The Lancashire Observatory
of David Ratledge
Observatory and Telescopes 53°37' 30"N; 2° 36' 50"W
Over the years it has housed 3 telescopes: a 12.5 inch Newtonian, a 16 inch Newtonian and now a 12.5 inch Ritchey -Chretien ( a true one!).
I chose a classical dome shape as being the best to keep out light pollution from neighbourhood lights and offer protection from the wind. It has been in operation for over 26 years and in that time has survived well and only occasionally needs a wash with washing soda to keep it clean. It was originally white but it has now changed colour to two-tone green to make it less conspicuous.
New Telescope and Mount 2007
The Lancashire Observatory has undergone a complete re-fit with new telescope and mount. After a lifetime of homemade telescops and and mounts I finally succumbed to the lure of high-tech commercial equipment. The new mount is a Paramount ME and the main telescope an RC Optical Systems (RCOS) 12.5 inch f/9 Ritchey-Chretien.
The RCOS is a true Ritchey-Chretien cassegrain. This design uses 2 mirrors to produce a coma corrected field with no false colour. There is an option of closed carbon-fibre tube or open truss. For Lancashire with its damp climate and light pollution then the closed tube makes much more sense. The carbon-fibre tube also, having less aluminium, is less prone to temperature induced focus shift than the truss. The optics are in astro-sitall and were made by Paul Jones.
Ritchey-Chretien telescopes are often quoted as having a flat field which usually they are not. The flatness of field for a cassegrain telescope is almost entirely dependent on the ratio of the 2 mirror curves ie the magnification or amplification of the secondary - the bigger the magnification then the bigger the resultant field curvature. This is why f/10 Schmidt-Cassegrains with their 5x magnification have very curved fields. The amplification of the RCOS is 2.8x which makes for a longer tube than Schmidt-Cassegrains but much less curvature of field. All things being equal then the RC has a a slightly larger field curvature than other cassegrains.
Note not all Ritchey-Chretien are real ones! A well known manufacturer has in recent years tried to pretend that a coma-corrected Schmidt-Cassegrain is an advanced Ritchey-Chretien. They may be good telescopes but they are not Ritchey-Chretiens and secondary amplifications of 4 or 5 mean they are certainly not flat-field telescopes despite many dealers advertising them as such. They also have a small amount of chromatic aberration due to that Schmidt corrector lens. With modern CCD cameras sensitive well into the infra-red then chromatic errors are to be avoided.
The old mount was sold in May 2007 leaving a big hole in the floor...............
The existing concrete foundation was retained and the idea was to construct on to it a new concrete plinth. This would in turn take a steel pillar for the Paramount.
The existing concrete was drilled for 8 vertical reinforcing rods. They were epoxied into the holes. Reinforcing hoops completed the cage.
Formwork removed after 3 days. Still quite "green". It had been kept wet.
False floor repaired and carpeted. Concrete starting to go a bit whiter.
Pier dropped on. Fortunately the bolts were in the right place! Pier is made from 150mm RHS square section - 6 mm thick. Gusset plates add to the strength.
The Paramount has now been mounted in place. Next are the counterweights and then the two tube assemblies.
RCOS and Takahashi and their cameras mounted. Maxim takes control.
TPoint has been used for this. At first sight it looks difficult to master but, at least for polar alignement, it has proved relatively straight forward. The first run showed a large error in azimuth (0.7 degrees) but after one run of TPoint, with 20 mapped stars, this reduced to under 2 arcmins. A second run (35 stars) reduced this to even less. The aim was toset the polar axis 40 arcseconds high as this is suggested as ideal (for my latitude) for better tracking. This is the result:
This is the associated scatter diagram for the TPoint model:
Periodic error results:
The periodic error is so small it is all but lost in the seeing noise.
The camera I settled on was the Apogee Alta U9. This range boasts a lifetime guarantee on the chamber seal and as I have had problems with this in the past this seems just the camera for me. The chip I chose was the KAF-6303E. The size of this chip is 28mm x 18mm with 9 micron pixels. The quantum efficiency is very high at H-alpha (65%) and into the infra-red. I have to use light polution filters so efficiency away from the visual midband is important for me.
