Aperture mask advice

[MartinB]

I'm increasingly interested in wide field imaging.  I would like to eliminate bright star diffraction spikes created when I stop down my lens a little.  I'm planning to make some aperture masks but wonder if anyone could give me some tips.  Is is best to fit it inside the lens hood, best way to fix in place, what material to use, how to gauge the correct aperture.  I'm totally cack handed!  Any advice much appreciated

[Psychobilly]

I use a cutting compass.

Just  remember to  cut the outside diameter first then the inner... For testing use black card and when you have found a size that works for you cut it out of black plastic folder material...

Mounting -  Size it so it fits in the lens hood or inside the filter threads... Micropore tape  is great for "fixing"

Initially I would base the aperture on 1.5-2 stops from the lenses wide open setting

http://www.ebay.co.uk/bhp/compass-cutter

http://www.ebay.co.uk/itm/A3-Coloured-Polypropylene-Plastic-Sheet-0-5mm-Model-Making-Arts-Crafts-/181920454908?var=481453188568&hash=item2a5b4dd8fc:m:mAg68sXNtWce5BGmPRhih8g

Peter...

 

[MartinB]

Brill, thanks Peter, just what I was needing

[ollypenrice]

This is how I did it, Martin.

The compass cutters come from graphics outlets and I thought it best to get it as close as possible. I held it in with draught excluder self adhesive foam and based the diameter on working out a variety of F ratios, opting for the largest aperture that gave a clean result. I see Peter's already replied but something I also learned from him was the usefulness of focusing on a star at the intersection of the 1/3 lines. This is the optimal position and will let you work with more aperture. If you want to finesse the mask then barbecue and stove paints use pigments which are not reflective in IR, unlike dye based paints. I doubt it matters in this application but mention it just in case.

Have you come across these Telescope Service gadgets? They made all the difference to my lens efforts. There was a set on sale, here I think, quite recently.

 

I only tried the aperture mask on the Canon 200L where it worked perfectly. How it would work on shorter FL lenses I don't know. Instinct tells me it might not be so good but that might be quite wrong.

Olly

[MartinB]

Thanks a lot Olly, I tried to find that focuser gadget once on the TS site but it seemed to have been discontinued, I'll have another look although for wider field I  can generally manage a good focus.  Draft excluder tape sounds like a good idea

[Psychobilly]

50 minutes ago, MartinB said:

Thanks a lot Olly, I tried to find that focuser gadget once on the TS site but it seemed to have been discontinued, I'll have another look although for wider field I  can generally manage a good focus.  Draft excluder tape sounds like a good idea

Here you go Martin...

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3286_Microfocuser-and-focus-lock-fuer-camera-lenses-up-to-D-135mm.html

Links to the other sizes are on that page...

Peter...

 

[MartinB]

Thanks Peter, I'm thinking it won't be necessary for my new Samyang 24mm but the 135mm should be good for my Canon 135.  Might give one a try

[Psychobilly]

I am not sure about front aperture masks on very wide lenses  I have only used them down to 50mm ...  Shorter FL especially those with highly curved front optics I will have to do some digging...

 

Peter..

 

[Psychobilly]

Perhaps MHFW could expand a bit more on the teaser from this post in an earlier thread...

https://stargazerslounge.com/topic/220645-front-aperture-mask-vs-aperture-blades/?do=findComment&comment=2371352

 

Peter...

 

[StuartJPP]

On 17/02/2016 at 12:31, PBS said:

Perhaps MHFW could expand a bit more on the teaser from this post in an earlier thread...

https://stargazerslounge.com/topic/220645-front-aperture-mask-vs-aperture-blades/?do=findComment&comment=2371352

Peter...

I read an old article about lens design a while ago, unfortunately I can't find it again. It definitely said that the aperture blades should be positioned in the correct place in the imaging train. Unfortunately it didn't really elaborate on it, if it did I would have certainly kept the article. Until I know better I will believe that putting a front aperture mask on a lens isn't ideal but it still gets the job done and it is the only real solution for us.

 

I prefer using a step-down ring if your lens is threaded (not my idea), nice and solid and you can still use some screw in filters as well...I use them on my Canon 135mm f/2 L and Canon 200mm f/2 L II lenses.

The focal ratio is obviously the focal length divided by the new aperture...so the Canon 135mm f/2 L, with an actual aperture of 67.5mm (135/2), with the above 72->52mm step down ring becomes 135mm/52mm ~ f/2.6.

 

You have to make sure that you work out the actual aperture and not just the filter adapter size as lenses such as the Canon 100mm f/2.8 L IS lens have a lot of dead space that isn't actual aperture at all...filter size is 67mm but the actual aperture is only (100/2.8) ~36mm...

[StuartJPP]

Here's an excerpt from "Geometric, Physical, and Visual Optics" By Michael P. Keating...(courtesy of Google Books)...