BOEING 747SP (N747NA)

This aircraft is actually a 747SP. It's about 40' shorter than the 747-200.

Not an expert, but I believe it's the one with the airborne telescope fitted under the large door at the back.

Yes, Colin Seftel you're right. It's a B747SP. We can see 3 doors on the port side (left) as one was removed due to the big door added for the telescope (which opens on flight).
The B747-200 model has 5 doors on each side.

Great shot! Would love to see the interior! NASA's website has a picture with the door open: https://www.nasa.gov/mission_pages/SOFIA/overview/index.html

Stratospheric Observatory For Infrared Astronomy, SOFIA
A significant rebuild from the original SP...
A unique aircraft with a unique job.

Engines seem extra long.

Fantastic observation instrument ! Great photo !

Was chief engineer on on this program for awhile after we got the aircraft mod done to install the telescope. The picture shows the aircraft with uprated -15 engines to provide the extra performance to get to altitude faster (as well as better high-altitude mission performance...). Since the telescope is pre-cooled on the ground prior takeoff, they want to get to altitude as quickly as possible before opening the door so the telescope won't go through thermal cycles. Plenty of interior pictures can be found at nasa.gov and search for "sofia interior pictures"

Thanks for the info Mark Henley!

Karl Jackson - You're welcome. It was a great program to work. Got to fly on a science mission out of Palmdale to see how it all fit together.