Diffraction spikes in newts

[Flame Nebula]

I was wondering what people's experiences are with diffraction spikes in Newts? Specifically, I read stories of these being less in some scopes than others. In one case, replacing the mirror with a premium one improved stars to sharp points! Refractor- like. 

Any thoughts on spikes, and minimising them? 

Thanks 

Mark 

[Tomatobro]

I believe that a curved secondary mirror support will remove the spikes. Changing the mirror itself will have no effect.

[Louis D]

It depends on the thickness of the spider vanes and their number.  Curved vanes distribute the spike around the entire field of view, slightly decreasing contrast by increasing background brightness.

The main time diffraction spikes can be a pain is if you're trying to find a faint companion to a bright star, and it happens to be right behind a diffraction spike.  If the tube is rotatable, you can reorient the spikes to possibly reveal the faint companion by putting it between spikes.

I would take diffraction spikes in a fast Newt any day over the unfocused light at either end of the spectrum in a fast achromatic refractor of similar aperture.

[Flame Nebula]

30 minutes ago, Tomatobro said:

I believe that a curved secondary mirror support will remove the spikes. Changing the mirror itself will have no effect.

Hi Tomatobro, 

Do you know if this be  available in any commercially available scopes? 

[Stu]

As said, changing the mirror won’t change the diffraction spikes, they are an artefact of the secondary support vanes.

Curved vanes need to go through at least 180 degrees in order to cancel out the diffraction and then it doesn’t get rid of it, it just smears it around the target so reduces contrast a little rather than having it concentrated in the spikes.

I’ve used a couple of scopes (including a 8” Portaball with a Zambuto premium mirror in it) with curved vanes and wasn’t totally convinced by the benefits. Plenty of info on the web about it, including some here:

http://www.astrosurf.com/viladrich/astro/instrument/sensitivity/spider-diffraction.htm

[Stu]

6 minutes ago, Flame Nebula said:

Hi Tomatobro, 

Do you know if this be  available in any commercially available scopes? 

I don’t think anyone sells them as standard now. OO used to, and Portaballs aren’t made anymore as far as I know. Protostar offer upgrade curved vanes:

https://www.fpi-protostar.com/crvmnts.htm

[Ratlet]

I think like a lot of things in astronomy they either bug you or you don't really notice them.  Generally I don't notice them, but I really started with a newt and moved onto a dob.  Just started with a refractor so check back with me in a year and see how I feel (assuming the clouds part at some point).

To be clear, they are there, but i don't find they interfere (I've not had one cover a double star yet).

I also grew up with Hubble and for me DSO images look more authentic with the spikes.  It wouldn't surprise me if there is a trend in a decade or so for spiders making the JWST diffraction pattern.

You can't get rid of them but you can make them worse by not having the spider aligned.

Stu beat me to some of the links.  More discussion here:

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

[Mr Spock]

I don't notice them on planets with the 12" as I use a polariser to cut down the brightness. You can see them without the PL.

I can't say they interfere with stars either.

They are one of those things you either don't notice or they get on your nerves 

[Flame Nebula]

27 minutes ago, Stu said:

As said, changing the mirror won’t change the diffraction spikes, they are an artefact of the secondary support vanes.

Curved vanes need to go through at least 180 degrees in order to cancel out the diffraction and then it doesn’t get rid of it, it just smears it around the target so reduces contrast a little rather than having it concentrated in the spikes.

I’ve used a couple of scopes (including a 8” Portaball with a Zambuto premium mirror in it) with curved vanes and wasn’t totally convinced by the benefits. Plenty of info on the web about it, including some here:

http://www.astrosurf.com/viladrich/astro/instrument/sensitivity/spider-diffraction.htm

Thanks Stu, 

So from reading the link, it seems that the best way to keep diffraction spikes to a minimum is to go for low %obstruction of secondary(approx 20%) in tandem with the thinnest possible vanes. Would that be correct? 

[John]

My OO 12" F/5.3 had these secondary vanes:

I didn't see diffraction spikes with this scope. The diffraction was still there but spread evenly across the image and, to my eye, did not degrade it. The CO in this case was 21% the diameter of the aperture.

Personally I was very happy with this arrangement. Orion Optics stopped offering it many years ago though so I guess uptake was low amongst customers. I didn't specify this - the scope had 2 owners before me.

The scope showed excellent planetary and lunar details, conditions allowing, and was the first scope that I managed to split Sirius with. 

