Welcome to the Curdridge Observatory Astronomy, Telescope and Astrophotography Blog

Welcome to the Curdridge telescope observatory astrophotography blog. We explore many aspects of astronomy and astrophotography especially including projects such as telescope mounts. We have a Newtonian reflecting telescope in our budget observatory made from fence panels. Most of our imaging is done using Astrodon narrowband filters and cooled astronomy CCD cameras.

Posted by: Tom How
A thought struck me today. Why don't telescopes have electric focusers?

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Posted by: Tom How
At this time of year you have to wait until quite late in the evening before it is dark enough to use a telescope. However, the long evenings can be used to refine the polar alignment of your telescope mount. Especially when you’ve just installed your new telescope mount in your observatory!

So last night was telescope mount polar aligning night at the Curdridge Observatory.

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Posted by: Tom How
It has taken a long time to make my own telescope mount. Over the years I've snapped quite a few photos of various parts of the telescope mount as they have been made.

I've gone through a few years of images and copied about 60 of them into a simple gallery of images.

I really need to do this properly, and add some description to each photo, but that is going to be a long job.

However, you can see my first collection here.

Homemade Telescope mount gallery

As you scroll through the images you will see various parts of the telescope mount in various phases of construction. There are some more recent shots of the mount in my observatory, and other shots where you can see parts of the telescope mount being made on my lathe.

Apologies if some of the older photos of the telescope mount parts are a bit fuzzy, I've only had a decent camera for a couple of years!

I promise to put in a better gallery soon!
Posted by: Tom How
The main 200mm diameter F5 telescope mirror in my Newtonian telescope is getting rather old. Almost 10 years old.

The telescope wasn't the greatest quality telescope in the world when it was new. It was branded Helios, which was the forerunner to the more familiar Skywatcher telescopes. Although a lot of time has passed, the fundamental design has not really changed.

Because the old mirror has spent many years out of doors in the observatory, it was started to seriously deteriorate. Looking though the back of the mirror, I'm surprised it reflects anything. With the new telescope mount, it seemed proper to get a better mirror.

One of the foremost telescope mirror shops in the UK is Orion Optics. They sell a 8 inch F4.5 mirror for about £300. I asked them if they would make me a one off 8 inch F5 mirror, and was told this would be an extra £160 re-tooling charge on top of the cost of the F4.5 mirror.

That didn't seem to be a sensible use of funds.

The next obvious route was to buy an 8 inch F4.5 Orion Optics telescope mirror and simply slice 10cm off the bottom of the telescope tube. I pondered this one for a while, but was still put off by the cost. At the back of my mind I know I'm going to build my own larger telescope at some point in the future and it seems a bit silly to invest in an expensive mirror now.

I asked a couple of other UK telescope mirror makers, and they all quoted impossible sums of money.

A new telescope OTA from Skywatcher seemed the simplest fix. However, buying a new 200mm OTA sounds like an inefficient use of cash – I’d end up with a lot of hardware I don’t need. The £300 cost of a new OTA is close to the cost of an 200mm F4.5 telescope mirror set from Orion. All I have to do is chop 10cm off the end of the OTA!

Spending £300 on an OTA from which I’m going to pinch the mirrors also has to be weighed against the option of getting one of their £400 10 inch OTAs. However, I don’t want to rush out and change instrument without consideration to my next camera.

The final option is recoating my mirrors – this doesn’t give much change from £100 after vat and postage. That seemed like a daft option as I don't know what else is wrong with the old mirrors.

After dithering about this for a few weeks, I contacted Bern at Modern Astronomy, a great UK supplier who often comes up with creative solutions to my problems. He actually managed to get a price out of Skywatcher for a replacement telescope mirror. Whilst it has a lead time of 2 to 3 months, the cost is acceptable.

This is just the primary mirror. There are various UK suppliers for secondary flat newtonian telescope mirrors, such as Orion, who do a 50mm flat for about £50.

So in a new months the new telescope mirror will turn up and we shall have some brighter images!

