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A new telescope for Christmas!

Posted: Fri Dec 20, 2013 8:52 am
by rwilkinson
Although I'd bought five different telescopes between 2001-3, I only kept one of them (the Celestron C8), so I've decided that after ten years maybe it's time to treat myself to a new 'scope?

Now SCTs are great instruments for visual observing and planetary imaging, but are not well-suited for imaging larger objects: even with an f/3.3 focal reducer my C8's focal length is around 700mm, and the field-of-view with a small (webcam) CCD is a mere 20 arc-minutes across.
But recently I've been doing some experiments with a little 80mm f/5 refractor borrowed from a friend. With a focal length of only 400mm it gives a much wider field (albeit with a fainter image), but it's just a low-cost achromat, so it can't provide really sharp results.

So I've been looking for a better-quality refractor - I was thinking of a compact 70mm APO, but this week I found an almost-new StarWave 80ED 80mm f/6.9 which was not much dearer. This is identical to the one which the Society bought in March 2012, and I was impressed when I looked through that (although I've never managed to try any imaging with it).
Anyway, it was delivered yesterday (Christmas came early!) and I soon had it set up pointing through an upstairs window at Bolton Town Hall clock (about a mile away). This enabled me to do some daylight tests with the webcam, and I found that I could achieve focus through my Celestron 0.63 focal reducer (designed for the C8), giving me an f/4.6 system with an effective focal length of around 365mm. This should yield a field-of-view around 43 arc-min across - about four times the area which I can get with my SCT operating at f/3.3.
Webcam image from prime focus (f/6.9)
Taken through a double-glazed window, and with lots of trees in the way!
clock_f6.9.jpg (21.88 KiB) Viewed 7986 times
Webcam image through focal reducer (f/4.6)
clock_f4.6.jpg (23.18 KiB) Viewed 7986 times
But then after 7pm the clouds started to break up, so I was able to try it outside (whatever happened to "the curse of the new telescope" weather?). I mounted it on my CG5 AGT and had a look at the Double Cluster, the Pleiades, Mizar & Alcor and the Trapezium (in M42). Then once they had risen above the trees, I could focus on the Moon and Jupiter through a 6.5m eyepiece. I was impressed with the clarity of the views, and the wide field made it much easier to find targets with my "Go-Near" system. Although it has a smaller aperture and focal length than my C8, it gave a really good view of Jupiter at high magnification - the benefit of looking through a smaller column of air?

And finally after 8:30 all the clouds disappeared, so I fitted the mono webcam and filter-wheel and collected some images of M42 through four different filters. I also managed to get some images using my original colour webcam, before the 'scope started to dew up and the clouds returned; so I've got plenty of data to play with over Christmas. And I need to sort out a finderscope for it (I have an old Celestron 6x30 which should do) and make a Bahtinov mask and dew-heater - yet more jobs for the Christmas holidays!

Re: A new telescope for Christmas!

Posted: Fri Dec 20, 2013 10:49 am
by cstone
Well done Ross. :)

Re: A new telescope for Christmas!

Posted: Fri Dec 20, 2013 6:18 pm
by dean_kos
My word Ross you had a busy day ;)

Time to get busy with the cardboard and scissors now then eh 8-)

Re: A new telescope for Christmas!

Posted: Sun Dec 22, 2013 10:42 am
by Amartin
Welcome to the APO club Ross!

Bahtinov mask for 80ED

Posted: Mon Dec 23, 2013 11:01 am
by rwilkinson
dean_kos wrote:Time to get busy with the cardboard and scissors now then eh 8-)
No Dean, this time I've used plastic card (from the model shop) and a craft-knife, so the result is more robust and won't go floppy on damp evenings:
A home-made Bahtinov mask for the 80ED
bahtinov.jpg (157.26 KiB) Viewed 7946 times

Further tests with StarWave 80ED

Posted: Mon Dec 23, 2013 11:45 am
by rwilkinson
Amazingly, two of the four evenings which have passed since I got my new 'scope have been clear (at least partly) - what a good job I didn't leave it wrapped up under the tree! ;)
Al-fresco imaging with StarWave 80ED on Celestron CG5-AGT mount, using a monochrome webcam with filter-wheel and Celestron 0.63 focal reducer.
sw80imaging_small.jpg (180.01 KiB) Viewed 7944 times
So last night I was able to try out my newly-made Bahtinov mask, using Bellatrix as the target.
First with the Hydrogen-Alpha (red) filter it was really clear, but not so good with the Oxygen-III (green) and then when I tried the blue filter, the diffraction patten was really indistinct:
Trying to focus on Bellatrix through the blue filter.
bellatrix_b.jpg (11.32 KiB) Viewed 7944 times
So recalling Dave's advice that the ED 'scopes may not be so well-corrected for the short wavelengths (violet), I got out my old Baader "contrast-booster" filter (aka "minus violet") and put that in-line too. That made a big difference:
Focussing on Bellatrix through the same blue filter, but with the Baader "contrast booster" in-line too.
bellatrix_b_minusv.jpg (10.63 KiB) Viewed 7944 times
The improvement can also be seen by comparing these two images of M42, both taken through the blue filter:
Single 20-sec image of M42 through the blue filter.
m42_b.jpg (127.85 KiB) Viewed 7944 times
Single 20-sec image of M42 using blue and "contrast-booster" filters.
m42_b_minusv.jpg (107.76 KiB) Viewed 7944 times
Now the stars aren't "bloated", and there's more detail in the nebula too.