Currently work is progressing on getting it attached to the RCOS complete with off-axis guider. A Van Slyke Targetron is used but this has had to be modified to (hopefully) make finding a guide star easier. With the standard version I just could not centre a guide star.
Webcam off-axis guider located ahead of the filter drawer. I tried the Opticstar PL130M autoguider but it behaved very oddly. Despite claiming 10 second exposures it actually recorded for 1.5 seconds then had a rest for 3.75 seconds and then another 1.5 seconds worth and so on! (test results here) Any exposure longer than 1.5 seconds is literally a waste of time. I am currently using a Starlight Xpress Lodestar which is very sensitive. It is however susceptible to electronic noise and its cable has to kept well clear of other power lines.
CCD Inspector curvature with Alta U9 (27.6mm x 18.4mm)
The telescope is operated remotely - well from 30 metres away in my (warm) house. The following should make it clear how this is achieved.
The observatory predates any thought of building-in ethernet connections so homeplugs have been used to get the observatory onto my house network. They work well but I found they don't like surge protectors so they are plugged in direct to the mains. UltraVNC handles the remote control and is very easy to set-up. It is magical watching images as they appear. As the images are taken they are copied over the network to the hard drive in the house thus satifying the rule that "digital images don't exist until they are in at least two places". I just need to get the dome motorised now for full remote control.
First Images with RCOS using Canon DSLR
RCOS at f/5.7 with a Canon 20Da and a Celestron 0.63x reducer. 13 x 4 mins exposures with Lumicon Deep-Sky light pollution filter.
RCOS at f/5.7 with a Canon 20Da and a Celestron 0.63x reducer. 23 x 5 mins exposures with IDAS light pollution filter.
RCOS using Alta U9 CCD Camera - HaGB image of M82
M81 and Holmberg IX - RCOS at f/9 and Alta U9 Camera - 150 minutes total.
Rosette Nebula: 190mins at f/9 with Alta U9 and H-alpha filter
I built the observatory in 1980 using fibreglass throughout for both the top and walls. Fibreglass is light, rot-proof and keeps out the famous Lancashire rain. It also has low thermal mass which is desirable for good seeing. The observatory is 3 metres high, 2.9 metres in diameter and the slot width is 750mm.
The dome was made in four quarters. This is the easiest segment of a dome to make, having three equal sides and three right angle corners. The segments were assembled together on a fibreglass ring beam and the slot cut last. Perhaps a bit wasteful but it guaranteed it all fitted together. The slot features radiused corners - these weaken the dome far less than square corners. The whole dome was then given another covering of glassfibre making a very strong structure. It can easily carry my weight.
The weight of the dome is taken by four timber posts at quarter-points on top of which are mounted the wheel assemblies. The assemblies comprise 2 vertical wheels (for the weight) and one horizontal wheel to guide it around when rotating.
The walls are again fibreglass and were made curved. Doing it again I would buy flat plastic sheets and bend them.
The slot cover is a single piece (fibreglass yet again) sideways sliding and is light enough not to disturb the balance when open. It runs on two angle irons - ex bedstead.
4 support posts with wheel assemblies
For many years the main telescope was a 40 cm (16 inch) f4.7 Newtonian with a 106mm Takahashi FSQ Refractor and a Astro Tech 66mm ED refractor piggybacked on top. The 16 inch replaced a 12.5 Newtonian which served me well for 17 years. The massive steel fork mount was designed to take a bigger telescope so when time came to upgrade just a new tube assembly was required. The drives were upgraded at the same time.
Originally my interests were astrophotography but increasing light pollution has meant this is all but impossible from southern Lancashire. Fortunately the arrival of CCD imaging kept my interest alive The upgrade to the 16 inch was very much a team effort with Gerald making the tube assembly, Brian the fantastic 400mm f/4.7 mirror and myself the electronic control. It is designed with CCD imaging very much in mind and with the Takahashi it gives me focal lengths of 1750mm and 530mm. The Astro Tech 66mm is for autoguiding. All imaging has to be done through light pollution filters - I generally use ones that pass infra red where light pollution is minimised. These range from a simple red filter to Lumicon's Deep-sky filter and, for the Takahashi, the IDAS filter.
For more constructional details see Telescope Making
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