Posted by: Tom How
I am a great fan of webcams and I have always used them in my astronomy and astrophotogaphy. One of the most effective places to put a webcam on a telescope is the end of your finderscope. Many people like to operate their telescopes from indoors or remote locations and I do not know how they manage without a webcam finderscope.

Telescope finderscopes are small telescopes attached to larger telescopes. The finderscope has a much larger field of view than the main telescope, and as such can be used to help the astronomer aim the main telescope at a target of interest.

Telescope finderscopes are often mounted at around head height on the top of the telescope OTA so that the astronomer can easily look down the finder without bending his neck in 6 different places, however, they are still be uncomfortable to use.

Once you start trying to remotely operate your telescope on a GOTO telescope mount you will often find that the telescope GOTO system does not always land your astronomical target on the sensor of the CCD camera. In these situations I find it useful to sync the telescope GOTO with a nearby bright star, and then do a GOTO my astro target. But how do you get the bright star on the CCD camera sensor? Well, you look up the finderscope! This doesn't sound very good for remote operation, so what I do is have a webcam attached to the finderscope where I could normally put my eye.

telescope webcam finderscope

Unlike most astronomical applications of webcams, you do not remove the standard lens, you leave it on. You just fix the webcam in place on the end of the finderscope with the webcam lens about where your eye normally goes. The output of the webcam now looks just like the view you get up the finderscope - cross hairs and everything!

I can normally see stars down to about Mag 6 using this method - more than enough to help me get a bright star on the CCD camera and sync the telescope GOTO system. You have to turn up the exposure etc, but it does not require any modifications to the camera.

I attach my webcam to the end of the finderscope using a piece of plastic tubing I found which slips tightly over the finderscope - I have simply glued this to the webcam.

Without the webcam finderscope I could not be able to remotely operate my telescope observatory.

A futher advantage is that you are free to position the finderscope where you like. I have mine bolted to the top of the tube rings down by the mirror end of the scope. Here it is more out of the way. In this position the finderscope helps to counter balance the weight of the CCD camera attached to the focuser of the main telescope. It is uncomfortable to eyeball the telescope finderscope in this position - I always use the webcam.

Posted by: Tom How
Fascinating paper from the Astronomy and Astrophysics journal on the most distant comet observation with a telescope.

Use the 2.2m ESO telescope at La Silla, the observers have stacked 26 x 180s exposures of the distant comet Hale Bopp. The distance is around 30.7 Astronomical units, or 2,850,000,000 miles in more local terms.

Comet Halley had been observered at around 27 AU distance, but these new observations beat that record.

Objects the size of Hale-bopp (around 30km) are very faint that this extreme distance, researchers give a magnitude estimate of around magnitude 24.5

Comet Hale Bopp has been extremely active, but these recent telescope observations show that the comet activity is now in rapid decline. Some scattering from a light coma was detected, but the comet is now almost in deep freeze.

We ought to be able to continue tracking Hale-Bopp with the worlds largest telescopes until around the year 2020 when the magnitude falls below mag 30 and pickign the comet out of the background galaxies becomes impossible.

Although I enjoy a naked eye observation of a comet as much as the next person, I've never been particular interested in imaging them. However, this type of observation is in a different class of astrophotography. Simply finding the damn thing must be a challange. The researchers noted that the comet appeared slightly out of the predicted position in the sky, but comet revealed its nature by a slow movement against the background stars.

Hale-bopp wil be back in our skys in the year 4385, so don't wait up!

Paper reference :A&A 531, A11 (2011)
DOI: 10.1051/0004-6361/201116793

Frozen to death? Detection of comet Hale-Bopp at 30.7 AU
Posted by: Tom How
After a long weekend out with friends (visiting Arundel Castle of all places) I've done some more work on the homemade telescope mount project.

After testing the entire system over the long Easter weekend, I dismantled the mount and did some painting and final adjustments. Today I've rebuilt the mount in the shed. Although it is easier to construct these things in the workshop, it is beyond me to move the entire mount in one go. It weights in around 35kg.

