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Astroscope + DSLR = Setups! (2 Viewers)

Tord said:
Hello Jules

Attached are two crops, 1:1 scale, showing what I mean.

The purple/pink-ish reflections in the beak and eye on the Wood Sandpiper, and the reflections in the water droplets in the feathering of the Grebe.

This CA is visible in bright light conditions only and only when light is concentrated.

Cheers
Tord
Click to expand...

Tord,

I also have photos where drops of water have this defect. However, it looks like ordinary purple fringing to me. I think we see it as a general purple reflection because the image portion causing fringing is very small. Looking at your 2 photos, I do not see any general purple tint which would be a sign of overall CA.

As I understand it, the overall CA would be axial (longitudinal) CA while the purple fringing around your droplets would be transverse(lateral) CA. See this article:
http://en.wikipedia.org/wiki/Chromatic_aberration

My SW80ED will sometimes show purple fringing around water droplets or small very bright objects like your photo and very rarely as a thin purple or magenta line around sharp lines. My Olympus 100-300 mm is much worse for CA - fortunately it can be corrected.

Regards
Jules
 
There was a huge flock of geese at the lake yesterday so I took the scope over there tonight. Heading into the golden hr a dark grey cloud bank blotted out 1/4 of the sky where the sun was falling. So this sample is in dark grey light adjusted in raw. ISO 540(auto)

100% crop also attached.

Scope is 1200mm 10" dobsonian.
 

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Tlapo 804

Just got myself a TLAPO 804, have had time to test it. Fernando wrote an extensive review a few years ago:
http://www.birdforum.net/showthread.php?t=190727&highlight=TLAPO+804. That thread is however old and I was recommended by the forum not to append to it so I start a new post.

The scope I got differs in design from the one Fernando has, the tripod mount is different, this one comes with two CNC rings.

The focuser is very precise, no play at all.

It is short enough to be transported as hand luggage.

It is light enough to be carried around. I can check the weight on a scale if anyone is interested.

The short length means that the centre of gravity is close to the focusing wheel so hand-held operation should not be too difficult. This was an issue with the SW80 that required a "third hand".

Monopod is a viable option (for hikes and longer walks). You need to pay attention on the torque exercised on the extension tube if using a loosely tightened ball head though. I guess better use with head tightened enough to avoid surprises/scope tilting and risking exercising excessive stress on the camera fitting.

With 80mm extension tube I am just able focus on infinity.

At F/6 the viewfinder image is clear as one would expect. And the depth of field is shallow, a blessing and curse at same time.

The few pictures I have taken so far look good. Will test deeper as soon as light conditions are OK.

The close focus range supported by the improved Crayford focuser and 80mm tube feels like real close, I have not measured exactly, I estimate to something like 2.5 meters meaning semi-macro at distance is a potential application area. I can measure if you want to know. Fully extracted, there was no noticeable play on a E5 body (850g).

I will think on optimizing the mechanics for easy conversion between tripod/monopod/hand held.

Stay tuned
Tord
 
Tord said:
Just got myself a TLAPO 804, have had time to test it. Fernando wrote an extensive review a few years ago:
http://www.birdforum.net/showthread.php?t=190727&highlight=TLAPO+804. That thread is however old and I was recommended by the forum not to append to it so I start a new post.

The scope I got differs in design from the one Fernando has, the tripod mount is different, this one comes with two CNC rings.

The focuser is very precise, no play at all.

It is short enough to be transported as hand luggage.

It is light enough to be carried around. I can check the weight on a scale if anyone is interested.

The short length means that the centre of gravity is close to the focusing wheel so hand-held operation should not be too difficult. This was an issue with the SW80 that required a "third hand".

Monopod is a viable option (for hikes and longer walks). You need to pay attention on the torque exercised on the extension tube if using a loosely tightened ball head though. I guess better use with head tightened enough to avoid surprises/scope tilting and risking exercising excessive stress on the camera fitting.

With 80mm extension tube I am just able focus on infinity.

At F/6 the viewfinder image is clear as one would expect. And the depth of field is shallow, a blessing and curse at same time.

The few pictures I have taken so far look good. Will test deeper as soon as light conditions are OK.

The close focus range supported by the improved Crayford focuser and 80mm tube feels like real close, I have not measured exactly, I estimate to something like 2.5 meters meaning semi-macro at distance is a potential application area. I can measure if you want to know. Fully extracted, there was no noticeable play on a E5 body (850g).

I will think on optimizing the mechanics for easy conversion between tripod/monopod/hand held.

Stay tuned
Tord
Click to expand...

Hi Tord,

Interesting ! Smaller and faster than the 600mm doublets and about half way between those and a 300mm M43 lens.

I wonder how it would compare with the 500mm telephoto lenses that can be had for not much money on eBay. They are quite fast, not too heavy, M43 adapters and TCs are available for most makes, the tripod mount is built in, etc.

Some examples:

569$ -- Tamron SP AF 200-500mm F5-6.3 Di LD IF
350$ -- Sigma 170-500 mm F/5.0-6.3 APO AF ASP Lens For Minolta/Sony
360$ -- Tokina AT-X　SD 150-500mm F5.6 F/5.6 for Nikon
380$ -- MAMIYA 645 C 500mm 5.6


Has anybody tried this type of lens as an alternative to the doublets most of us use ?
 
