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Soft focus on terrestrial targets

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#1 BarryBrown

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Posted 19 August 2014 - 05:11 PM

I just got a T-mount and adapter for my DSLR. I thought I'd try taking some pictures of terrestrial targets during the daytime, so I focussed on a tree about 1/4 mile away.

 

No matter how much I try to dial in the focus as sharply as possible, everything is soft -- just slightly out of focus. If I swap the camera for an eyepiece, the view looks as sharp as a tack. But primary focus on a DSLR is soft. I'm not anywhere near the SCT mirror's limits of travel.

 

My scope is a Meade LX200 Classic 10". Camera is a Nikon D700.

 

What's causing this?



#2 jrcrilly

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Posted 19 August 2014 - 05:15 PM

It's likely that you are seeing the difference between visual focus, where you get it close and your eye accommodates to finish the job, and imaging, where it is either perfectly focused or it isn't. It's much more critical.



#3 BarryBrown

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Posted 19 August 2014 - 05:22 PM

The nice thing about a tree is it has depth. If I focus on the trunk as best I can, then even if I'm off a bit, something (such as a branch or leaf) ought to be sharp. But nothing was. Since the scope is f/10, I should have a bit of depth-of-field to work with, too.

 

Still, I'll try a target further away to increase the DOF.



#4 Ed Holland

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Posted 19 August 2014 - 06:29 PM

Barry,

 

If your object is too close, the scope will exhibit spherical aberration, because the optics are designed to focus light from "infinity". The closer a target, the more things deviate from this ideal. That is why one must be careful about interpretation of star tests and optical quality when using artificial stars.

 

Also, it's hard to focus - I know this after trying to use my Mak for some nature photography. Despite what the camera's sensor thought, very few if any shots were properly sharp.

 

Lastly, any heat haze will upset the view - just like seeing does at night, and it can be impossible to take a sharp picture!

 

I hope that is helpful,

 

Ed



#5 PowellAstro

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Posted 19 August 2014 - 07:27 PM

This is normal with a 10 inch during the day. The speed your eye sees changing detail is much faster than the shutter on the camera. So the camera catches the image shifts as an average and the result is a blurred image. 


Edited by PowellAstro, 19 August 2014 - 07:34 PM.


#6 BarryBrown

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Posted 19 August 2014 - 11:19 PM

Thanks for all your suggestions.

 

My latest test shots were of a tall pine tree approximately 1/2 mile away. It's amazing how just the top 12 inches of the tree nearly fills the frame!

 

I was shooting at 1/2000 sec, which should eliminate all motion blur. What image shift are you referring to?

 

There is a Moonlite focuser attached to the scope. Its dual-speed knobs should allow for some very fine focus without mirror shift.

 

I'm not going to rule out heat on the mirror or that heat rising off rooftops could blur the image.

 

Regarding the spherical aberration, how would an SCT differ from so-called "mirror lenses" available for SLRs?



#7 Ed Holland

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Posted 19 August 2014 - 11:54 PM

1/2  mile might well rule out aberrations. Heat disturbance of the air between you and the subject is a likely cause.

 

I've played a bit with my scopes in the daytime and it is a very different prospect than night time viewing. High powers are hard to achieve due to disturbances, and one doesnt resolve as much as expected.



#8 Kevdog

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Posted 20 August 2014 - 10:14 AM

 

Regarding the spherical aberration, how would an SCT differ from so-called "mirror lenses" available for SLRs?

 

The mirror lenses are figured to focus at much closer distances (and the focal length is much smaller too).  The telescope is made to work at infinity focus, so it is optically best there.



#9 Jon Isaacs

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Posted 20 August 2014 - 11:32 AM

1/2  mile might well rule out aberrations. Heat disturbance of the air between you and the subject is a likely cause.

 

I've played a bit with my scopes in the daytime and it is a very different prospect than night time viewing. High powers are hard to achieve due to disturbances, and one doesnt resolve as much as expected.

 

 

I agree..  During the day, large scopes at long distances (and even a few hundred yards or even less) are unlikely to provide crisp, sharp views..  It's like looking at an object low on the horizon at night, it's never good.. 

 

Why does it look good in the eyepiece.. what eyepiece, what magnification?  What exit pupil. It may be that your pupil is not dilated and the exit pupil is masking the aperture.. 

 

Jon



#10 Bob D

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Posted 21 August 2014 - 06:42 PM

 

 

Regarding the spherical aberration, how would an SCT differ from so-called "mirror lenses" available for SLRs?

