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Old 12-01-2010, 11:34 AM
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alistairsam
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Talking Moving primary mirror on a Newt for DSLR astrophotography

Hi,

I have an 8" F6 Newtonian with a 2" Crayford focusser.
I'm very keen on using a DSLR with the T adaptor at prime focus for astrophotography.
I read that the primary mirror has to be moved forward to bring it to focus, my question was if anyone knew how this can be calculated or approximated?
Will the effective F/L reduce from 1200mm if the mirror is moved forward, and thereby reduce overall magnification? Not sure if this is relevant as there is no eyepiece with prime focus.

I presume the same can be achieved by using focal reducers, but i'd rather avoid using additional glass (lenses) if i can.

I also wanted to know what the difference was in the eyepiece projection method, afocal method and prime focus method. I read that in eypiece projection, the greater the distance between the CCD plane and the eyepiece plane, the higher the magnification.
But would'nt that be fixed depending on the focal length of the mirror and eyepiece?

I'm working on a split fork ring mount for the OTA with autoguided tracking, as i'm very interested in long exposure deep space imagery.

Thanks
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Old 12-01-2010, 12:44 PM
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DavidU (Dave)
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Welcome mate. There is a great article (to the left) of moding a Newt for DSLR imaging.
http://www.iceinspace.com.au/63-441-0-0-1-0.html
There is a few fellas doing Newt imaging here and the coma correction and field flattening is a thing you will have to address.
IIS has at least 4-5 world class planetary imaging folks here using Newts.
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Old 12-01-2010, 01:33 PM
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Hi and welcome to IIS. Moving the primary doesn`t change focal length but brings the focal point further out of the OTA so your DSLR can achieve focus. I would forget eyepiece projection, barlows etc for Deepsky imaging, just use prime focus for this and if you want to, get a Baader MPCC, Your f/5 primary will show coma in the corners and the coma corrector will give you fairly round stars to the corners. The only thing with moving the primsry is that you shall have a slightly smaller field of illumination (vignetting like) but taking flats will fix this.
Hope that helps.
Gary
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Old 12-01-2010, 03:14 PM
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Smile

Thanks for that. Am just deciding on the DSLR.

Could you'll please elaborate a bit on field flattening. I'm guessing this is to reduce field curvature. how is this done.

Had a look at the Baader MPCC and Paracorr. Will have to get one of these.

Came across explanations on how colour sensitive cones in our retina are limited and therefore we see less colour through the eye than through a camera.
This would mean that to view nebulae and discern colours, we have to use a camera.

Threfore, does normal viewing without a camera through a large aperture reflector, say 12" and above, provide satisfactory viewing?

Would'nt it be pretty much like viewing in black and white, add to it the need for long exposures to get details of galaxies and nebula, can we really see galaxies with just good eyepieces and 8" and above reflectors?

What objects are best viewed through a large aperture F5 or F6 reflector without cameras. I mean with 12" and above primaries.

I might have to move this question to a different thread, but are equatorial platforms sufficient for exposures of say 15 to 20 minutes for deep sky objects?

Does dark frame subtraction make much difference if using a DSLR?

Sorry about the series of questions, just that its so fascinating and broad.
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Old 12-01-2010, 09:44 PM
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mswhin63 (Malcolm)
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I also use a DOB tube to do webcam imagery, most webcams I am unable to acheive prime focus as the CCD element sit too far back in the camera. I use 1 webcam VAGENT the focuser has about 1mm of travel left. images done are here http://picasaweb.google.com/mswhin63

I have just purchaced a DSLR 450D and wating for my T Adaptor to arrive. I found the 1.25" adaptor unscrews to expose the T-Thread so I don't need to purchace a 2 or 1.25" tube to slot in.
With that it looks like I have managed to reduce the focal length but about 1" giving a better opportunity to reach prime focus.

It's a waiting game until the EOS - T adaptor arrives
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Old 13-01-2010, 08:49 AM
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pics look good malcom. Were all of them taken at prime focus?
Did you remove the lens of the webcam?
the VAgent V5 specs show its 2M capable.
I saw their new one which says its got a full HD Sensor at 1920x1080 or 8MP.
I have'nt seen any long exposure mods for this webcam though. May suit planetary imaging.
Do post pics when you get your DSLR and how much you had to move the mirror by.
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