How Much Focal Length Is Too Much For Planetary Imaging

[JayTee]

Wow, great image size and great resolution. I've gone as far as 5600mm for FL. I would need a 4X barlow to get close to your 13,650. Seeing your results, it may be worth a try.

Thanks for sharing this amazing image.

Cheers,
JT

This brings up an interesting point. At some point, the magnification will be too high. For visual it is something like 50x per inch aperture I thought. Isn't this the same for imaging? I never have seen this addressed.

Isn't this an interesting question, is there such a thing as too much focal length for planetary imaging. Luc, who won today's TSSAPOD, used a focal length of 13,650mm or for us USA types that's almost 45 feet!!!. The most I've ever used is 5600mm (FL = 2800mm with a 2X barlow). If I were to use a 4X barlow I could get that up to 11,200. But managing that much FL and dealing with centering the object, the ROI (is it big enough), and the inherent, unavoidable "shakes" might make this much FL very problematic.

I see this as both a physics question and a physical question. At what point does FL supersede resolution and light grasp for a camera? And is your mount stable enough to handle that much (for want of a better term) magnification?

To start off with using visual ROTs (rules of thumb), regarding the human eyeball, just doesn't apply to imaging with a camera. Our eye is a rather small aperture and slow video camera and has well documented limitations. But has anyone seen any limitations as to how far you can push FL in an imaging system?

Please, let's discuss this. I'm eager to hear your thoughts.

Cheers,
JT

PS, This quote is from another forum (solar system astrophotography). I brought it over here because this tangential topic might "steal" the topic away from the OP.

[KathyNS]

I have used 5600 for planetary. I am tempted to try 8400 (2800 x3). But even finding the target would be tricky, and then there's the seeing which is hardly good enough to justify 5600, let alone 8400. I might try it some day. Maybe at Mars opposition this fall.

[sdbodin]

I use 8000mm with the big 16 on planets, 2x barlow. It depends on your arc-sec/pixel scale for the camera you have and the theoretical resolving power of the scope. Then apply the 3x sampling criteria. My math work out to f18.3, 7329mm. So 2x and 8000 is close enough for amateur work.

http://www.wilmslowastro.com/software/f ... D_Sampling

From the above reference. Hope the link works.

Clear skies,
Steve

[JayTee]

Steve,

I forgot about that link. Nicely done. For my planetary imaging setup - (C11 and ASI 120MC) it says using 5800mm is as far as I should go. Interesting. I'm still tempted to go bigger just to see what happens. Also, I wonder if his assumptions are correct/reasonable.

Cheers,
JT

[sdbodin]

Steve,

I forgot about that link. Nicely done. For my planetary imaging setup - (C11 and ASI 120MC) it says using 5800mm is as far as I should go. Interesting. I'm still tempted to go bigger just to see what happens. Also, I wonder if his assumptions are correct/reasonable.

Cheers,
JT

I checked over on that 'other forum' and f20 seems to be a popular choice among the best of the planetary imagers. There is no reason you can't mess around with other enlargement, just time and patients, neither of which I have much of.

Steve

[gcisko]

To start off with using visual ROTs (rules of thumb), regarding the human eyeball, just doesn't apply to imaging with a camera. Our eye is a rather small aperture and slow video camera and has well documented limitations. But has anyone seen any limitations as to how far you can push FL in an imaging system?

Please, let's discuss this. I'm eager to hear your thoughts.

Cheers,
JT

I never considered that visual ROTs wouldn't apply to imaging with a camera. My alternative to this "problem" may be to use software to magnify the image effectively increasing the focal length.

Here is a new image I just created. It is taken with my 8SE with a F/6.3 focal reducer, 2.5x Tele Vue Powermate, then magnified with Topaz Giga Pixel, 2x, then again another 2x. My calculations put this image as 12000mm focal length (42 feet). I see no "optical" way to get 12600mm focal length... Suggestions?