What's causing the poor image quality?

[RtOaNn]

I guess Isisaxon already explained it all.

 

[Murcielago]

A likely reason is the presence of high clouds which you cannot see with your naked eye. This happens all the time to astrophotography especially in SG where clouds prevail.

Since you used 70-300 on D70s, your images are probably cropped and your lens may not be sharp enough to resolve the moon surface also. The difference in quality you obtained with P mode and M mode may be due to the aperture in use also. Selecting f/8 or f/11 may give you a better image since most lenses are slightly better when stopped down a couple of stops.

For example, this moon below is shot with D2X using 300/4 and resized to show the entire frame and the relative size of the moon within the frame. It's quite small.

This is the crop. As you can see, even with more megapixels from the D2X, because the magnification due to the cropping is tremendous, the image is not really sharp because the lens was not able to resolve.

This is the entire frame resized shot with D2X through a 1250mm SCT telescope around the same time. It already has so much more detail than the 300/4 because the lens is designed to give this kind of magnification.

A crop from this image would yield something like this.. you can see that the focus is not spot on and there is some camera shake also. Again, this is pushing beyond the limits of the telescope.

The absence of clouds helps in both cases, shot within minutes of each other.

what's magnification due to cropping?

so are u saying telephoto lens have a 'range' for images it can resolve or resolve to a point where the 'blur parts' are negligible???

thanks.

btw is the sct telescope yrs ?

 

[PsychoV]

Bro, the lowest ISO setting on the D70s is ISO200.

Oops sorry! :embrass:

Not a Nikon user, lol.

 

[lsisaxon]

what's magnification due to cropping?

so are u saying telephoto lens have a 'range' for images it can resolve or resolve to a point where the 'blur parts' are negligible???

thanks.

btw is the sct telescope yrs ?

2 reasons.

1) the telephoto lens cannot resolve. If you look at the moon image shot with the 300/4, the diameter is only about 1/10 the frame width, so it's only about 2.3mm wide on the sensor. To have so much details, the lens must be able to resolve to within microns, very near optical limits.

2) the sensor itself has a limited resolution, some of the details on the moon may be within one or 2 pixel width already. So you can't get anything better than that.

The SCT is an old Nexstar 5 which I had for quite a number of years already.