything from 1/2 hour to a couple of hours, all depending on the temperature differential between the ambient and that of the mirror, and the size of the mirror - small mirrors will lose heat faster than large ones.

Dew control

There are three ways to prevent dew formation, heat, air flow and shelter.

Solid tube Newt's have their primary mirror sheltered down the end of a long tube. Those long 'light shrouds' that are often seen wrapped around open tube Newt's do the same thing, but they introduce other complications as they get wet, and can also trap convection currents while the primary mirror is still warm.  On my scopes I use what I call a 'cuff', or dewshield, that comes up only about half way up the open tube from the mirror box.  Since using this on my 17.5" I have never had dew problems plague it. My 12" the same, and save for impossible nights when even fog forms it has also not had dew problems, but when this occurs it also coincides with the time to pack things up for the night as seeing conditions have gone pear shaped too.

Air flow is the way professional observatories stop dew formation on their mirrors. Vibration from the fans is the biggest hurdle here.  Once the primary is at thermal equilibrium with the surrounds, this 'boundary layer' does not exist that can form from a warm primary. The fans then help prevent dew formation.  This method is trickiest to apply to most Newtonians.  Many big dobs made by specialist dobs builders incorporate a battery of fans that blow across the face of the primary mirror as their mechanism of dew control.  A small fan can also be used to control dew formation over the secondary.  For both primary and secondary mirrors, vibrations from the fan/fans needs to be dampened.

There are some instruments that employ fans that blow across the face of the primary, and draw air out the rear of the primary mirror cell.  These are typically high end photographic instruments.

Heat can be used on scopes, but it needs to be carefully employed.  Secondary mirrors benefit from gentle heating as it eliminates potential problems from fan vibration.  But heating a secondary is more complicated than just sticking a heater onto it.  The secondary mirror holder plays a vital part in this.  The wrong holder and it will render any heating efforts futile. Heating of the secondary is very effective as it is small and the heating system can be designed to quite uniformly and gently heat it.  Heating of the primary mirror I would suggest is not the best option, for the optical problems un-uniform heating creates is the same as for fan cooling.

But there is one method of dew formation that incorporates both air flow and heat – the good old hair drier!  Heating of moving air increases the evaporative capacity, but it also needs to be applied carefully.  If the hair drier is focused too much on one spot, it will affect the optical properties of the optics.  The hair drier needs to be constantly waved over the primary mirror and surrounding structures, and from a distance, and for no longer than is required to remove dew.  This will soften the intensity of the heat, and applies a gentle and minor heating to the face of the mirror’s face that won’t compromise optical performance.  And to state the bleeding obvious, if the ambient temperature is very frigid, extreme care needs to be taken if a hair drier is to be used lest thermal shock is induced to the corrector plate of an SCT or Mak which in extreme cases could cause the plate to fracture.  In ALL instances, the use of a hair drier needs to be a gentle one.  12V hair driers are excellent as they have limited heating capacity.  The box of my own 12V unit is in a very sore and sorry state I have been using it for so many years.  I should look at replacing this long serving little cardboard box.

Note that the hair drier solution only has intermittent effect as dew will form again requiring the use of the drier once again.

~ x . X . x ~

If you look at professional observatories, they do use fans. But looking closely at how they are used, not one single fan is blowing onto the primary mirrors directly.  The fans are used to blow across the face of the mirror to blow away any dew that can form on it.  Also, these mirrors are not allowed to get warm during the day.  These observatories are giant cool rooms, air conditioned to the temperature the evening is expected to drop to, so these huge mirrors do not get warm, ever.  Such massive mirrors may take all night to reach equilibrium, if not several days.  Allowing these to get warm is something professional astronomers cannot afford to do.  In an ideal world we might all like to be able to keep our instruments under temperature controlled conditions.  But this is not possible for most of us.  So if we do consider the cooling aspect of our scopes, we need to do so respecting the thermal properties of the materials being used, and the way that air does and does not move.

It took me a while to rationalize the misuse of fans on scopes. Particularly now that I also build scopes, how glass behaves thermally has become more significant in my mind in order to best design the instruments I build.

If you would like more specific information regarding your own particular requirements, please follow this link to the discussion thread of this article in the Australian amateur astronomy Ice In Space:

Article: Cooling of Newtonian Optics, Ice In Space

There you will find people with specific skills within visual and imaging who would be well placed to assist.  This article is intended as a collection of the current ideas of best practice, and a starting point for understanding the principles behind the cooling of optics.  It is beyond its scope to offer singular information for any one instrument.  That can be addressed at the link above.

Clear skies, sharp pencils and cool cameras,

Alexander Massey.


(4th Dec., 2014)

SCT fine focusing - cheat's way

One problem I've encountered routinely with my lunar sketching is the problem of focusing at high magnification.  It is unavoidable as I swap around eyepieces until I find what's optimal for the conditions.  And it can be a real drag!  To compound this, the focusing knob on my old C8 SCT is small in diameter, so even my smallest hand movements translates to an excessively large angular movement of the focusing knob.

Sure, there are after market focusing motors available, but all of these take away from the simple-to-use aspect that I want out of my equipment.

Then, the solution presented itself to me one day while playing with my son's Lego.  Playing with some gear pieces, a long forgotten article on modifying a focusing knob was dragged out of the depths of my memory!  Just increase the diameter of the focusing knob!  Oh so simple!

I have no idea where I read that, but it would have been some astro magazine when I was a boy.  But obviously it made an impression, and nearly 30 years later it came back to help me out!  WOOHOO!

I had a circular plywood cut-out from another project lying about.  Chance had it that its diameter easily cleared the space available between the focuser knob and the 2" visual back on my C8.  The diameter of the scope's focuser knob is a shade under 22mm, so after using a 22mm forstner bit to hollow out the off-cut's center, a little strip of sandpaper between the knob and the plywood extension and I have a brand new focusing knob!

The new knob is oh so sensitive to the touch.  It just needs a baby's breath to move it.  And as the new knob is five times the diameter of the original, my smallest hand movements now translates to 1/5th the angular movement of the original.  Focusing is now a whole lot less of a task, and less time consuming.

A friend suggested I make it look like a ship's wheel.  Talk with another mate was around the Steampunk fashion.  I took both of these and came up with a neat little decorative touch to the new knob.  Nothing beats a lick of varnish too.  I'm happy with the result.

No comments:

Post a Comment