90mm astro telescope for birdwatching?

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A 90mm f/11 refractor makes a decent spotting scope at the expense of greater length and weight than the usual specialist instrument. At 7lbs with Vixen, closed rings and dovetail it is not ideal for carrying far but many 'birders' arrive in their car and just set up their tripod mounted telescope nearby. 

A 45 degree Amici prism 'diagonal' corrects the inverted and L-R reversed image. An astro star diagonal can only invert the normally inverted image but cannot manage the left to right reversal.  

This particular Amici "terrestial" prism is from Baader with a larger claimed clear aperture. It has a bronze compression sleeve to avoid marring a standard 1.25" eyepiece barrel. It is an attractive and solidly made piece of equipment which seems not to damage the image up to at least 50x. Which is the highest magnification I have tried so far with my familiar no-name 20mm Plossl and plastic, detergent bottle top, camera adapter. See below for visual magnifications. 

Here is the Vixen 90M 90/1000mm on the Bogen video tripod with original, rise and fall, pan and tilt head. The tripod was bought secondhand decades ago and is quite firm provided all the head adjustments are well tightened. 

I suppose a purist birder would paint the main tube and dewshield green or black or even camouflage. Or even drape a camouflage cloth over the entire telescope. Padded camouflage case, anybody?

Though all the birds will ever see is a distant and very dark lens hidden deep within a blackened dewshield. Not that one should ever underestimate the remarkable visual acuity of birds. The person standing behind the telescope is likely to present are far larger target for nervous creatures of all kinds.

A 32mm EP in the Vixen produces a 31x90mm with options to push the magnification to ridiculous levels if needed. This is quite a decent telescope for birding on lakes, wetlands and bigger ponds. The image is bright and razor sharp with incredible contrast and detail. At 45 meters I could see every detail of a Greenfinch's plumage as he swayed back and forth on some nearby larches in a gale.

At 80 meters I could easily see the detail in the threads of seeds hanging from a neighbour's tree. At 135 meters I could easily see the texture on birch catkins and the detail on the bark. A fly would have been just as easily seen if it was daft enough to venture out in a gale in falling rain in the early evening. I pushed the power up to 100x, with the 10mm EP, but this was the limit under such a heavy overcast. I shall try again in bright sunshine to see how far the Vixen holds up for terrestrial use.

I took a couple of afocal 'snaps' through the 20mm Plossl to give an idea of image quality with both the Amici prism and "straight through." 

I have posted the un-retouched images here. [With the inverted one re-inverted in PhotoFiltre.] It might have helped if both images were of the same tree but I assure you they were at exactly the same distance.

Which is the laterally correct and upright and which was inverted and reversed should be obvious from the date and time stamp.

Remember that these images have a nominal magnification of 1000/20mm  = 50x. The view seen directly through the eyepiece is of much better quality. Camera shake may have something to do with this but the eye is very forgiving compared with a camera.

Perhaps I should try lower powers more in keeping with typical spotting scope powers? Though I would need a different [bottle top] adapter to manage the slightly larger body of the Meade 4000 32mm Plossl for ~31x. The rest of the 4000 range actually seem to be slightly smaller in body diameter than the no-name 20mm Plossl EP.

The conditions were heavily overcast and very windy but dry. The trees imaged are just over 600 meters/yards away and visibly moving in the wind. Shutter speeds on the Canon Ixus 117 HS, short zoom, compact digital camera lay in the range of 1/300-500th which is likely to freeze any movement.

Don't ask me to work out the true f/ratio or equivalent focal length of an afocal telescope/ digital camera system. These will both will depend on the camera's imaging sensor size. Picasa tells me that I used no zoom so the nominal "film" lens focal length is shown as 30mm. The human eye has an equivalent focal length [in camera terms only] of 50mm. So, perhaps I should reduce the 50x nominal telescope magnification by 3/5? Or increase the zoom to 50mm equivalent?  The former reduces magnification to a [very nominal] 30x. Or 50x with modest zoom. You do the maths. ;-)

I can't see much [any] difference in image quality from this simple image capture test.The light may even have changed slightly between taking images. Terrestrial seeing conditions are often inferior to astronomical observation due to thermal currents rising from the sun-heated ground over the entire length of the view.

Click on any image for an enlargement.

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