Aperture rings to vary the focal ratio of a Skyliner 250px

[Avocette]

Wanting to get to the bottom of an observing issue with my 250px Dob, I was looking for a way of simulating the performance of the Skyliner 200p and 150p Dobs by masking the 250px mirror. 

Firstly I removed the primary mirror and made some careful measurements. The basic glass cylinder is 258mm diameter. The glass top edge has a small amount of chamfered grinding all the way round which in my scope example leaves the mirror silvering itself measuring 255mm across one direction and 256mm at right angles. I decided that I wanted to mask the mirror to make it circular and chose a 250mm diameter circle (f4.8) rather than 254mm as quoted in the sales literature, thinking that the last couple of mm might be the least well figured. I was going to place this mask on the mirror itself but I came up with the alternative of placing it just outside of the secondary mirror mounting struts. This has the disadvantage that it assumes that the incoming light to the primary mirror is exactly parallel to the tube so that the mask shadow is well aligned with the primary mirror. However it has the major advantage of allowing easy and flexible access to the masks during experimentation.

The rings are made from 3mm foamex sheet which is strong and light but can be cut with a balsa modelling knife. I used cream coloured sheet. I have some black foamex sheet, and I thought of flocking the rings but decided that this was not necessary for the early testing.

So here are a series of photos.

The first shows the initial ring which is a snug fit in the tube between the secondary mounting bolts and the tube end casting. The further images show additional rings defining 230mm, 200mm and 150mm apertures, and their accompanying masking paper rings to ensure that no light passes through the gaps between the rings.

I had the opportunity for first light last night and initial tests with quite an interesting couple of hours looking hard at the Moon, Jupiter and its moons, and in Orion, Betelgeuse and the Great Nebula trapezium. This included comparing the use of an ND0.9 filter when looking at the Moon with the use of aperture rings to 'stop down' from 250mm to 200mm and 150mm. Of course the secondary mirror stayed the same size so, if you like, the the progression of focal ratios was f4.8 to f6+ to f8++ .

I need more time to gather evidence but first impressions are that for some brighter objects aperture rings could be a useful weapon in the toolkit.

[Alien 13]

Aperature rings are very useful i use them on my scope if im doing daytime nature shots, one of the big benifits might be that your eyepieces have an easier life with the slower F ratio.

Alan

[Laurie61]

Interesting experiment. As the light from an astronomical target is at infinity stopping at the tube front, instead of at the mirror, makes almost no difference. As the secondary is sized for the 10" scope it will be larger than one used in a 8" f6 and this would have some affect on image contrast in a direct comparison. Once the aperture stop has reduced to a small enough size using an off axis mask becomes possible creating an unobstructed scope.   

[Avocette]

Interesting experiment. As the light from an astronomical target is at infinity stopping at the tube front, instead of at the mirror, makes almost no difference. 

That's what I was hoping.

As the secondary is sized for the 10" scope it will be larger than one used in a 8" f6 and this would have some affect on image contrast in a direct comparison.  

What I found so far was actually that the sky background went darker with each smaller aperture ring - I'm still trying to reconcile this because it seemed that the contrast with the foreground stars/nebula etc was better!

Once the aperture stop has reduced to a small enough size using an off axis mask becomes possible creating an unobstructed scope. 

True - but then I'd start to be concerned about loss of resolution.

My next project is a Bahtinov Mask which can simply drop onto the first ring... 

[Rustysplit]

Loss of resolution with an off axis mask is one by product. The biggest advantage is the refractor like images due to the lack of diffraction spikes.The contrast is also superb. Also the reduced aperture can improve the view on nights of bad seeing. The

[michaeldurban]

very interesting this thread..

Would you propose (as the first 3-4 mm around the diameter of the

primary mirror are 'subjected/compromised') to put a small circular

mask around the top opening of the scope to improve contrast ?

I have a 254 flextube...

[michaeldurban]

ofcourse...

you're percentage obstruction increases..

[ollypenrice]

What about an off axis mask with two holes in it, one at each side? A small interferometer? This should give some of the benefits of aperture for resolution while losing the obstruction and the excessive light on bright targets. Someone wiser can doubtless shed light on this... 

Olly

[Avocette]

What about an off axis mask with two holes in it, one at each side? A small interferometer? This should give some of the benefits of aperture for resolution while losing the obstruction and the excessive light on bright targets. Someone wiser can doubtless shed light on this... 

Olly

Quite a thought that! Why not four holes or three holes avoiding the quadrant where the focus tube extends/protrudes.  I've got plenty of black foamex ready for when I start to make my short 'production' run. I'm open to any useful suggestions, especially if the optical experts can persuade me of the scientific arguments! 

