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Old 2018-01-19, 18:41   #1
xilman
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Default First light

At long last I now have 7 images of the night sky through my DSLR. They are of absolutely no scientific value whatsoever and you would need to be seriously weird to consider them to be of any aesthetic significance.

To take them I set a 80-200mm lens wide open (f/4) and at 200mm FL. Focusing consisted of winding the lens' control until it indicated infinity. The contraption was fixed to a light-weight tripod and 30s exposures taken of three different randomly selected portions of the sky.

The images show stars which are about 18 pixels (110 arcsec) across and 110 pixels (650 arcsec) long. The sky background is very noisy and vignetting is obvious. Nonetheless stars as faint as 9th or 10th magnitude are visible.

The focusing isn't actually too bad for photometry though I'd prefer a diameter closer to 9-12 pixels. It's clear (and I was aware of this beforehand, of course) that a driven mount will be needed.

First light images from any telescope are rarely useful to anyone but the commissioning engineers.
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Old 2018-01-19, 23:00   #2
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Quote:
Originally Posted by xilman View Post
At long last I now have 7 images of the night sky through my DSLR. They are of absolutely no scientific value whatsoever and you would need to be seriously weird to consider them to be of any aesthetic significance.

To take them I set a 80-200mm lens wide open (f/4) and at 200mm FL. Focusing consisted of winding the lens' control until it indicated infinity. The contraption was fixed to a light-weight tripod and 30s exposures taken of three different randomly selected portions of the sky.

The images show stars which are about 18 pixels (110 arcsec) across and 110 pixels (650 arcsec) long. The sky background is very noisy and vignetting is obvious. Nonetheless stars as faint as 9th or 10th magnitude are visible.

The focusing isn't actually too bad for photometry though I'd prefer a diameter closer to 9-12 pixels. It's clear (and I was aware of this beforehand, of course) that a driven mount will be needed.

First light images from any telescope are rarely useful to anyone but the commissioning engineers.
Can you approach this in a passive manner, such as placing one of the tripod legs on a Ziploc bag of water with a small hole that allows the water to leak out at a rate corresponding to the shift? I know it wouldn't be as easy to calculate the water escape rate as it would a drive unit, but it would be less expensive...
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Old 2018-01-20, 16:57   #3
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Ernst, in a PM, suggested that posting the images might be a good idea. I can't see why myself but here's a compromise. The raw images are far too big (3100x2064x16 bits) and are almost completely black to the unaided eye. The ones attached have been resized to 600x399x8 bits and histogram equalized. The vignetting, bias and sky noise are now easily visible.

im1 is of the sky immediately overhead here (+52o08', +0o07') at 2018-01-16T21:24:18+00:00

im2 is of somewhere in the vicinity of M45 and im3 is a random chunk of Ori or possibly Eri.

Added in edit: im1 contains 5 Cam. Persuading astrometry.net to solve that field was a bit of a challenge. It expects star images to be roughly circular.
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Last fiddled with by xilman on 2018-01-24 at 17:19 Reason: Doh! Selected images but failed to upload them.
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Old 2018-01-20, 17:06   #4
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Quote:
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Can you approach this in a passive manner, such as placing one of the tripod legs on a Ziploc bag of water with a small hole that allows the water to leak out at a rate corresponding to the shift? I know it wouldn't be as easy to calculate the water escape rate as it would a drive unit, but it would be less expensive...
Probably easier, and much less messy, is to get the mount under my 25cm Meade powered up and properly aligned. That's my next major commissioning task.

The major downside is that it is much less easily movable than a camera tripod. The trees in and around my garden severely restrict the view of the sky from the telescope.

Tom has was sounds like an elegant compromise solution.. I'll investigate further because it sounds like it should be good for off-site observations.
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Old 2018-01-22, 20:13   #5
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Well, trying prime focus photography threw up a number of opportunities. I just hope that none are insurmountable.

One for which I don't yet have a solution is that the focus can't be achieved, even when the telescope's rack & pinion is at the minimum position. Not sure how to fix this one yet. Any suggestions?
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Old 2018-01-24, 10:21   #6
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It's possible that a solution to the focus opportunity has been found. Web searching suggests that a Barlow lens is often needed with reflectors. A test in broad daylight, attempting to focus on a contrail, showed promise. 10/10 cloud cover every night has delayed a definitive test.
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Old 2018-01-24, 10:50   #7
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Quote:
Originally Posted by xilman View Post
Probably easier, and much less messy, is to get the mount under my 25cm Meade powered up and properly aligned. That's my next major commissioning task.