Martin Mobberley went for a similar approach with his OO 10 inch F/6.3:

At some point in the future I would consider a scope such as the above with OO's top quality mirror in it. I'd try and find one pre-owned though, to avoid having to deal with OO 🙄

Other folks have different opinions of course - I can only relate my experience 🙂

Edited March 9 by John

[Flame Nebula]

3 minutes ago, John said:

My OO 12" F/5.3 had these secondary vanes:

I didn't see diffraction spikes with this scope. The diffraction was still there but spread evenly across the image and, to my eye, did not degrade it. The CO in this case was 21% the diameter of the aperture.

Personally I was very happy with this arrangement. Orion Optics stopped offering it many years ago though so I guess uptake was low amongst customers. I didn't specify this - the scope had 2 owners before me.

The scope showed excellent planetary and lunar details, conditions allowing, and was the first scope that I managed to split Sirius with. 

Martin Mobberly went for a similar approach with his OO 10 inch F/6.3:

At some point in the future I would consider a scope such as the above with OO's top quality mirror in it. I'd try and find one pre-owned though, to avoid having to deal with OO 🙄

Other folks have different opinions of course - I can only relate my experience 🙂

Hi John, 

I'm surprised more people weren't interested in that design - seems perfect, and as you said, it may have made the difference between splitting sirius or not. That's the kind of design I'd go for, if I went for a large newt. 

[John]

13 minutes ago, Flame Nebula said:

Hi John, 

I'm surprised more people weren't interested in that design - seems perfect, and as you said, it may have made the difference between splitting sirius or not. That's the kind of design I'd go for, if I went for a large newt. 

Well, many are not convinced by it and that's fine as well of course. I have owned newtonians with conventional secondary support vanes and been happy with those as well, despite the diffraction spikes. Orion Optics obviously stopped doing the curved ones for a reason and I guess lack of demand was a part of that.

There was a US company called Destiny I think - don't know if they are still in business ?:

1800 Destiny On Line Store for Curved Vane Spiders (destinycomp.com)

Another approach is to use an optical glass window to support a minimal size secondary, rather like the meniscus that the  maksutov-newtonian design uses. Now those things really do perform well on the planets, double stars and other high-res targets 🙂

 

Edited March 9 by John

[Flame Nebula]

14 minutes ago, John said:

Well, many are not convinced by it and that's fine as well of course. I have owned newtonians with conventional secondary support vanes and been happy with those as well, despite the diffraction spikes. Orion Optics obviously stopped doing the curved ones for a reason and I guess lack of demand was a part of that.

There was a US company called Destiny I think - don't know if they are still in business ?:

1800 Destiny On Line Store for Curved Vane Spiders (destinycomp.com)

Another approach is to use an optical glass window to support a minimal size secondary, rather like the meniscus that the  maksutov-newtonian design uses. Now those things really do perform well on the planets, double stars and other high-res targets 🙂

 

Thanks John

I'm intrigued by the optical glass window. This sounds interesting!

Further investigation needed. 

Best wishes 

Mark

[Stu]

32 minutes ago, John said:

My OO 12" F/5.3 had these secondary vanes:

I didn't see diffraction spikes with this scope. The diffraction was still there but spread evenly across the image and, to my eye, did not degrade it. The CO in this case was 21% the diameter of the aperture.

Personally I was very happy with this arrangement. Orion Optics stopped offering it many years ago though so I guess uptake was low amongst customers. I didn't specify this - the scope had 2 owners before me.

The scope showed excellent planetary and lunar details, conditions allowing, and was the first scope that I managed to split Sirius with. 

Martin Mobberley went for a similar approach with his OO 10 inch F/6.3:

At some point in the future I would consider a scope such as the above with OO's top quality mirror in it. I'd try and find one pre-owned though, to avoid having to deal with OO 🙄

Other folks have different opinions of course - I can only relate my experience 🙂

I would like to try a spec like that John. The scope I used had three curved vanes, presumably summing to the 180 degrees required to cancel diffraction. ‘Your’ design has two complete 360 turns so whilst it likely has more overall diffraction, I think the cancellation probably works better so you got excellent results.

[Stu]

36 minutes ago, Flame Nebula said:

Thanks Stu, 

So from reading the link, it seems that the best way to keep diffraction spikes to a minimum is to go for low %obstruction of secondary(approx 20%) in tandem with the thinnest possible vanes. Would that be correct? 

