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Author Topic: Astrophotography and MFD  (Read 21167 times)

Fine_Art

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Re: Astrophotography and MFD
« Reply #40 on: January 25, 2012, 12:56:38 AM »

I'll be using tilt screen live view so the strange camera angle of a newtonian is not a big deal. I'd expect to use it like the waist level ground glass of my Mamiya RB67 on tripod with cable release. I will have to consider the options.
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Fine_Art

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Re: Astrophotography and MFD
« Reply #41 on: January 25, 2012, 03:51:58 AM »


Most cassegrain systems for photography and astronomy have very large central obstructions. This mirror in the front of the scope is big and it kills contrast. Take a look at how big the mirror is on the front of the RC you listed. I bet its central obstruction is at least 40%. Anything above 25% and the impact of a large secondary mirror really kicks in.


You are right, the manual said 47% by diameter, 22% by area which is a massive diffraction hit. Not worth it at this price level IMO. If it was a 30% i'd be all over it.


I have no problem with pointing a 32" tube. If the distributor tells me the lower end refractors arn't up to the challenge so be it. It's much easier to correct reflector problems that color problems in a raw conversion.
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PierreVandevenne

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Re: Astrophotography and MFD
« Reply #42 on: January 31, 2012, 06:02:30 AM »

I wanted to get back to this with a practical example, but the weather has not been cooperating to say the least. Still, these extremely poor pictures show the issues to expect. Wide field shows the area in front of my house, with uncorrected vignetting. The white spot is the area we are aiming at. 1st zoom level is taken with the Canon 300 2.8 + TC 1.4 (420mm) 2nd zoom is the TMB 805/115 at prime focus. The two flat fields (that's the plus of bad weather, you can take flat fields ;-)) show that the amount of vignetting is, expectedly, much higher with the TMB than it is with the Canon Zoom. This being said, compared to the wide angle shot, which can be corrected in lightroom, it remains quite acceptable. The problem is of course that you'll have to make a model of the vignetting yourself if you care. The second image shows a roof on the edge of the field of view and, imho, distortion is kept well under control in the TMB (no need for a field flattener).

Now, about the constraints... The Canon + TC was handheld with IS on (I made sure exposure was quick enough so vibration wouldn't impact the image too much) The TMB was on a 26kg mount, quite unpractical to catch a bird in flight, and I had to take 5 shots around teh focus point to be sure I would hit something decent (can't trust my eyes too much).

With even more constraints, I can bet better results with afocal images because the camera (using a macro lens in my case) is doing the autofocus on the image that gets out of the eyepiece, but the conditions weren't worth the trial
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Fine_Art

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Re: Astrophotography and MFD
« Reply #43 on: January 31, 2012, 04:59:40 PM »

This is not clear to me. The TMB is about 800mm focal vs the canon with 1.4 at 420mm.

So the 1st zoom at 100% is the TMB not the canon. Correct? I can see some color fringing on the smaller canon shot. The TMB looks like a real APO.
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PierreVandevenne

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Re: Astrophotography and MFD
« Reply #44 on: February 01, 2012, 07:39:44 AM »

Correct, relative sizes were preserved: bigger magnification equals TMB. I meant "second image", but had a slip of the mind and keyboard.

Yes, the TMB is a triplet. And in fact it is even better than the Canon than the images show because the edge of the roof was quite close to the edge in the TMB shot, and a bit less so in the Canon shot (because both shots were centered on the same location and, of course, the field is wider on the shorter focal length.

I'll try the Skywatcher doublet as well when the weather clears a bit.
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