Pex Hill Observatory Telescope Tests, April/May 1999

M3 CCD image of 70 Seconds Exposure. 22nd April 1999

M3 CCD image of 70 Seconds Exposure. 22nd April 1999

M64 "Black Eye Galaxy" CCD image of 2 Mins Exposure. 22nd April 1999.

M64 "Black Eye Galaxy" CCD image of 2 Mins Exposure. 22nd April 1999.

Telescope Mount Extension

Report by David Galvin

The telescope mount extentsion and Steve Southern, Pex Hill, 1999

The telescope mount extension and Steve Southern

The Telescope mount extension is constructed from a ½ thick 7″ inch diameter mild steel pipe that has two 3/8th” thick plates welded either end. These plates are same length and breath as the orginal grey top plate of the telescope pillar. There are four tubular sections, 1¼ diameter, welded at the four corners. The pillar stands at 3 feet (1 metre) high. The weight of this extension requires at least two people to carry it!

The purpose of the extension is to test out several different telescope designs/sizes to see if the vibration problems inherent in the building can be visibly reduced by using a more compact telescope such as a Meade 12″ Schmidt-Cassegrain instead of the tall 16″ Robertson Newtonian Reflector that we have struggled with over the years.

In recent weeks, we have tested out an 8″ Meade LX200 at X165 Mag and found that direct vibration from the floor stops at 5 secs, whilst from the pillar, dead at 7 seconds. This I feel is a vast improvement from the 20 seconds plus with the 16″ telescope. Finding and tracking was much easier. Over a dozen deep sky objects were observed in my first hour of testing, and they stayed in the centre of the scope. Bare in mind that it was not polar aligned and therefore not working at its optimum design.

The newly purchased CCD video camera was also used at this time, and showed that stellar objects could be positioned on the CCD chip and tracked, further testing in the coming weeks will hopefully include a 10″ and 12″ Meade LX200.My Thanks must be noted to LAS members Christos Spyrou who made available his telescope, and Steve Southern who helped me install the telescope mount at the Observatory.

CCD Imaging Trial – April 22nd 1999

Report by Rob Johnson

Would it or wouldn’t it?

Since the opening of The Pex Hill Observatory 5 years ago the big question has remained, with a suitably equipped instrument would the stability of the building be sufficent to allow serious astrophotography or CCD imaging through a telescope. As most members are now aware, despite extensive work, the 16″ Robertson reflector would never be a first class observatory instrument so the search was on for a replacement. Dave Galvin and Geoff Regan had started to arrange for different sizes and types of telescopes to be placed in the observatory and assessed for their potential usefulness. This would be an essential step before the Observatory sub-committee and council would splash out on a new telescope.

8" f10 Meade LX200 belonging to Christos Spyrou

8" f10 Meade LX200 belonging to Christos Spyrou

Now the interesting bit. I had a call tea-time from Geoff Regan “It’s going to be clear – will you come up to Pex Hill and try out some CCD imaging?”
Christos Spyrou had very kindly brought his 8″ f10 Meade LX200 and set it up on the new steel test pillar put in place by Dave Galvin & Steve Southern. Dave Owen, Chris Banks, Geoff Regan, myself and DG assembled under a clear sky with a first quarter Moon. The CCD was plugged in and Christos Spyrou started to align the telescope – you can guess what happened next – clouds swept in from the South-West to cover the sky! Chris frustratingly attempted to get an alignment for the alt-az mount on two stars, eventually he obtained an approximate alignment, which enabled the telescope to be roughly pointed at any object with the software.

For a couple of hours we dodged the clouds to capture brief exposure tests on various objects, M3, M64, M66, M95. It Quickly became clear that the supporting structure was stable enough to allow images of at least 30 seconds duration to be taken, this may not seem very long but CCD images can be easily “stacked” together to produce much longer effective exposures. Towards midnight we had a good clear slot sufficient to complete our tests. The image of M3 was surprisingly good considering the poor conditions and goes to prove what could be done. The Pex Hill Observatory was not quite as jelly-like as some of us had feared after all.

Now the technical bit. The CCD was a Starlight Xpress mono parallel port camera with 12×16µ pixels, this gave an image scale of around 1.4 arc seconds per pixel, this is considered about the correct sampling size for typical seeing conditions. At f10 the telescope was slower than ideal for imaging, a faster f-ratio, by design or by use of a focal reducer, would produce a higher rate of signal and allow a good signal to noise ratio to be obtained in a shorter time.

Exposures were taken increasing from 5 seconds upwards. At 30s images showed round stars with no trace of vibration even when Chris Banks exited via the observatory trap door during the exposure. Despite high winds, at times entering the dome slit, the system proved able to cope. Exposures over 30s were slightly trailed due to the lack of accurate telescope alignment and stacked images showed field rotation because the telescope was mounted alt-az. Both of these problems would be absent in a permanently set up equatorially mounted telescope. Most of the images showed somewhat bloated stars due to poor seeing conditions, this I suspect was mainly due to the incoming weather system, though observatory domes are notorious for producing turbulent air, this will always be a factor at Pex Hill.

It was not possible to fully assess the pointing accuracy of the Meade because of the lack of correct alignment, however, most objects were very close to if not on the CCD chip most of the time. The periodic error of the drive was assessed from the image movement on multiple CCD frames and found to be no more than a few arc seconds.

Other Images

M103, M34, NGC2158

AND finally…..

It was a very exciting and enjoyable evening at Pex Hill where we answered our long-standing question – yes we will be able to do serious imaging – supernovae searches, asteroid positional measurement and lots more. All we need now is a first class telescope!

Many Thanks to everyone who helped with the testing
Geoff Regan, LAS Director of the Observatory