stopping down camera lenses on CCD

[Stardust]

I recently aquired an Atik 314L+  My intention is to try some wider views in HA of the main scope target.

I have tried with lenses and want to loose the 8 point star spikes. I realise the best way is with a front aperture mask.

I can see when on the dslr what f4 looks like, but what size should the front mask be? I first thought it should be the same size looking down the lens as the aperture blades. I tried with step down rings but 37mm diameter was way too small to be of any use, so I realised that wasn't going to work.

Here is the set up so far, I made the plate at the weekend and can remote focus the geared motor. 

[Psychobilly]

Nice setup

50mm for f4...

I had a set of different diameter laser cut that would fit in an old filter holder...

Have you tried thirds point focusing - I could pretty much use my 200/2.8 wide open using that method and that was on an APS-C sized sensor...

Peter...

[Stardust]

ok thanks, discs in a filter holder sounds good, I'll be using the 200/2.8   100/2.8 macro    50/1.4

What is thirds point focusing ?

cheers

David

[Psychobilly]

Instead of focusing on a star in the centre of the FOV place it at the any one of the "imaginary"  intersections of lines 1/3 of the way in and parallel from the edges of the frame... this can often result in "optimal" focus across the whole FOV..

[Stardust]

ok . I'll try it wide as that's one of the reasons for using a fast lens, but I have read it performs much better at f4. I can use the rings to step down to 50mm then up again to fit  a large old Hoya rubber lens hood. There'll also be a dew band in place. Ready to go but don't think it's going to happen this week looking at the forecast.

[Psychobilly]

My set went from f3.0 to 5.0 in 0.2 steps they were laser cut from 0.8mm stainless steel and blackened...  For "web sized" images I found wide open was OK with 1/3rds focusing but around f3.2-3.4 was better for larger size images...

You can also make corrections in the PP...

Peter...

[Alien 13]

I have bought a few very cheap filters for use with aperature masks but you can use step down rings if you dont want the hassle of making one.

+1 for thirds focussig.

Alan

[Stardust]

That's all very helpful thank you. The next sizes I can get with these rings is 52 & 55mm which one of those would be nearer f3.2/3.4. The more of the front element exposed the better.

[Psychobilly]

Use 200/clear aperture to give the Focal Ratio...

[Alien 13]

55mm would give you F3.6 its easily worked out by Aperature size = FL (200mm) / F ratio or F ratio is FL/Aperature size.

Alan

[Stardust]

brilliant, I've learnt a few things here, thanks guys

[Ewan]

Excellent setup there Dave, pretty constructive hands you have mate.

Looking forward to your first captures with that setup.

ATB

Ewan

[Stardust]

Excellent setup there Dave, pretty constructive hands you have mate.

Looking forward to your first captures with that setup.

ATB

Ewan

Thanks Ewan, think it's been cloudy since I bought your camera, itching to give it a spin out under some clear skies. I'll let you know when I have something

cheers

David