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frustrated!!! (getting to grips with a 6" Newtonian)
http://www.boltonastro.co.uk/forums/viewtopic.php?f=8&t=236
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Author:  Amartin [ Sat Mar 10, 2012 9:19 am ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Thanks for the advice David, (I've no idea what vignetting is!!!!) Ross has pind'g me an IM telling me to post some pictures as there are likely to be others in the same predicament as me. Basically i have taken some shots to demonstrate how Ross & I discovered that i could bolt my 1100D EOS directly to my camera. The attached pictures show the 5 steps it takes to do it.

1. extend the eyepiece focus aperture a little
2. unscrew the eyepiece holder
Attachment:
swthread1.jpg
swthread1.jpg [ 107.29 KiB | Viewed 6823 times ]

3. screw in the T ring adaptor assy
Attachment:
swthread2.jpg
swthread2.jpg [ 137.54 KiB | Viewed 6823 times ]

4. Fit the T-piece adaptor to the scope (loosen the small allen screws to allow free movement of outer ring)
5. et voila! officially the longest camera lens in the world (not strictly true, but you get the gist of my enthusiasm!!!)
Attachment:
swthread3.jpg
swthread3.jpg [ 126.75 KiB | Viewed 6823 times ]


Hope this helps others in the same situation/with the same kit - DSLR & a Newtonian that want to do a bit of imaging...
Cheers
Andy

Author:  rwilkinson [ Sat Mar 10, 2012 11:55 am ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Amartin wrote:
I've no idea what vignetting is


Vignetting is a sort of "optical bottleneck" where the light-cone is constricted by having something with too narrow an aperture in its optical path. It appears as a darkening of the corners of the image:
Image

Now if you'd used a 1.25" adaptor to slide your camera into the existing eyepiece-holder, its inside bore (around 25mm) would have been too narrow to let the light-cone from your telescope fully illuminate the sensor in your camera.
But the "mouth" of your T-adaptor is wider (>35mm) so none of the cone is blocked. :D

Author:  DRatledge [ Sat Mar 10, 2012 5:21 pm ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Andy,
I see your set-up now. You will get a bit of vignetting from the 1.25 inch focuser tube. Ross is correct the T mount is fine but the focuser tube is still only 1.25 inches - less ID. However, the full Moon should be OK as you have a shortish focal length. Vignetting will only show when you attempt deep-sky objects but that's another story! If you are interested in shooting the Moon then make sure you don't miss April's Astronomy Now Magazine - out later in the month.

Author:  Amartin [ Sun Mar 11, 2012 7:52 am ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Is my focal length calculated by D/F??? If so, i can work it out that my focal length is 5. i don't however know what that means in practical terms lol & probably i should as i guess (stab in the dark) it defines the limitations of what can be seen through the scope.
I'll check out Astronomy Now, I'm already subscribed to Sky at Night (good deal of 3 publications for £5), will see how that goes!!!

Weather was a total washout last night, so no opportunity to get some imaging done!
what would be the best ISO setting for imaging the moon through my setup Ross/David??? Ive really no idea of where to start - buy you guessed that already!!!!

Author:  rwilkinson [ Sun Mar 11, 2012 8:52 am ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Amartin wrote:
Is my focal length calculated by D/F??? If so, i can work it out that my focal length is 5. i don't however know what that means in practical terms

I posted some comments on f, D and f/D last week in response to Phil's questions:
http://www.boltonastro.org.uk/forums/viewtopic.php?f=8&t=234#p610
but unlike the planets, the Moon is a large subject (~30 arc-minutes diameter).

Now the Moon is also a sunlit subject, so it's very bright, therefore you should use a slow ISO setting and a fast shutter-speed. Try the camera in aperture-priority mode or manually select different shutter speeds and use the camera's histogram function to check that you're not over-exposing.
And you must minimise vibration from the camera, so use the self-timer and mirror-lock functions when firing the shutter (and keep very still if you're standing next to the 'scope!).

Author:  Amartin [ Sun Mar 11, 2012 9:55 am ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Thanks for the link Ross, i understand why the ratio is so important when viewing different objects (even if the practical application is yet to be discovered!!!). Thanks for the edit on the images i posted too - much better now!!!

I may have to get out the manual (shock horror) & learn the flash points of how to use my camera. Please don't tell me that my camera is no good for astro-photography now lol.

Cheers
Andy

Author:  DRatledge [ Sun Mar 11, 2012 4:12 pm ]
Post subject:  Re: frustrated!!! (getting to grips with a 6" Newtonian)

Andy,
Good news you have bought the best camera. The Canons are extremely low noise and their raw files have not been messed about with by camera internals. Nikons are notorious for deleting faint stars because the camera thinks they are noise!
Your focal length is printed on the the scope - 750mm. For that focal length the Moon will be about 7mm in diameter so will easily fit on your camera's chip without any vignetting. Try and get the Moon dead middle where the telescope performs best. I use ISO400 as a standard. Under expose rather than over-expose. Once an image is burned out then there is no recovery.

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