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First photos from friends telescope - but tracking problems


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Hi all - four of us had a fairly successful astrophotography night last Friday up in Ashdown Forest (which included a drive by from the Police to check on what we were up to lol). Moon was pretty bright and there was a lot of air traffic about, but we saw some decent objects and a couple of planets plus got some nice shots (See Log in | Facebook).

One question we had is the mount my friend has is a Skywatcher 127 (5 inch cassegrain) and although it has a Goto function on it, from what he understands from checking with a few people (Possibly even from here), that this sort of mount is poor for long exposure photos of more than 10 seconds.

I can't fathom out why a telescope manufacturer would create a cracking scope, with a GoTo system that basically points you at the nebula, star, planet etc but doesn't allow you to photograph it! It's tantamount to There it is, but you can't see it lol.

Is there anything we are doing wrong or should be doing to set this scope up so it can do long exposure pictures to show up galaxies, nebula etc?

many thanks

Frank

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As its altaz (the mount) you need to get it on a wedge (Google is your friend) As the AltAz can on move in Alt and Azimuth it tracks in a strange, zig zag fashion instead of a smooth, RA track. The Wedge (Im unsure myself of how) allows the fork style AltAz mounts to track well in RA.

Its a tracking mount, just not accurate enough for photography, for visual use it will track fine :)

All the best

Eddy

**EDIT** After a quick google search the problem is 'Field Rotation' on AltAz mounts ;)

Edited by eddy
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The short answer is yes you can get quite reasonable DSO shots out of alt-az telescopes. My advice is the following

(1) Alt-az mounts do suffer from field rotation (don't try imaging straight up!), but usually tracking inaccuracy will get you first. Stick to no more than 30sec exposures.

(2) The Skywatcher 127 has a longish focal length (1500mm), which is going to give you sub-arcsec pixels on a typical DSLR. This is too small for the tracking accuracy of these mounts, even in 30 secs, so bin up the final picture to bigger pixels (i.e. scale the image smaller in photoshop or whatever). This will reduce the effect of tracking errors - scale down by at least a factor 2 and maybe as much as 4.

(3) You need to take lots of subs and stack them together. As well as getting better signal-to-noise, you will be amazed how the rubbish tracking gets smoothed out to give round(ish) stars!

NigelM

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Frank, I used to use the Celestron version of that mount for deep sky photography. it's not impossible, just infuriating. Your biggest issue with it for that task, is the focal length of the scope, which is going to work brilliantly on the moon and planets, and with a webcam you should be able to get good results (check out the spc880 from Morgan Computers).

If you keep the focal length shorter, camera and lens perhaps on that mount, then, as long as you stay low in the east or west, you can.. get exposures of up to 2 minutes in length. As your target gets higher or is further north or south, that comes down. Near the zenith, I was able to get 30 to 40 seconds, take plenty of exposures, and you'll be amazed at the fuzzies you can get (even from where you are).

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Frank, I used to use the Celestron version of that mount for deep sky photography. it's not impossible, just infuriating. Your biggest issue with it for that task, is the focal length of the scope, which is going to work brilliantly on the moon and planets, and with a webcam you should be able to get good results (check out the spc880 from Morgan Computers).

If you keep the focal length shorter, camera and lens perhaps on that mount, then, as long as you stay low in the east or west, you can.. get exposures of up to 2 minutes in length. As your target gets higher or is further north or south, that comes down. Near the zenith, I was able to get 30 to 40 seconds, take plenty of exposures, and you'll be amazed at the fuzzies you can get (even from where you are). I can post some exmaples if you want.

Edited by jgs001
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If you keep the focal length shorter, camera and lens perhaps on that mount, then, as long as you stay low in the east or west, you can.. get exposures of up to 2 minutes in length.

Alt-az image rotation is independent of focal length.

NigelM

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Agreed Nigel, but the shorter focal length, is likely to be lighter and helps alleviate tracking errors, it worked with mine. My C80ED was just too heavy for the mount to track accurately for longer than 15 seconds. I think a Skymax 127 is actually heavier, the Skymax 102 is pretty close.

Here's some examples Frank, they were taken with a 450d on an ST80 clone from my back garden. My camera has not been modded and therefore isn't particularly sensitive to infrared (Ha).

m38adj3.jpg

m52bubble.jpg

m27.jpg

I think they all have the details in the border.

Edited by jgs001
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