The major downside is that it is much less easily movable than a camera tripod. The trees in and around my garden severely restrict the view of the sky from the telescope.

Tom has was sounds like an elegant compromise solution.. I'll investigate further because it sounds like it should be good for off-site observations.
I am happy to lend you the iOptron SkyTracker for a few months of evaluation - I have the necessary rails for mounting the DSLR on the EQ5 mount, and am doing that on the (sadly at present very rare) occasions that a clear night and an awake Tom coincide.
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Old 2018-01-24, 17:21   #8
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I am happy to lend you the iOptron SkyTracker for a few months of evaluation - I have the necessary rails for mounting the DSLR on the EQ5 mount, and am doing that on the (sadly at present very rare) occasions that a clear night and an awake Tom coincide.
Many thanks, most generous! I'll take you up on that one. More comms via email.
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Old 2018-02-11, 19:07   #9
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Here is an image of the Ξ²CMi region (the bright star at the bottom) taken with the aid of Tom's camera drive. It's a stack of 10 images taken on 2018-02-09 with a Canon EOS 10D with a 200mm lens at f/4. The faintest stars visible on the original FITS image are around mag 12.5. By chance the images are around 8 pixels FWHM, which is just about ideal for photometry but nowhere near sharp enough to make a prettier picture. The sky isn't black because of histogram equalization and a gamma tweak to make the faintest stars visible. Some streaking in the sky shows that I didn't do dark and/or bias reduction properly --- need to investigate that.



Pleased! as The Donald would tweet.
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Old 2018-02-12, 12:57   #10
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Pleased! as The Donald would tweet.
Hurray! Glad the drive is proving itself useful; it was a good clear night last night, though I curled up with a duvet and hot chocolate and sore-throat lozenges rather than setting up a scope.

Last fiddled with by fivemack on 2018-02-12 at 12:59
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Old 2018-02-22, 21:35   #11
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Now that it seems I'm going to continue with the astrophotography lark I followed advice and purchased a camera with LiveView (i..e using the display for target acquisition and focusing) and a more sensitive detector. The venerable Canon EOS 1000D has been gaining rave reviews for astrophotography almost since it came out. Mine cost Β£22 on eBay, roughly 25% of the median price, because it has an irritating habit of over-exposing itself in usual usage. Indeed, every picture of the moon I've tried taking has been over exposed. When exposures are measured in seconds or minutes this misbehaviour is quite unimportant.

Anyway, the attached image is a mildly contrast-stretched stack of three 30" images of the region of Rigel, all in the Gi channel (see below) and of an intervening yew tree at the bottom of the image. The faintest stars are around 12.0m and focusing yields a FWHM of around 2-3 pixels on unsaturated stars, which are far too small for photometry. LiveView focusing works amazingly well!

Lessons learned:

1) it's a real pig to align the iOptron's polar axis but it tracks at diurnal rate very well. The trailing in the image is virtually pure N-S and indicates that I couldn't find Polaris in the spotting scope. Once, some time ago, when it was properly aligned the result was beautiful circular star images. I need a permanently mounted equatorial head, preferably with GOTO, which can be adjusted and left properly aligned. Luckily I've a Meade LXD-75 but, unluckily, don't have enough round tuits to install it.

2) Color separation (I use rawtran) really, really should be done in the detector's raw colour space (Ri, Gi and Bi in rawtran jargon, the 'i' indicates "instrumental") or interpolation and cross-talk errors are legion. The sub-images are also half-size compared with the 3-colour or interpolated-colour versions because the Bayer mask (RG/GB for instance) replicates every other pixel. Canon boast, with some justification, of a 3906x2602 pixel sensor. However, only 1953x1301 are red detectors and the same for each of green and blue.

3) The 80-200mm lens used can't be focused at 200mm with the 1000D. The test shots had to use a notional 100mm focal length. "Notional" because adjustment wasn't very precise and the adapter for the elderly manual lens contains a Barlow of (about) 1.3x. The image scale in arcsec/pixel and the known pixel pitch (remembering to double the documented value because of the issue above) will provide an accurate figure --- not that it particularly matters.

4) Bias, dark and flat correction is important, even for aesthetics let alone scientifically useful measurements, as can be seen from the vertical streaks in the image.

5) The camera has absolutely no hot pixels at all and only a very few which are even mildly warm. They should be easily treatable with dark correction. Until flat frames have been acquired it's impossible to say whether there are dead pixels

6) A cable release is essential, even for 30 second exposures. I have one but didn't use it tonight.
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Last fiddled with by xilman on 2018-02-22 at 21:39 Reason: Fix 2 minor tyops
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