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Spider Diffusion Spikes?

Started by fblue, 07/26/2003 04:03PM
Posted 07/26/2003 04:03PM Opening Post
I took out my 15" f/4.9 DOB out last night. I have only owned it for a short time. This is my first DOB and I don't know a lot about observing with one. I have owned SCTs in the past.
I did a lot of looking at Messier objects and NGCs before Mars came up and the views were very good. Others checked it out and said that it had good looking optics. (Swayze Mirror)
When Mars came up I tried to look at it with my 31mm Nagler and I had a very large starburst looking bright spikes in a + pattern. I tried several other Naglers and a couple of Meade UWAs and basically saw the same thing. A filter suppressed them quite a lot. Is this what the diffusion spikes look like when looking through a DOB at a bright object? If not, what else could cause this. The scope was well collimated with a LaserMax collimator. It looked textbook perfect.
These spikes were not apparent at all until I tried to look at Mars. This is the first time I looked at Mars with it.
Any help or suggestions would be appreciated.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 07/27/2003 05:35AM #1
The diffraction spikes are, as others have mentioned, caused by the spider support for the secondary. The are too faint to see on fainter objects, but, as you see, show up quite vividly on bright planets and stars. While they are unattractive, they really do very little harm to the ability of the scope to show planetary detail.

A three vane spider would show six spikes, although each would be fainter. Some folks use a curved spider to spread out the effect and render it invisible, but that might be difficult with a larger scope if you want to keep the vanes thin. Most folks learn to live with it.
Posted 07/29/2003 01:32PM #2
To de-fuse terminology confusion and avoid mental distraction, let's change the subject name to its correct form, Spider Diffraction. Diffraction is a well-known optical phenomenon that has nothing to do with diffusion. Pardon me if I'm a nitpicker, but I cringe every time President Bush says "nucular".