[Avocette]

very interesting this thread..

Would you propose (as the first 3-4 mm around the diameter of the

primary mirror are 'subjected/compromised') to put a small circular

mask around the top opening of the scope to improve contrast ?

I have a 254 flextube...

Reviews of SW telescopes often suggest that the paraboloid 'mirror is slightly overcorrected'. I'm wondering if any of our SGL mirror grinding experts concur with my naive intuition that the outer rim of a Sky-Watcher manufactured primary mirror is likely to be the least well figured? 

[RAC]

With one two or four offset holes you now just have very thick curved spider vanes.

[michaeldurban]

i am intrigued enough to start experimentation..

will post my progress later..

thanks for all the tips guys...

[Moonshane]

I have been bestowing the virtues of off axis masks for larger scopes for ages

they really work.

http://stargazerslounge.com/topic/143780-aperture-mask-for-16-f4-dobsonian/

I even used a 60mm one on my 6" f11 the other day, creating a 60mm f27 on the sun. it made no difference to detail visible and helped the stability too.

[Avocette]

I've made one offset 9cm aperture mask for Moon observing (for the Sun I use the SW cap aperture with Baader Solar Film).

But just learned that an aperture mask with two or three circular holes is a Hartmann mask and can be used for focus adjustment as an alternative to, or in combination with Bahtinov (striped) mask. 

[John]

One question that sticks in my mind about using off axis mask relates to coma. All newtonians produce coma and it increases as you move away from the optical axis of the primary. With an off axis mask, you are gathering all your light from away from the optical axis (by definition) and it will be, presumably, more comatic. Would this affect the quality of the resulting image ?

[Moonshane]

maybe in principle John but I'll hopefully convince with my 16" at SGL9. In catchphrase style I can only 'say what I see'.

[John]

maybe in principle John but I'll hopefully convince with my 16" at SGL9. In catchphrase style I can only 'say what I see'.

I don't need to be convinced that it works Shane as I've tried it myself. A 4" off aperture mask used with my 12" dob gave me the same sort of performance that my Vixen ED102 refractor gives so I've seen if for myself 

I was just wondering about the optical principles thats all. 

PS: I'd still like to look through your 16" dob though, please 

[Moonshane]

LOL of course! I start to 'edge towards the cliff of ignorance' when it comes to optical theory

[John]

LOL of course! I start to 'edge towards the cliff of ignorance' when it comes to optical theory

You will be able to see me clinging to the edge of it by my finger tips then ! 

[John]

I've made one offset 9cm aperture mask for Moon observing (for the Sun I use the SW cap aperture with Baader Solar Film).

But just learned that an aperture mask with two or three circular holes is a Hartmann mask and can be used for focus adjustment as an alternative to, or in combination with Bahtinov (striped) mask. 

Interesting article on a multiple aperture mask here:

http://www.cloudynights.com/item.php?item_id=548

[ollypenrice]

With one two or four offset holes you now just have very thick curved spider vanes.

 By Gad, I thnk you have a point!

Olly

[Naemeth]

One question that sticks in my mind about using off axis mask relates to coma. All newtonians produce coma and it increases as you move away from the optical axis of the primary. With an off axis mask, you are gathering all your light from away from the optical axis (by definition) and it will be, presumably, more comatic. Would this affect the quality of the resulting image ?

You bring up an interesting point - perhaps this effect is mitigated somewhat by the increase in effective focal ratio? I'm not sure myself .

[Ags]

One question that sticks in my mind about using off axis mask relates to coma. All newtonians produce coma and it increases as you move away from the optical axis of the primary. With an off axis mask, you are gathering all your light from away from the optical axis (by definition) and it will be, presumably, more comatic. Would this affect the quality of the resulting image ?

No... The whole mirror collects light for the on-axis target, not just the middle of the mirror collecting for the targets in the middle :-) . Coma applies to light that is coming off-axis (i.e. not parallel to the mirror axis). So you should not see an increase in coma with an aperture mask.

[John]

No... The whole mirror collects light for the on-axis target, not just the middle of the mirror collecting for the targets in the middle :-) . Coma applies to light that is coming off-axis (i.e. not parallel to the mirror axis). So you should not see an increase in coma with an aperture mask.

Yep, that sounds logical ! 

I guess this covers it but I have to confess it's over my head 

http://www.telescope-optics.net/tilted3.htm

I've read elsewhere that the off axis mask results in a slightly tilted optical axis so, ideally, one should tilt the eyepiece / focuser axis to match. I think the above touches on this too.

In yet another article it suggests re-collimation when using an off axis mask because of the tilted optical axis but thats going too far I guess