Congrats on the new scope, it's a very good choice IMHO ;)

Tord said:
At F/6 the viewfinder image is clear as one would expect. And the depth of field is shallow, a blessing and curse at same time.
Click to expand...

That's why I went with the trouble of putting an iris in mine. F/6 is very shallow if we shoot upclose.
 
JGobeil said:
I wonder how it would compare with the 500mm telephoto lenses that can be had for not much money on eBay. They are quite fast, not too heavy, M43 adapters and TCs are available for most makes, the tripod mount is built in, etc.

Some examples:

569$ -- Tamron SP AF 200-500mm F5-6.3 Di LD IF
350$ -- Sigma 170-500 mm F/5.0-6.3 APO AF ASP Lens For Minolta/Sony
360$ -- Tokina AT-X　SD 150-500mm F5.6 F/5.6 for Nikon
380$ -- MAMIYA 645 C 500mm 5.6


Has anybody tried this type of lens as an alternative to the doublets most of us use ?
Click to expand...

Excluding the Mamiya because I've never tried it, the TL is largely superior to any of theses lenses in anything related to image quality.

Also, keep in mind these lenses Tstop, the Tamron 200-500 has a very comparable Tstop to the ED80 at infinity. So it's actually not that fast.
And the telescope has no focus breathing witch is a good advantage for anyone shooting upclose.
 
I doubt very much that any of those "cheap" telephoto lenses would come close to a good ED triplet scope in IQ. Maybe easier to use and lighter, but sharp? I doubt it...
 
Is it not so that all lenses have what you call "focus breathing"?

Wikipedia:
--------------
When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.

To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length (f), the distance from the front nodal point to the object to photograph (S1), and the distance from the rear nodal point to the image plane (S2) are then related by:

1/S1 + 1/S2 = 1/f
---------------
 
Breathing refers to the change of angle of view of a lens when shifting the focus. Some (often higher quality) lenses are designed to lessen the degree this effect. Lens breathing does not prevent one from racking focus or following focus with this lens, but it lessens the desirability of any type of focus adjustment, since it noticeably changes the composition of the shot.
Also wikipedia under breathing (lens)
 
Tord said:
Is it not so that all lenses have what you call "focus breathing"?

Wikipedia:
--------------
When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.

To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length (f), the distance from the front nodal point to the object to photograph (S1), and the distance from the rear nodal point to the image plane (S2) are then related by:

1/S1 + 1/S2 = 1/f
---------------
Click to expand...

Actually that section of wikipedia article explains exactly what happens with our telescopes (or any lens that focus by extension), and why it needs to be focused in or out as the distance of the object to focus varies. Basically it shows the mathematical ratio between the distance of focus and how much the focuser needs to be adjusted. I don’t see it as a reference to focus breathing.

I don’t think breathing can be generally quantifiable. It’s a variable aspect of each photo lens and depends greatly on its design, it happens due to the change in the distances between the internal elements on each lens.
A lens like a telescope have no change at all in the focal lenght since there’s no lens elements to vary the distance between each other, it’s just a lens and a focal plane (sensor).

OTOH is not only upsides, a lens that doesn’t “breath” will lose light when focused upclose.
A photo lens that breaths loses focal length, but for the most part it keeps it’s F/ratio, throughout it’s focus travel.

You can see the same effect of the telescope in most macro lenses too, as they also lose light when focused closer than infinity, some more than others too, depending on design.

BTW, here’s two test shots I’ve done sometime ago of the TL against a bigma OS.
This one it’s just to compare the Tstop of both, as you can see the scope is nearly a stop faster.
http://photos.smugmug.com/photos/i-6JM5Prk/0/O/i-6JM5Prk.jpg

And this compares the breathing effect:
http://photos.smugmug.com/photos/i-BPRjNt6/0/O/i-BPRjNt6.jpg
They were all done from the same place, the bigma was at 500mm, but breathing make it drop to a 400mm FOV. I had to upsize it’s image by 125% to get the same FOV as the TL APO.
 
Fernando,

I have a quick question related to the TLAPO 804:

Does your scope have a splittable tube?

If so, have you been able to unscrew the threaded ring holding together the front and rear tubes? I could easily unscrew the back tube from the ring using gentle hand force, but the front tube feels like semi-permanently fixed to the ring.

The reason is that I want to slide the dew shield past the tube in order to remove the felt inside. This operation requires free access. This is needed to wrap the scope in camo tape, the tape builds about as much thickness as the felt and sliding is no longer possible.

Otherwise I will have to think of another solution.
 
No, mine is the older version with the single piece tube.

Looking at the new scope, I assume that if the middle ring doesn't come out you can take the dew shield off by unscrewing it’s rear ring (the part you need to remove the felt). Then you’ll be able to unscrew the lens cell off the main tube and have access to the needed ring.
At least that’s how it looks from the images, I may be wrong though.
 
Thanks Fernando,

I tried, but no success, I don't want to excercise too much stress risking deforming my scope. I am thinking about putting a collar of neoprene on the rear section instead of wrapping with tape. Not beautiful, but functional.
 
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