Finally a better spider...

[Dave_D]

Finally got round to making a new spider for my 10" newt. the original orion optics spider (30ish years old i think) was horrible... the opposing vanes were offset from eachother by a couple of millimeters, were badly warped when looking down etc. hopefully, this will get rid of the fan-shaped diffraction spikes i was getting.

Feels a lot more solid now as i used 2mm aluminium angle to fix the vanes to the centre piece as the original was just a 90 degree bend in the vanes which wasn't too stable, even considering the original vanes were 1.5mm thick and these are less than 0.9mm

Just need to narrow the vane ends and give it a black paint job...

Now just need to clear the back garden and build an obsy 🤣

Edited May 9, 2022 by Dave_D

[Stu]

Looks good! I need to do something similar with my 12”. What did you use for the vanes themselves?

[Dan_Paris]

Very nice job ! Regarding the vanes several experts recommend mirror-polished vanes over blackened ones (lower emissivity).

Edited May 9, 2022 by Dan_Paris

[Dave_D]

2 hours ago, Stu said:

Looks good! I need to do something similar with my 12”. What did you use for the vanes themselves?

brushed stainless steel... got them here https://www.ebay.co.uk/itm/164917219621?ssPageName=STRK%3AMEBIDX%3AIT&var=464436248323&_trksid=p2060353.m2749.l2649

[Chriske]

On 09/05/2022 at 19:21, Stu said:

Looks good! I need to do something similar with my 12”. What did you use for the vanes themselves?

You could consider curved spiders.

[Stu]

5 hours ago, Chriske said:

You could consider curved spiders.

I often consider doing this Chris. I get concerned over how you centre the secondary (how would you adjust this, or is it just about getting everything right in terms of dimensions etc so it is centred without adjustment?)

[Space Hopper]

It look very smart. Nice work.

 

[Chriske]

1 hour ago, Stu said:

I often consider doing this Chris. I get concerned over how you centre the secondary (how would you adjust this, or is it just about getting everything right in terms of dimensions etc so it is centred without adjustment?)

Well first of all whether you would go for curved vanes depends. Installing classical vanes you end up with spikes in the field of view around bright stars. Diffraction you know. 4 spiders -->>four spikes in case of three spiders you end up with six spikes around stars.
To get rid of these spikes you need to install curved spiders.
To be completely correct. The diffraction of these curved spiders is not gone at all. It's diffraction is spread evenly all over the field of view, invisible of course.
You need to bend these spiders correctly to achieve the correct result. Many curved spiders out there have the wrong curve, even commercial spiders.

There's also another way to get rid of these spikes. The very old ATM-I book of Albert Ingalls also deals with this issue. He does mention a strange pattern is glued on top of the spiders. There's even a drawing of these patterns in his book. Never tried it.

On the other hand many amateurs do love these spikes around bright stars, so up to you...😉

[Louis D]

22 minutes ago, Chriske said:

It's diffraction is spread evenly all over the field of view, invisible of course.

Not completely invisible.  The diffuse diffraction adds a bit of glow to the entire field, lowering contrast a bit.  I can't recall any reports of side-by-side testing of otherwise identical scopes except for spider type on low contrast objects to actually quantify the difference, however.

[Stu]

1 hour ago, Chriske said:

Well first of all whether you would go for curved vanes depends. Installing classical vanes you end up with spikes in the field of view around bright stars. Diffraction you know. 4 spiders -->>four spikes in case of three spiders you end up with six spikes around stars.
To get rid of these spikes you need to install curved spiders.
To be completely correct. The diffraction of these curved spiders is not gone at all. It's diffraction is spread evenly all over the field of view, invisible of course.
You need to bend these spiders correctly to achieve the correct result. Many curved spiders out there have the wrong curve, even commercial spiders.

There's also another way to get rid of these spikes. The very old ATM-I book of Albert Ingalls also deals with this issue. He does mention a strange pattern is glued on top of the spiders. There's even a drawing of these patterns in his book. Never tried it.

On the other hand many amateurs do love these spikes around bright stars, so up to you...😉

Thanks Chris. I have a decent understanding of the theory, it’s the practical bit I struggle with! 🤪

[Chriske]

Some tests have been performed if I recall correctly at CN forum years ago.
These test were done side by side comparing two different spiders.
There was no mentioning of loss of contrast at all.
In our workshop we did the same test only a few years ago. During telescopemaking course exactly same telescope different spiders. No loss of contrast, Maybe there was some but not visible. To be clear all telescopes are identical. I asked course(I always do) members to switched mirrors every 10 minutes or so. They end up with exactly the same mirror/scope. So that is why we could compare these different spiders without any doubt  at all.

[fwm891]

You could always twist the vane 180 degrees along its length which will give you curved edges. OK for narrow ish vanes…

[Dave_D]

I literally threw my scope together tonight with a set of 'Masque Aigrettes' on the spider vanes I just printed as I finished the new spider... Hardly collimated, very rough polar align, focused by eye on my laptop screen, half the lights on in the house and a fair bit of high cloud but yeah, they really work. just 20 second totally unprocessed pics of Vega, with and without the masks.

Can't wait for my back garden to be done so i can build my obsy... been away from imaging for far too long.

Edited May 28, 2022 by Dave_D

[Dave_D]

On 09/05/2022 at 18:21, Stu said:

Looks good! I need to do something similar with my 12”. What did you use for the vanes themselves?

430 Grade Brushed Stainless Steel FLAT BAR Metal Strip Trim - 0.9mm off ebay

[skybadger]

On 20/05/2022 at 18:41, Chriske said:

Some tests have been performed if I recall correctly at CN forum years ago.
These test were done side by side comparing two different spiders.
There was no mentioning of loss of contrast at all.
In our workshop we did the same test only a few years ago. During telescopemaking course exactly same telescope different spiders. No loss of contrast, Maybe there was some but not visible. To be clear all telescopes are identical. I asked course(I always do) members to switched mirrors every 10 minutes or so. They end up with exactly the same mirror/scope. So that is why we could compare these different spiders without any doubt  at all.

There's a much deeper investigation on CN that compares a while range of spider types including  different curves and numbers of arms. Also Suiter takes look at this too. 

I used steel belting on my first spider on the 12" and then have built a wire spider for my next. The steel works fine under tension. That one is 0.5mm thick and free.

However I have learnt that it doesn't matter if the limbs are offset from each other,  they just need to be in the same alignment to prevent extra spikes being introduced. 

[skybadger]

Some references:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.researchgate.net/publication/47718414_Diffraction_effects_of_telescope_secondary_mirror_spiders_on_various_image-quality_criteria&ved=2ahUKEwjO262O6JL8AhUVg1wKHRMWCuMQFnoECBIQAQ&usg=AOvVaw3dTg6XO6Ke3bmkisGpvRLZ

https://www.cloudynights.com/topic/495707-spider-and-secondary-diffraction-what-to-do-what-to-avoid/

Apodising masks:

https://www.cloudynights.com/RMF51116.pdf

More CN:

https://www.cloudynights.com/topic/491137-how-to-minimize-spider-vane-spikes/page-3#entry6513473

 

Hth