Although it is raining a bit today, I've got on quite well and got the whole mount put together and the telescope attached.

Next step is re-wiring. Lots of options here - I must keep it tidy!

First picture here is the scope in the shed. Second picture is the parts of the scope prior to putting it all back together.

You can now see the various covers I have fabricated from 1mm aluminium sheet. I do not sheet metalwork at the best of times, but it is important to cover the shafts with something. The shafts of the telescope mount are made from specially hardened steel, but it is not stainless steel, so it will corrode over time. To prevent corrosion, I've covered the shafts in thick protective grease. The shaft covers are required to prevent dirt and dust sticking to this grease, and to prevent the grease getting on the astronomer!

The covers have been painted a simple matt black and fixed with stainless steel screws so they can be easily removed for telescope mount inspection and maintanence.

You can also see the circular covers on the worm wheels. The greasy worm wheels are made from aluminium which corrodes less than steel, but still needs protecting to prevent dirt. The covers also give a level of physical protection to prevent accidently knocking the delicate edges of the worm wheels. These round covers may appear a little odd. The reason for this is that I couldn't face the idea of making something like that, so I simply purchased a couple of cheap cake tins. These round baking tins from a kitchen shop were the perfect size to make worm wheel covers!

diy telescope mount in observatory

parts for homemade telescope mount
Posted by: Tom How
What's the point of training your mount to reduce PEC which is effectively taken out when you autoguide anyway?

Some say yes, some say no... I say... back to basics....

PEC stands for “periodic error correction”. PE is “periodic error”. As you say recall from my photos, those big worm wheels (7, 8 or 11 inches in your case!) are turned by a small threaded shaft called a worm gear, with a diameter of about 20-30mm. The worm gear is turned at a constant rate. If the worm gear (the little threaded shaft) is perfect, then the mount will track in RA perfectly.

That threaded shaft is held at each end by two bearings. These bear on a flat part of the shaft. But, however good you are at machining, the flat part of the shaft will never be perfectly concentric with the threaded bit.

Read the tolerance issues here http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml

Now, that slight eccentricity results in the speed of the tracking to go over-speed or under-speed in a cycle.

e.g. http://www.freewebs.com/eq6mod/D1.jpg

If the worm shaft is well made, this error will be smooth, and highly predictable. For a 360 tooth gear it will repeat every 86164/360 = 239.3 seconds. A paramount ME is 576 teeth, so 149.6 seconds.

This is the gross error arising from the gears in the drive chain.

Higher frequency error can also be superimposed over this low frequency cycle due to reductions gears closer to the motor, but ignore that for a moment.

Because this periodic error is supposed to be highly predictable around the entire worm wheel (because it arises from the fast rotating worm gear shaft), the mount can be taught the shape of the curve and correct for this error by speeding up and slowing down the drive motor accordingly. The mount will learn the shape of the curve by watching a star during the cycle of a worm rotation (149s) – actually you normally average many cycles of observation. The mount will also contain a sensor so it knows what position the worm was in at startup. (so it knows what point in the correction table to start from)

Once this gross error is mostly corrected by the periodic error correction (PEC) then the strain on the autoguiding system is much reduced. The guiding only has to cope with the smaller errors which arise from
• Tooth – to – tooth differences on the worm wheel
• Flex in the mount/scopes etc
• Diffraction
• Error in polar alignment
• Transients (E.g. speck of grit in the system)
• Drift in DEC (cause by all of the above)
• Higher frequency periodic errors. These have different periods. Go play with an FFT program if you want to analyse them.

If the periodic error arising from the gross eccentricity of the worm shaft is both smooth and predicable, then periodic error correction will reduce the strain on the autoguiding system and therefore make things better. Think of PEC as correcting tracking errors before they have occurred, unlike autoguiding which can only correct error after it has occurred, the former is obviously preferred. The PEC is also averaging and smoothing the correction over a few seconds instead of the autoguiding making a series of sharp correction every few seconds.

However, if the PEC training is poor, or the periodic error is unpredictable due to poor engineering, the PE Correction will do a poor job. The autoguiding and the Periodic error correction will fight each other and produce an inferior result.

