SkyViking
01-05-2011, 08:46 PM
Hi all, I finally had a chance to put the new Serrurier truss OTA to the test the other night.
(I have recently finished building a new OTA, details and photos of the beast are in this thread: http://www.iceinspace.com.au/forum/showthread.php?t=74690)
I wanted to test the performance of the new scope on an open cluster. I thought it would be a good target for evaluating the (hopefully reduced) diffraction pattern and to see if there was any improvement in the resolution I could achieve.
Attached is an image of NGC 6231 in Scorpius taken with the new OTA. Also an inverted version which shows the faint diffraction spikes better. The last image is one I took last year of the Gem Cluster, I attached it for comparison. Notice how the halos and spikes are much more prominent and how details are lost near the brighter stars.
With the new scope I even had to stretch the NGC 6231 image considerably to bring out the diffraction spikes! I also like how even faint stars are easily resolved right next to the brightest cluster members. I could definitely not have gotten such a result before. This scope seems to deliver refractor-like images now, which quite surprises me.
It's not that I don't like spikes, they look pretty :P, but they also divert precious light away from the target and reduces the overall image contrast - which may make all the difference when trying to capture some faint quasar or other obscure target.
The most interesting aspect is that this new image was taken with the same optics as before, the only difference is that I have mounted the mirrors in a new OTA where I have removed all obstructions such as mirror clips, eliminated protruding focus barrel etc, and replaced my traditional 4 vane metal band spider with a wire spider using 0.23mm guitar strings.
All in all, I now have better focus and less difffraction. I think the mirror was slightly pinched in the old OTA too, something I took care to avoid when building the new mirror cell. I believe the biggest single improvement comes from the wire spider. I had massive diffraction spikes and halos before, and they are virtually invisible now, which translates to tighter stars and better resolution.
As an added bonus I seem to have gotten completely rid of the focus drift that plagued me before. The NGC6231 image was taken as a 5 part mosaic, each 100x15.5 seconds. So total imaging time was just over 2 hours. During this time I did not once have to readjust focus, which is something new to me! :lol: The last frame of the last panel was as sharp at the very first frame.
I also noticed that my finder now stays perfectly parallel with the optical axis. It used to shift slightly even when just changing direction of the scope. Now I can center a star on the crosshairs and it will be in the center of the eyepiece at 400x magnification. It means that I can center a target in the finder and it will be right on the camera chip the first time. Excellent :D.
Comments and critique is most welcome.
The OTA design itself is documented in detail in the other thread mentioned above.
(I have recently finished building a new OTA, details and photos of the beast are in this thread: http://www.iceinspace.com.au/forum/showthread.php?t=74690)
I wanted to test the performance of the new scope on an open cluster. I thought it would be a good target for evaluating the (hopefully reduced) diffraction pattern and to see if there was any improvement in the resolution I could achieve.
Attached is an image of NGC 6231 in Scorpius taken with the new OTA. Also an inverted version which shows the faint diffraction spikes better. The last image is one I took last year of the Gem Cluster, I attached it for comparison. Notice how the halos and spikes are much more prominent and how details are lost near the brighter stars.
With the new scope I even had to stretch the NGC 6231 image considerably to bring out the diffraction spikes! I also like how even faint stars are easily resolved right next to the brightest cluster members. I could definitely not have gotten such a result before. This scope seems to deliver refractor-like images now, which quite surprises me.
It's not that I don't like spikes, they look pretty :P, but they also divert precious light away from the target and reduces the overall image contrast - which may make all the difference when trying to capture some faint quasar or other obscure target.
The most interesting aspect is that this new image was taken with the same optics as before, the only difference is that I have mounted the mirrors in a new OTA where I have removed all obstructions such as mirror clips, eliminated protruding focus barrel etc, and replaced my traditional 4 vane metal band spider with a wire spider using 0.23mm guitar strings.
All in all, I now have better focus and less difffraction. I think the mirror was slightly pinched in the old OTA too, something I took care to avoid when building the new mirror cell. I believe the biggest single improvement comes from the wire spider. I had massive diffraction spikes and halos before, and they are virtually invisible now, which translates to tighter stars and better resolution.
As an added bonus I seem to have gotten completely rid of the focus drift that plagued me before. The NGC6231 image was taken as a 5 part mosaic, each 100x15.5 seconds. So total imaging time was just over 2 hours. During this time I did not once have to readjust focus, which is something new to me! :lol: The last frame of the last panel was as sharp at the very first frame.
I also noticed that my finder now stays perfectly parallel with the optical axis. It used to shift slightly even when just changing direction of the scope. Now I can center a star on the crosshairs and it will be in the center of the eyepiece at 400x magnification. It means that I can center a target in the finder and it will be right on the camera chip the first time. Excellent :D.
Comments and critique is most welcome.
The OTA design itself is documented in detail in the other thread mentioned above.