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Contains:  NGC 2252, NGC 2244, Rosette nebula, NGC 2239, The star 12Mon
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Rosette Nebula with a Sow's Ear Telescope, 


            David McClain
Rosette Nebula with a Sow's Ear Telescope

Rosette Nebula with a Sow's Ear Telescope

Technical card

Resolution: 3379x2703

Dates:Feb. 22, 2016

Frames: 15x300"

Integration: 1.2 hours

Avg. Moon age: 14.52 days

Avg. Moon phase: 99.93% job: 972716

RA center: 97.962 degrees

DEC center: 4.947 degrees

Pixel scale: 1.791 arcsec/pixel

Orientation: 179.334 degrees

Field radius: 1.077 degrees

Locations: Backyard Observatory, Tucson, AZ, United States


This image is from last night, using my new system for removing differential guide flexure, and my "Sow's Ear" telescope, as has been described by another expert.

The image is barely modified, but only stretched into visible contrast regime. (MURE denoising, color calibration, nonlinear luminance stretch to visible) For the first time, I'm unashamed of people looking too closely at the image. So here it is in full resolution.

Each sub was a guided 300s exposure under Full Moon conditions on the Tucson sky toward the SouthWest, overlooking the lights of downtown Tucson, about 5 miles away. The sky measured about 19 mag/arcsec^2. Each sub had more than 50% of well capacity taken up by the sky background, despite my using an IDAS LPR filter to remove sky pollution. So there isn't much point exposing for any longer than 5 minutes.

But exposing fully for 5 minutes gives the deepest background a chance to overcome the read noise and Poisson noise from the sky background. And that makes a lot of difference -- provided you can keep them focused tightly on the image plane.

WIth a HyperStar system there is nowhere to mount an OAG or an on-axis guider. The only choice is a separate guide scope. And with my particular "junk" mechanical arrangement (Televue Pronto (heavy!) mounted atop the C8 with Celestron/Losmandy adapters), there is plenty of differential flexure. Normally about 120 arcsec / hour during tracking.

But I created a "kicker" App that restarts the guiding every minute with a modified guide star center location, fooling the autoguider into compensating for the effects of changing differential flexure. And last night's images showed star cores that were as good as I have ever seen at just 5 sec exposures, over the full 5 minute duration. Normally, a 5 minute guided exposure would have produced star trails about 10 pixels long.

[ so why has it been deemed a "Sow's Ear" of a telescope? I did an all sky survey of differential flexure and the results showed thermal drift, and at least 4 zones of something changing horribly badly along the way. My attempts to accurately align the mount are confounded by 100 arcsec+ image shifts after focus, which moves the primary mirror. While the mount is very fine, my mounting is mechanically poor, there are no mirror locks on the primary. In short it is a piece of consumer junk. Yet, despite all that, it is capable of spinning silk when helped along by a computer. ]



David McClain
License: None (All rights reserved)

Sky plot

Sky plot


Rosette Nebula with a Sow's Ear Telescope, 


            David McClain