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View Full Version : Is it me or my eyepiece



seeker372011
2003-Nov-05, 02:15 AM
I have a small 5 inch Newtonian. When I look at bright objects-Venus or Mars I have noticed recently that I see multiple rays-that is star points around the disc, particularly when I use a modified webcam for real time viewing on a PC, but also when I use a normal eyepiece

Distant stars seem OK. I seem to see them as points of light, it's the bright planets that are giving me this grief, even with my lowest power eyepiece

Deep space -nebulae and globulars and so on-too, look faint but OK.

Is it a problem with my mirror? my collimation? my eypiece quality? or is that just the way it is and something one has to put up with?

Is this diffraction and can anything be done to control it?

I would appreciate any help anyone can provide....

Thanks in advance
Seeker

Dave Mitsky
2003-Nov-05, 04:01 AM
What you're describing sounds like diffraction spikes produced by the vanes of the spider (secondary mirror support). The brighter the object observed the more prominent these spikes will be.

Dave Mitsky

kashi
2003-Nov-05, 05:10 AM
Does this problem occur when you don't view objects via the webcam?

Dave Mitsky
2003-Nov-05, 07:16 AM
See the section on diffraction spikes at http://www.chapman.edu/oca/gallery2/artifact.htm

Dave Mitsky

seeker372011
2003-Nov-06, 02:12 AM
Dave:

Thanks for your response, you've nailed it in one. It is diffraction spikes -very similar to the photo in the link you provided,

In my colour webcam the spikes actually breakdown into the colours of the spectrum, but for sure, that's the problem. Kashi, I still see spikes when I look at say Mars through an eyepiece.


Question then is: can anything be done about this? Or is this the bane of a reflector? I have seen some ATM sites where people have used single strut spiders and maybe that's why they chose to do so , to avoid diffraction spikes?


The funny thing is when I bought the scope a reflector rather than a refractor was suggested because observing from light polluted suburbia, I was told I'd be more likely to do planetary observing and needed the larger aperture for the same money..but if a reflector isn't good for bright planets because of diffraction from spider vanes.....Is this a Catch 22 for small scope users?

Seeker

starrman
2003-Nov-08, 10:59 PM
Don't be too quick to abandon your Newtonian, seeker. In some instances those diffraction spikes can actually be an advantage. An example is in the case of a double star in which the primary is significantly brighter than the secondary (or tertiary, in the instance of a multiple system). Positioning the diffraction spikes in the proper orientation can enable observation of the fainter companion, where the larger Airy disc of the primary might overwhelm the companion in a refractor or Schmidt telescope.

Clear skies.

seeker372011
2003-Nov-15, 10:06 PM
Ed wrote:


"Have an Intes Micro Maksutov-Newtonian - The beauty of this model is that there are no supporting vanes to the secondary mirror as it is mounted in the lens at the front end of the scope.

No vanes = No diffraction spikes.

Question from a person who does not have the problem. Would it be possible to modify the vanes in some way to reduce the problem? My first toughts are to use only a thin wire or to lessen the surface area of the vane by putting holes in it."

Ed J

It's a good question, can some one advise if there is a way to reduce diffraction spikes in a newtonian?

seeker

PS mine is a commercial OTA, so I am terrified to think about tinkering with the spider.

Even the "c" word-(collimation), makes me afraid, very afraid!!

--------------------

starrman
2003-Nov-17, 09:55 PM
Seeker, please don't be afraid of the "C" word. Collimation of your scope is not only possible, but absolutely necessary in order to achieve the best optical performance. You'll be a much happier observer with a well-collimated instrument, and the process isn't difficult or dangerous. There are numerous resources online that will supply all the technical information you need to do the job, and lots of other astronomers out there who'll be glad to assist you along the way. Do you belong to a local astronomy club? If not, get in touch with your local group. There are bound to be experienced observers there who'll be happy to help you learn the twists and turns of collimating a Newtonian.

It's almost a given that a newly purchased scope (especially a Newt) is not in perfect collimation and thus not giving the best image of which it's capable. Now that Saturn is well-placed, it provides a great target for comparison purposes. Take a good look at Saturn now, before you make any adjustments, and take a look at a bright star at the highest magnification your eyepiece collection allows, on both sides of focus. Then get together with a collimation mentor and get your scope adjusted, primary, secondary, and focuser. Take another look at Saturn and compare. I think you'll be amazed at the difference.

Here's a link to a very good resource for collimation. Even if you decide to get help from someone with more experience, it'll be good to have some information about the process, and Carlin's procedures are well thought out and clearly presented. Have fun with your scope.

http://zebu.uoregon.edu/~mbartels/kolli/kolli.html

Good luck, and
Clear skies.

John

corkft
2003-Nov-18, 01:54 AM
you might try using a filter for the bright objects.

ORSkywatcher
2003-Nov-20, 01:24 AM
By the sound of it you are seeing diffraction spikes. It is normal that they are more noticable on brighter objects. They are caused by your spider's veins. Check and make sure that they are not angled. Thicker spider veins will cause biger difraction spikes. If your Spider viens are turned/twisted it will appear as if you have thicker spider viens. There are ways to reduce this effect but you can not get ride of it completely.

exAstro
2003-Dec-21, 08:59 PM
Check out curved vane spiders instead of four vane. There's a vendor on the net who sells them (can't recall name, sorry). These are supposed to eliminate the diffraction you observe with standard vaned spiders. I'm going to replace the 4 vane on my starhopper as soon as I have the time ;-)