Diffraction spikes

[Moonshane]

hi all. I have just realised that my 6" f11 newt creates much more obvious diffraction spikes on Jupiter than my 12" f4. I have just ordered a three vane curved spider for the former to combat this as I have been after one for ages but is the reason for this effect / difference the vane:surface area of mirror ratio?  anyone know please?

[Laurie61]

The surface area ratio sounds reasonable, the thought that pooped into my mind was a possible contrast  issue. The F 11 scope has a small secondary and a more contrasty image, the f4 scope a bigger secondary and less contrast. Maybe the higher contrast image is emphasizing the diffraction spikes a bit more. ?  

[Peter Drew]

Shane. I don't know the answer to the spider diffraction difference although your reasoning seems sound. As far as spider design is concerned I do have some experience. During my time with Astro Systems we experimented with pretty much all forms of configuration, particularly with regard to a design for our best selling FG1 6" F6 Newtonian. The winner for planetary performance was a single radius strut. The curved vane spiders we tried gave minimal visual diffraction effects on stars but did appear to lower the contrast of planetary detail by "smearing" the diffraction across the field. The choice boiled down to good stars or good planetary detail.    

[Moonshane]

cheers both. thanks for the comments on the spiders Peter. as it happens the 6" f11 is what I consider my main binary scope so that might help me. it does give good planetary images too of course and will be interesting to see the difference when fitted.

[John]

I've got this from the Protostar website - you may have already seen it:

"Total diffraction from the secondary support vanes is a function of the edge-on area of the vanes divided by the area of the mirror. This value is called the obstruction ratio. The appearance and intensity of diffraction artifacts (like spikes) is a function of the obstruction ratio alone, and not the thickness of the vanes by themselves...."

[Moonshane]

cheers John

That might be where the thought in my mind came from - hidden in the depths somewhere!

[Moonshane]

Shane. I don't know the answer to the spider diffraction difference although your reasoning seems sound. As far as spider design is concerned I do have some experience. During my time with Astro Systems we experimented with pretty much all forms of configuration, particularly with regard to a design for our best selling FG1 6" F6 Newtonian. The winner for planetary performance was a single radius strut. The curved vane spiders we tried gave minimal visual diffraction effects on stars but did appear to lower the contrast of planetary detail by "smearing" the diffraction across the field. The choice boiled down to good stars or good planetary detail.  

hi Peter

do you think the reduction in contrast of planetary detail was actual or 'apparent'? By this I mean that I thought the smearing (for e.g. three curved vanes) was a result of the vanes' diffraction being spread across the space in between the vanes with the planetary disc unaffected?

either way I am looking forward to seeing the results.

[AlexB67]

Moonshane.  You may be interested in this paper I came across some time ago, just for my own edification. It has quite  few diagrams in it discussing different arrangements and shapes  of spider vanes where you can see how the diffraction patterns are affected. 

http://adsabs.harvard.edu/abs/1959AJ.....64..455E

I found it to be very informative.

Straight to the pdf

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1959AJ.....64..455E&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

if I were a scope builder ( which I am not ),  I'd be very tempted in the arrangement of using ultra thin wires as shown in the last figure 

[John]

In the past a couple of manufacturers offered the option of mounting the secondary on an optical glass window so dispensing with support vanes all together. I think this would affect the cool down time of the scope and dewing could be an issue as well if not addressed though a deep dew shield and / or heated dew band.

[AlexB67]

Levitating mirrors, control the magnetic field to set the tilt and collimate that way at the touch of a few buttons, no bob knobs, no screw drivers, no more diffraction spikes. Now that's Crazy talk, though possible in theory on a  scope  perhaps   . This technique has already been used to control mirrors in this way in some research level experimental setups    

[Caldwell14]

Have you considered a refractor  

[Moonshane]

LOL. who sir? me sir? NO sir!

[Moonshane]

then again, show me a 6" f11 frac that I can mount on a dob mount, weighing about 10kg and costing less than £200 and I might consider it

[John]

then again, show me a 6" f11 frac that I can mount on a dob mount, weighing about 10kg and costing less than £200 and I might consider it

I've got a 6" F/12 frac but it won't meet your other criteria   

Off axis newts might be of interest to you though. There have been one or two commercial ones but they did not seem to take off. Might be fun to get a 6" F/8 mirror set and experiment though 

[nightfisher]

I must admit to being out of my depth on this spider vane issue, other than having used my 150 f5 newt on Jupiter a couple of times i find the diffraction spikes on a planet not to my liking, and a strange sight indeed  

[John]

I must admit to being out of my depth on this spider vane issue, other than having used my 150 f5 newt on Jupiter a couple of times i find the diffraction spikes on a planet not to my liking, and a strange sight indeed  

The irony is that imagers are buying software to add the spikes in !: 

http://www.prodigitalsoftware.com/StarSpikesPro3_News.html

[bambuko]

well... it takes all kinds

[Stu]

Just researching curved vane spiders a little, with the thought of fitting one to my newt.

Shane, did you ever fit the 3 vane one to your 6"?

Stu

[Brian Nordstrom]

I don't like diffraction spikes at all , I dislike them so much I sold a Takahashi Mewlon 210 and now have a Celestron C9.25 and find the views much more pleasing in the C9.25 . To me of course .

I have built a few Newtonion's in my time and made curved spiders on a 5 inch f13 planetary and my mighty 10inch f10 and don't see any diffraction spikes on either , I used 15mm wide 12 inch stainless steel ruler's the type that open out to 24 inches long as these are light and strong .

I will post a photo of the 5 inch secondary set up if I can find one .

Here is the 5 inch , just gotta find the spider set up photo .....

Brian.

[Stu]

Thanks for the info Brian, interesting. I had been considering making one, but not sure I could get the rigidity without a lot of experimenting on a 12"?

Stu

[Mike73]

Just curious guys...What objects do you see diffraction spikes on?

[Brian Nordstrom]

The brightes stars , Jupiter , Venus  it is very distracting and blurs  tight doubles and Murphies Law dictates that the fainter secondary star you are trying to find beside the bright primary is "  In line with the diffraction spike"

Brian.

[Mike73]

Ahhh ok thanks Brian.

[faulksy]

i only seem to see them on bright stars, and i quite like them. no problems on planets

[Moonshane]

Just researching curved vane spiders a little, with the thought of fitting one to my newt.

Shane, did you ever fit the 3 vane one to your 6"?

Stu

hi Stu

I did actually. it really dies provide superb views on Jupiter, the moon and sun. I have not reallt tested it yet on double stars but expect it to be just as good as before if not better.   not sure about rigidity on  12" secondary but should be ok.