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Newbie Question on Dobsonians

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#1 Atl

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Posted 07 January 2013 - 12:47 PM

Is a video camera useful with a dobsonian or do you a precise tracking mount like one you would use for photography?

#2 mclewis1

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Posted 07 January 2013 - 01:11 PM

There are some folks who have successfully used a shorter focal length dob (or other scope) with a focal reducer (so the overall focal length is quite short and the focal ratio is very fast) and no tracking at all for shots in the 4-8s range. Useful for showing folks a little color in the brighter dsos and bunches of stars in the globular clusters but the lack of tracking quickly becomes a pain. I seem to remember comments a few years ago from a bigger dob owner who could see the arms of M51/81 with a non tracking setup.

There are quite a few dob owners with tracking tables using video cameras for shots in the 5-30s range (and sometimes more). This (big fast dob and a tracking table with a sensitive video camera) is IMHO a very effective combination for near real time viewing.

It's not until you want to get into the >60s exposure range that an accurate EQ mount (and even autoguiding) is required or desired for viewing. Now if you are actually doing any imaging (stuff you want to keep and display) then the more accurate EQ mount would probably be required for even the shorter exposures.

#3 Atl

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Posted 07 January 2013 - 01:18 PM

I have been looking at the Mallincam Junior which only allows four second exposures. So I guess what I was wanting to know is if this will enhance my observing experience using a dobsonian telescope or would the lack of tracking make the images too blurry for visual observation? I am interested in imaging but observing is my prime concern right now.

#4 mclewis1

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Posted 07 January 2013 - 02:57 PM

Have you asked the same question on the Mallincam yahoo group? You'll probably get a few dob owners chiming in there and maybe even a few Jr. owners.

#5 S.Boerner

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Posted 07 January 2013 - 06:40 PM

Part of the answer deals with the size of the sensor. I have a Samsung SDC-435 (now relabeled SCB-2000) with a 1/3" chip that I piggyback on my dob. I pretty much get the same FOV with the Samsung and a f/3.5 135mm telephoto camera lens (45.6x94.6 arcminutes)as I do with my 12" f/5 dob and a 26mm ep (72 arcminutes). Playing around with CCD Calculator will show you what I mean.

It will do fine as an finder and you'll see some detail you won't get visually, but the big thing to remember is that you are not going to get the resolution you'd get with a CCD camera and you won't want to hang the results on the wall.

#6 wcstarguy

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Posted 07 January 2013 - 07:13 PM

Another newbie here, I have an Orion XT8i dob scope with a 2" GSO crayford focuser(not the low profile) and an Atomic tracking platform. Thinking of getting one of the Samsung (SCB-2000)video cameras to try out video astronomy without the full Mallincam investment. I have read that I might have to move the primary mirror or get a different focuser to have the proper in bound focus. Has anyone tried video on an XT8i with a Samsung camera or would I be wasting my time. Thanks, WC

#7 S.Boerner

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Posted 08 January 2013 - 09:10 AM

wcstarguy...

The Samsung's sensor needs as much light as you can give it. Try to be faster than f/4. Your XT8i's f/6 will be a bit slow and the focal length of 1200mm is going to give you small fov 5.2x10.8 arcminutes with a SCB-2000. A .5x focal reducer will get you to a good f/3 light wise, but still the fov would be 10x22 arcminutes. I've used my camera on a C14 (3900mm fl) to do a good splitting double stars down to about 1 arcsecond. For a quick conversion, think of the 1/3" sensor in a Samsung as a 6mm eyepiece. Your 1200mm focal length is going to give you about 200x, more than most large DSOs are going to need.

Consider used camera lenses on ebay between 135 and 250mm that are faster than f/4. The 135mm lens I use frequently sells for less than $20. First go to a big online camera store like Adorama or B&H and search for c-mount & Canon, or c-mount & m42, or c-mount & Nikon and you'll find adapters that can be used to connect the lens to the Samsung in the $20-40 range. Decide on the brand of lens you want to use and get the adapter first. If you already have a DSLR and lenses so much the better, all you'd need is an adapter.

Using a zoom lens can be fun too, but they are a bear to get spaced correctly so they hold focus throughout the zoom range.

Check out CCD Calculator at:
http://www.newastro..../camera_app.php
to get an idea of what your fov with various focal lengths will be. A guy here got a SDC-435 a few years ago and stuck it on his ETX-125. He complained that the only thing he could see were craters on the moon. It was f/15 & 1900mm. It didn't give the camera's sensor enough light (remember faster than f/4) and his 1900mm fl gave a tiny fov of 3.3x6.8 arcminutes. I don't think he's used it since.

You'll also need a way of seeing the output from the camera. The two normal choices are either an lcd tv or frame grabber to your computer.

If you are really interested in doing video you might want to take a look at a web page I put together about two years ago:
http://www.asemonlin...ic-viewing-w...

Video is fun, but it does take a bit of time to work out the kinks.

Good Luck

#8 wcstarguy

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Posted 08 January 2013 - 02:26 PM

Sounds like I might be better off getting one of the camera lenses or maybe using my 80mm Orion goscope refractor, it is f4.3 and I already have a .5 focal reducer,this should put it at f2 or so (I have a 90mm mak I was trying it with). I also have a portable dvd player (the dvd laser doesn't work anymore but the screen and input from cable seems to work fine) for viewing or my asus netbook(with a cheap capture device (I have a 15" laptop too). If I use the 80mm refractor, I could mount it on my dob, this would allow me to use the Orion object locator and the tracking platform, hmmm or I have a meade ds2000 mount with autostar that will hold the 80mm (bought it used a couple of years ago and have never fired it up (supposed to work great). So, using my current inventory, I would need the Samsung camera and power supply (I also have a deep cycle 12v marine battery), c adapter, cheap image capture device, maybe a pollution filter and a cable or two.....sounds fairly inexpensive to start. Just trying to get my feet wet in the video area with minimal investment for now....later, more scope and mallincam? Thanks for the excellent reply...WC

#9 S.Boerner

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Posted 08 January 2013 - 05:01 PM

I think you'd be much happier with the results with your 80mm! That, a camera, and an 1.25" c-mount nosepiece will get you going quite well. Agena Astro has Baader metal nosepieces ($38)or you can go to ebay for plastic($15).

BTW if you already have a 12v battery all you need to hook up power to the camera is a hunk of wire. There are two screw terminals on the back of the camera.






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