You must always autoguide over longer exposures. The items in the list above dictate this. No mount can correct for transients and flex!

With a good mounty, the PE will be smooth and regular, thus PEC will work well with autoguiding, but to be honest you are only going to visually notice the difference if you are imaging around 1”/pixel instead of your wide field stuff. That said, even in wide field, the better the tracking, the better the signal to noise ratio in the fine detail of nebs and galaxies, even if you can’t detect the drift by eye.

In summary, if the mount is well made, PEC reduces the strain on the autoguiding and ultimately produces better SNR, but if the PE is unpredictable, or the guiding system poor (flexure, seeing etc) then the two will fight and cause a problem.

Practically I would start with the autoguiding and play with PEC when your are bored one night in the full moon.

Posted by: Tom How
After some recent tests with a pin hole on my CCD camera, I realised that the edges of my secondary mirror were causing a lot of stray light scatter in the big Newtonian imaging scope.

More on the tests another day, but I didn't realise that the simple frosted glass sides of the telescope secondary mirror were reflecting a lot of light into the wrong place. This coincided with some observations made by Richard Crisp about problems with flat fields in a friends telescope to convince me that an unpainted and unshielded secondary was a very bad idea for the standard of imaging I'm hoping to reach this coming season.

Painting the secondary mirror of a Newtonian tlescope is a pretty scary idea. Certain parts of telescopes do not respond well to paint. Silvering of mirrors is quite high on this list. After selecting some slow drying black enamel spray paint I set about solving this this problem.

My solution was to made a long sausage of blu-tak which I stuck around the rim of the silvered part of the mirror. I then laid this face down on a piece of thick card (one half of a birthday card) and pressed firmly. Hopefully this will work!

I'm not too fussed about blu-tak on the silvering. That secondary mirror is 10 years old and the silvering is starting to come off and I'm generally well overdue for a new set of mirrors.

Anyhow this approach seems to work - see the piccie! In a few days I will reinstall in the telescope tube and see what it looks like.

painting mirror
Posted by: Tom How
Another long 4 day weekend here in the UK. Of course, the royal wedding was splendid and very tastefully done, but we can't keep partying all weekend, so I've been concentrating on painting the homemade telescope mount.

As I have always said, it doesn't really matter what a telescope mount looks like, because nobody is going to see it, because it is dark! However, when you've spent 5 years (on and off, mostly off) making something, pride kicks in a little bit and you want to make it look pretty. People will come from far and wide (sometimes as far as the next village) to be impressed by the telescope mount. These folks will appreciate a bit of paint.

Another factor is the weatherproofing. Unfinished machining grade aluminium doesn't respond well to years of exposure to the weather. It tends to become pitted and dirtied with a dusting of white aluminium oxide. Some grades of aluminium are less effected, but these are harder to work with in the workshop.

Making a perfect job was never going to happen. Some of the aluminium parts have had a hard life in the workshop. Whilst I could spent a couple of days sanding and polishing every surface of every part of the mount, I'm not *that* fussed. A can or two of car paint is perfectly suited to giving the large aluminium parts a coat of paint. The small parts are of course anodised but I don't have the setup to anodise the larger parts. Whilst I could probably sort myself out with the required big bucket of acid, I don't really want to, so I will settle for some paint.

The first picture below are the two sides of the equatorial wedge part of the homemade telescope mount. In my on-going homage to the Paramount ME I've chosen red for these. A nice satin finish red.

Secondly are the longer bases for the axis of the telescope mount. These I've simply done in matt black car paint. If I had my way, it would all be matt black, but I would get a lot of protests from various advisors! Hmm, methinks they should cut the 'effing worm gears, and then they can have a choice of paint. *ROFL*

In case you were wondering, at some point in the past, one part got stuck in another part and I needed a lot of hammers to fix it.

I'm also have a secret desire to get some part of it painted purple, but I don't know what yet.

Anyhow, a couple of piccies.

telescope mount
telescope mount

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