Yes, that’s about the size of it. Minimal secondary size (whilst ensuring sufficient illumination of the eyepiece) and nice thin vanes. Some people even use very fine wires instead of vanes to keep the diffraction to a minimum.

[Flame Nebula]

2 minutes ago, Stu said:

Yes, that’s about the size of it. Minimal secondary size (whilst ensuring sufficient illumination of the eyepiece) and nice thin vanes. Some people even use very fine wires instead of vanes to keep the diffraction to a minimum.

 

2 minutes ago, Stu said:

Yes, that’s about the size of it. Minimal secondary size (whilst ensuring sufficient illumination of the eyepiece) and nice thin vanes. Some people even use very fine wires instead of vanes to keep the diffraction to a minimum.

Thanks Stu

Do you know if these scopes are handmade by the owners or bespoke manufactured? 

[Stu]

20 minutes ago, Flame Nebula said:

 

Thanks Stu

Do you know if these scopes are handmade by the owners or bespoke manufactured? 

Pretty sure they will be custom made by their owners, although some specialist builders may do stuff like that, not sure.

[900SL]

Speaking as a Structural engineer, one potential issue with curved spider vanes is the inability to pretension them (unlike blade or wire type straight vanes, which behave like bicycle spokes in some ways)

This may effect stiffness and require thicker plate to reduce deflection etc

Edited March 10 by 900SL

[Stu]

4 minutes ago, 900SL said:

Speaking as a Structural engineer, one potential issue with curved spider vanes is the inability to pretension them (unlike blade or wire type straight vanes, which behave like bicycle spokes in some ways)

This may effect stiffness and require thicker plate to reduce deflection etc

Very true! Centring of the secondary is also trickier than with a traditional tensioned four vane spider.

I actually quite like three vanes as each diffraction spike is less pronounced than with four vanes so is a little less intrusive. Still easy to centre the secondary as they are tensioned vanes still and can be thin.

[mikeDnight]

 Nothing really removes the effects of diffraction, which is caused by anything that crosses the light path. In refractors the diffraction pattern is caused by the edge of the lens cell. In Catadioptric's by the edge of the primary and the edge of the secondary, intensifying the brightness of the first diffraction ring. In scopes using a spider, both sides of the spider vein whether single, double, triple or quadruple create diffraction spikes thet travel right across the field of view. So no matter how thin your spider veins are, both sides of each vein will still cause a diffraction spike across the field of view. Using curved veins doesn't remove the diffraction, just alters the shape so it becomes less obvious. The stronger or more numerous the spikes, the more damaging to definition they become. Better to have four vein spiders than three, as each vein caused a spike across the entire field, so a four vein spider will show four spikes, but a three vein will create six spikes. A single vein or twin vein spider is better in my view, but that's why I like refractors!

Edited March 10 by mikeDnight

[DaveS]

Did someone say "diffraction spikes" lol. This is with an OOUK ODK which has a spider that's machined from the solid. Yes, I can see the spikes aren't quite even. No, I'm not 100% sure of the cause.

That searchlight is actually mag 6.3 I think.

[Tomatobro]

I wonder if the secondary could be mounted on a glass disc on the tube end. Like a catadioptric?

[Louis D]

50 minutes ago, Tomatobro said:

I wonder if the secondary could be mounted on a glass disc on the tube end. Like a catadioptric?

Yes, @John mentioned that possibility above.  However, the glass disc must be plane flat and free of bubbles or other flaws.  It must also be multicoated to avoid a fairly serious loss of light due to reflection.  I would think centering has to be done at the factory as with catadioptrics.  There are hybrid scopes such as Mak-Newts that use spherical curves for everything but the flat secondary to avoid figuring a parabolic primary.  There's also a Schmidt-Newt, but I know very little about what's going on with it.

I would also think that glass secondary mounts would be size limited at a certain point, just like refractor primaries, so probably not the best choice for giant Dobs.

[mikeDnight]

1 hour ago, Tomatobro said:

I wonder if the secondary could be mounted on a glass disc on the tube end. Like a catadioptric?

If it was a true optically flat window it would work well. The old AstroScan reflectors used this method of holding the secondary. Another alternative would be to use a Herschellian design, where the observer looks directly down the tube and there's no need for a secondary. A Brachet reflector design also uses an off axis method but they are long focal length.

[JeremyS]

It’s not just the diffraction spikes you have to worry about.