 

The mirror lenses are figured to focus at much closer distances (and the focal length is much smaller too).  The telescope is made to work at infinity focus, so it is optically best there.

 

In addition, the typical mirror "lenses" for SLRs are a compromise in optical quality vs. cost compared to the same-aperture all-refractive lenses.  I tried a few over the years and have never been happy with the tradeoff.



#11 Ed Holland

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Posted 21 August 2014 - 07:28 PM

I have a Tamron 500mm that is really quite good indeed, found as a used bargain. There is a lot more in the optical design than in a typical Maksutov. The additional constraint to provide a flat field for 35mm film means that this includes corrective optics - kind of an "Edge" Maksutov if you will. The trade off will likely involve "acceptable over a wide range of focus lengths" (down to about 6ft for my example!) rather than the telescope with the single constraint of correction at infinity.

 

Used on my crop sensor SLR, the results are quite impressive. It's also easier to carry around than a conventional long lens. Pros and cons of course - there is no aperture control (fixed at f/8), and also the issue of out of focus highlight "doughnuts".

 

I've played with my 127mm Mak a few times in daylight, as a 1500mm camera lens! Focus is tricky to control in perfect conditions, and the image scale easily reveals thermal disturbance. However, it can work well if the conditions are right. Baffling/stray light control was initially very inadequate and contrast quite dramatically affected by this. Flocking the baffle tube made a vast improvement - there's a thread somewhere in Cats & Casses in which I presented pictures documenting the effect on image quality.

 

I must have another play around with this :)



#12 orion61

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Posted 22 August 2014 - 08:18 AM

For one you are looking through the optics of your Camera, If your camera lenses had an eyepiece adapter they would look sharper too.

As long as the pictures are coming out good I wouldn't worry about it. It is the image on the Sensor or (shiver) film that is important.



#13 Hesiod

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Posted 22 August 2014 - 09:06 AM

I had the same issue with my Celestron C8. The big culprit was air turbulence, simply too strong for such resolving power (a 100/1000 plus Barlow 2x provided slightly better results, as a 90/1250 Barlowed did: in other word, lower resolving power and longer f/, a known "remedy" for bad seeing).

 

Another issue was scatter light: the scope does well under a night sky (even with full Moon), but noon summer Sun was simply too much, and the pics exhibited a lowered contrast (I noticed this fact shooting at the same target -a wall with some colored lichens- in cloudy and sunny days).



#14 Ed Holland

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Posted 22 August 2014 - 11:29 AM

One big issue with Cassegrain designs in daylight is that the baffle tube can be illuminated by quite a significant amount of stray light e.g. from a bright sky. Although there is no direct path to the image plane, the inside of the baffle tube can readily scatter this light, resulting in a bright "veil" across the images that obscures dark details.

 

This thread: http://www.cloudynig...ffle-mod/page-3 has some pictures I took showing the large improvements made by improving stray light control.

 

Cheers

 

Ed



#15 PeterR280

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Posted 22 August 2014 - 12:47 PM

I have the same issue during the day with my C11 on distant terrestrial objects. Everything seems soft visually  as compared to a large refractor.

 

I attribute it to thermal currents and lower contrast due to all the ambient scattered light in the baffles.



#16 Nippon

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Posted 23 August 2014 - 05:15 PM

Most likely mirror slap vibration. That is an extremely long telephoto and your shutter speed would have to be very high to cancel out vibration from the camera's reflex mirror. Try locking the mirror up before the exposure. I believe the D 700 has that ability.



#17 james7ca

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Posted 23 August 2014 - 05:28 PM

As others have said, a lot of the problem is from light scatter in the optical system which reduces contrast in the image. Then there is the turbulence when looking through air that is being disturbed by daytime heating. Try the same test very early in the morning before dawn when the sky isn't so bright and the ground and air are nearer to the same temperature.



#18 azure1961p

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Posted 23 August 2014 - 10:35 PM

I didn't read the entire thread so here goes...

 

the dslr is a time exposure where as the eye is a continuous and more accommodating *exposure*.  You're seeing a smeared image from the dslr caused by the seeing.  If you want stunning long distance details  observe over water on a cloudy day.

 

Pete



#19 kozmik frakture

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Posted 23 August 2014 - 11:38 PM

I think part of this is the heat waves in the air you are looking thru.  I don't think that takes much of a distance to raise it's head.  I used to shot Nikon 35mm film thru my old Celestron 8" SCT, doing mainly birds.  The photos came out sharp but the distances were no more that 300 to 500 feet








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