After improving the backlash on the telescope mount last week it was time to test again with some longer exposures.
The testing needed to see if I had improved the guiding in declination (DEC) with my mechanical adjustments. I also need to test the CCD camera. I'd had the camera apart a few days ago to clean the sensor and hopefully cure a noise problem. I've also made some mechanical adjustment to the telescope mirror cell, so I need to check the field illumination.
When we are this close to summer solstice, the nights are very short. It never really gets properly dark, and the sky glows brightly all night. For this reason a hydrogen alpha filter has been used for all testing. The Ha filter only lets through light at the extreme red end of the spectrum. The sky is blue - therefore this filter helps block out the skyglow. Another advantage is that the longer wavelength of light are less effected by the turbulence in the atmosphere.
First target was a shot of M13, a large bright globular cluster in Hercules. This image is two exposures of 300s which have been combined. Total integration time is 10 minutes. Obviously such a puny integration time is going to give a very noisy image, but it would appear I'm better focused than on previous tests, and guiding looks improved.
Click on the images for the full sized version.
I then moved across the sky a bit to the Elephant Trunk Nebula region and did a single 1200s exposure. Again, everything seemed to be working about right.
Now it was getting on for midnight, and about as dark as it was going to get, so I moved back to the same pelican region I'd use on the previous test. I managed to stay awake long enough to complete 3 x 1800s exposures.
I've tried to do some further post processing on this dataset but the it seems that the sky glow has really washed out the contrast. The moon being around didn't help either.
Moving on to the other tests. During twilight I pointed the telescope at the sky and took a few very short exposures to so how the flat field was looking. The answer would appear to be "quite good". Although there is some dust and scratches, plus a smudge, there is hardly any classical Newtonian vignetting.
Finally, I've been suffering some some nasty diagonal banding on my images from the CCD camera over the last couple of years. The camera is a kit build Artemis 285 with a Sony ICX285 sensor. This camera became the Atik 16HR. The bias frames used to be very good, but deteriorated a few years ago. I think I've pinned some of the problem down to a dodgy earth on the 5v regulator.
Below is a before and after image of a cropped region of the bias frame. The one on the left is before and the one on the right is after. The diagonal banding is still visible on the right hand image, but is much reduced.
During the night time test the other weekend, I discovered huge backlash in the gears - both in the RA (right ascension, i.e. left and right) and the DEC (declination) directions. The backlash in the DEC was making it impossible to autoguide the telescope properly.
On measuring the values the other day, I found the DEC backlash was a huge 362 and the RA was at 169. Terrible.
However, after some tweaks, I seem to have improved it.
Setting the backlash is a bit of a phaff, you have to sight the telescope on a distant object. Such things are in short supply in my garden, so I have to find a chimney pot behind a tree. You then have to move the telescope around a bit under computer control until its little brain has learn the backlash in the gears.
Anyhow, DEC backlash now measures 27 and the RA 52... so greatly improved. In a month or so we might see some stars to try it out on!
With a project like the homemade telescope mount, nothing ever works first time. After each major test you have a dozen things that need improving and tweaking.
This time around, most of the problems seemed to involve backlash.
The DEC backlash was horrible. Really bad. Much worse than the test I did at Easter.
So last night was telescope mount polar aligning night at the Curdridge Observatory.
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!
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!
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.
In the UK we are running out of darkness. From now until mid-August the stars do not come out until late in the evening and don't stay out for very long. I'm hoping to start using the homemade telescope mount full time later this year, but this weekend, with clear skies and a few days holiday for Easter, is probably the last chance for an all up test of the system. So I decided to pop the new mount into the observatory for a complete test with camera and auto-guider and see what happened.
After nearly killing myself lifting the thing into the observatory and mating it to the pier (the mount weights 30Kg) I got everything setup and waited for the stars to come out.
This test was pretty important as it gave me a chance to find out all the niggles and snags in the homemade mount, and give me a chance to work on them over the next couple of months before I move the DIY mount into the observatory permanently.
First of all the mount seemed to slew around the sky quite well - no problems with the full load of the scope and cameras. Given that my polar alignment is abysmal (I've not polar aligned anything for 5 years - out of practise!) GOTO worked remarkably well. I was able to sync the GOTO to a Alkaid at the end of the Plough, and goto M51 and get it onto the sensor of the CCD camera.
So far so good, but what about guiding? Disappointingly I didn't have a good guidestar anywhere near M51 in the homemade OAG (who wants to image galaxies anyhow!), so I travelled a bit further to M63 and picked up a good guidestar with M63 just on the edge of the frame. Parts of the sky which contain galaxies do not, as a rule, contain many useful guidestars.
Guidedog started working on the guidestar and the Artemis 285 CCD camera started firing off 3 minute exposures. The RA guiding seemed ok - rarely recording errors much more than about 1.5 arc seconds, and the occasional glitch. Due to the dreadful polar alignment, the drift in DEC was huge in this part of the sky - something lie half an arc minute per minute, so the DEC guiding was working overtime. The resulting image has got the stars egging slightly in DEC - although this is hard to tell in the image below due to my esoteric camera angle. DEC runs roughly 10 o'clock to 4 o'clock on the image.
After tweaking around with the guiding parameters I decided this wasn't an entirely fair test due to the massive drift in DEC, however it highlighted that things could be tightened a little in the DEC axis of the mount so reduce sudden guiding jerks as the slack in the gears is taken up, but considering the drift I'm happy.
All the remained was to leave it running for a while. It behaved itself well - so I left it to gather 20 x 3min frames of the region.
Below you can see the image after processing - click here for the full sized one
Below M63 there are some photos of the mount in the observatory - I have to go and lift it back indoors now!
As an added bonus, whilst sitting in the observatory I watched the ISS float over!
For many years I've imaged with my F5 8 inch newtonian reflector on my LXD55 mount. A poor mount - but I made it work. The new homemade telescope mount is obvisouly designed to be a step forwards, and this summer is going to see the new mount in the observatory.
I've been testing the new GEM equatorial telescope mount with my small achomatic 400mm FL refractor on a tripod, but today I stripped down the main imaging rig and brought the 8 Inch Newtonian indoor for probably the first time in three years! Some adjustments to the dovetail bar and I had the big heavy Newtonian firmly attached to the homemade mount - and I'm pleased to say everything worked ok!
As you can imagine this is a rather terrifying experience - the first time I've trusted the big scope on something so, ahem, homemade! The motors drove the load quite happily, and I was pleased to be able to shorten the counterweight bar by about 8 inches - so it will now fit in the observatory.
By adjusting the clutches I was able to move the mount around on the mount with enough friction to hold it in position - this means if I run the mount into the pier, it will just slid on the clutches instead of something more terrible happening!
The mount is run from a Meade Autostar DS motor kit. These motors are designed to run a much smaller telescope, but can be adapted to run a much larger telescope mount because the Autostar controller lets you program in the ratio of the worm gears.
A consequence of this means the slewing is very slow - but it gets to the target in a few minutes, and I can wait a few minutes as I normally only image one or two targets a night.
The noise is still pretty dire, but sounds much more healthy than my previous video due to the better mounting.
Tomorrow, if it says clear, I will take the mount outside for some real GOTO and PEC tests. If the PEC looks acceptable then I am on the home run - I just need to make some covers and other weatherproofing bits and bobs . Then we can move the new mount into the observatory shed.
The main weatherproofing conerns are keeping the dirt off the worm wheels and gears, and protecting any of the mild steel parts. Most of the scope is made from stainless steel or aluminium but some parts (e.g. the shaft) are going to need some heavy grease and covers to prevent unsightly corrosion.