Field stop

[globular]

Hi all,

If a 2" eyepiece has a field stop of less than 30mm then can it be used with 1.25" filters (suitably mounted in the light path) without any issues across the entire fov?

Thanks.

[vlaiv]

56 minutes ago, globular said:

Hi all,

If a 2" eyepiece has a field stop of less than 30mm then can it be used with 1.25" filters (suitably mounted in the light path) without any issues across the entire fov?

Thanks.

Not sure about that.

First, 1.25" is 31.75mm but filters often have clear aperture of only 27-28mm (mounting ring). Filter thread for 1.25" filters is 28mm if I'm not mistaken.

Second, light rays coming in to a field stop (assuming field stop is in focus plane) are coming at an angle - faster the scope shallower the angle of incoming light. This means that there will be some sort of vignetting at the edges even if 28mm field stop is used.

Third - amount of vignetting depends on where filter is placed - further away from the field stop - more vignetting will be caused. It is best to mount it as close to field stop as possible for the least vignetting.

Maybe option to use filter after eyepiece is actually better. What size is eye lens of the eyepiece? If it is small enough - place filter in rubber eye cup if you have enough eye relief. This trick is often used when wanting a quick check with / without filter - like when hunting for planetary nebulae or similar - just put filter on top of eyepiece. Do be careful not to use it too much (too long time) as you can forget about it and it can fall off easily as it's not held in place with anything but gravity and friction.

 

[globular]

Thanks @vlaiv. So I can't push it too close 28mm but should be fine below 20mm or so, say?

I want to get some coloured filters for planetary use - which also tend to be at higher magnification and hence narrower fields stops (15mm and less in my scope).  
So I figured the extra real estate on 2" ones will never be used and I can save a few £ by getting 1.25" ones.
(Baader ones are £52 each for 2" and £36 for 1.25" - so it's like 3 for 2 if I go 1.25").

(Note I have a very nice DGM NPB filter which is 2" - and it needs to be because it's used on DSOs at medium to low power on EPs with field stops right up to 46.5 (Pentax XW40)!)

[vlaiv]

Yes, get 1.25" filters.

You are right about field stop - but then, question arises - what sort of eyepieces you'll be using for high power views? If eyepiece has small focal length, like 11mm or less, even as wide field - such EPs are almost always 1.25" eyepieces. In that case 1.25" filters will be proper match for them anyway.

 

[globular]

You're right, of course.  I was thinking about the ES100 range - which are all 2" even the 5.5mm.  I've not yet built up my full range of EPs but want to try and make sure any purchase I do make (e.g. these colour filters) last me wherever my EPs purchases take me.

[Louis D]

Are you thinking using a 2" to 1.25" Filter Adapter?  If so, let us know how well it works for you.  I haven't read any actual reports on their usefulness.

[globular]

I'm actually thinking of a filter slider... so I'd slide in 2" or 1.25" filters (or no filter) as required.  This is independent of the EP barrel size (other than making sure the field stop of a 2" EP is < 20mm if I'm using a 1.25" filter in the slider) so I wouldn't use an adaptor like the above as such.

It amounts to basically the same thing though - except it happens a bit earlier in the light path - so I'll post back with results once I've tried it.

[Louis D]

On 15/09/2020 at 13:55, globular said:

I'm actually thinking of a filter slider... so I'd slide in 2" or 1.25" filters (or no filter) as required.  This is independent of the EP barrel size (other than making sure the field stop of a 2" EP is < 20mm if I'm using a 1.25" filter in the slider) so I wouldn't use an adaptor like the above as such.

It amounts to basically the same thing though - except it happens a bit earlier in the light path - so I'll post back with results once I've tried it.

It will vignette the light path more as you would expect.  Most Dob filter slides are 2" for this reason.