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Old 02-03-2019, 01:29 PM
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Diffraction spikes eliminator

Idea is here....
Realization is on it's way.
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Last edited by bojan; 02-03-2019 at 03:39 PM.
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  #2  
Old 02-03-2019, 04:48 PM
astro744
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I've seen this or something very similar before in an old book on ATM. Whilst the diffraction spikes appear smaller or even appear gone the energy is simply spread out over a wider area impacting on contrast.

Whether a loss of contrast bothers you depends on what you're looking at as does the appearance or not of a bright spike. The only way to eliminate the diffraction caused by the spider vanes is to not have any whether they be straight, curved or otherwise masked and that's not possible with a standard Newtonian design.
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Old 02-03-2019, 04:53 PM
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I am aware of all that.

But sometimes, spikes are really in the way, like when observing/imaging Sirius B.

This is the main reason I am doing this - the mask is not mounted permanently, to be used as/if required.
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Old 02-03-2019, 05:17 PM
glend (Glen)
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Does this approach not increase obstruction, thus further reducing resolution?
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Old 02-03-2019, 05:50 PM
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The proof will be in the pudding so to say. I will be interested in the results of your experiments Bojan. Where did the design come from? Or how did you come up with the design?

Regards,

Mike
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Old 02-03-2019, 05:51 PM
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Ah, sorry Bojan I have just noticed the link. Got it!
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Old 02-03-2019, 05:59 PM
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Quote:
Originally Posted by glend View Post
Does this approach not increase obstruction, thus further reducing resolution?

Yes... So this should be used with due care
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Old 02-03-2019, 06:44 PM
Wavytone (Nick)
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Bojan

I’ve seen that before. The way this works is by having curved edges, where the radius of the curve is the same as the semidiameter (ie radius) of the primary mirror. The result being light diffracted will contribute a circular diffraction pattern concentric with that of the mirror, and if done right, no spikes.

The basic principle is that a curved edge in the light path produces a circular diffraction pattern, which must reduce to a circle at the focus. Whereas a straight edge (conventional vanes) produces a straight spike.

The snag with that beastie however is the hideous obstruction incurred. By all means try it, though I suspect the end result will be worse, not better, than without it.

There is a better solution - a curved vane spider - which achieves same without the obstruction penalty.

Though I know you are well aware of the ultimate solution ...

Last edited by Wavytone; 02-03-2019 at 06:55 PM.
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Old 02-03-2019, 08:41 PM
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Quote:
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...
Though I know you are well aware of the ultimate solution ...
Yes... my C11 (maybe)
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Old 31-03-2019, 09:52 AM
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Maybe some useful here:
https://astronomyhints.braintidbits.com/spider.html

/Lars
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  #11  
Old 31-03-2019, 10:46 AM
glend (Glen)
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Quote:
Originally Posted by bojan View Post
Yes... my C11 (maybe)
I don't understand why people buy SCTs, having owned one. I think the C11 also has a big obstruction problem, among others. Now a refractor would not have those issues, but of course the cost factors become considerable, but that is the price of high resolution without spikes. The middle ground might be a small secondary Mak-Newt, better resolution and no spikes. It is a shame the market is neglecting Mak-Newts.
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Old 31-03-2019, 11:28 AM
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Quote:
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I don't understand why people buy SCTs,...
Money mate.. I am not that rich.
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Old 31-03-2019, 03:17 PM
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Hi,
I like wide field and do not like spikes.

I have a 5" APO refractor. I like that telescope very much. For the money I paid for that telescope I could had bought a 12" RC telescope, then I get a f/8 or with reducer f/6.3 system. The field or image circle about 55 mm. Something you can not use today, maybe in future with medium format sensors. My 5" telescope too have a big image circle, maybe 55 mm, but only 910 mm focallength and a slow f/7.

If I buy a new telescope in future it will be either a 6" APO refractor or a 10" RC telescope. But I live in a city with high light pollution so I use my not so heavy Star Adventurer and camera lenses more, that equipment I easy can take with me out to darker places.

Small telescope without spikes or big telescope with spikes. I think Mak-Newt normally have a limited image circle and can get uggly reflections in the correction lens.

I'm not made of money and I always buy used equipment.

This was out of topic and I stop here.

Lars
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  #14  
Old 01-04-2019, 04:22 PM
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I used a six inch Meade for years where the secondary mirror sat on a glass disk and the had magical properties to correct the image.

I wonder say one stuck in a disk off glass and hang the seconary as with the meade...and if you do need a corrector plate where do you get that sort of glass..but I do wonder the effect on the image...I expect there are folk doing it if I look...theres always someone doing something like you think of doing☺
Alex
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Old 01-04-2019, 04:30 PM
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I did not think more about this but looking at posts all over about the idea ...someone said just add a long dew tube ... did he mean just get out there or does a dew tube reduce spikes?
Alex
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Old 01-04-2019, 04:34 PM
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If you did put in a glass disk of high quality and with a dew tube the internal reflection in the glass window would be reduced you could think.
There must be high quality glass so I wonder.
Alex
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  #17  
Old 01-04-2019, 07:54 PM
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Of course you guys know that there are some serious refractor users out there who use Photoshop plug-ins to add in diffraction spikes.....
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  #18  
Old 01-04-2019, 09:50 PM
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Quote:
Originally Posted by xelasnave View Post
If you did put in a glass disk of high quality ... Alex
Alex you're quite right that would eliminate the spikes of course but a disk of plane parallel optical glass to quarter-wavelength wavefront error is not cheap in the size needed for a reflector. Strain, bubbles, ripples, curvature and wedge (non-parallelism) just to name a few defects in plate glass that just don’t work well in a scope.

I did see one 8" newtonian made this way once, in the mid 1980's - the owner asked me to look at it, unsure if it was a schmidt-newtonian or just a newtonian with flat disk (it was the latter, and the glass was uncoated). Optically the scope was OK and it looked like a DIY affair made by someone competent with a machine-shop, though I have no idea who made it.

Last edited by Wavytone; 01-04-2019 at 10:33 PM.
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  #19  
Old 07-04-2019, 07:18 PM
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Tried this weekend from Mt Pleasant visually on 10"Newt at Sirius B- it works, but it did not help to detect B-component... at the end, I glimpsed it but only when jetsteeam calmed down for couple of minutes around 22:00..
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  #20  
Old 08-04-2019, 04:46 AM
sutekh (M)
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No one has yet mentioned another way to eliminate diffraction spikes:
the Pfund telescope

https://www.cloudynights.com/topic/4...nd-telescopes/

sometimes also called the periscope telescope:

http://www.designwizardry.com/telescopes/index.html

--sutekh
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