1.25" filter with 2" eyepiece

[Stu]

Bear with me as I explain the convoluted way I got to even ask this question....

Having read all the recent Horsey posts, I was thinking about how I could try my 1.25" Hb filter in the Tak using the kit I currently have. Eyepiece wise I have a 24mm Pan and then a 2" 40mm TMB Paragon. Intention was to be able to try the filter on some of the other Hb targets at my next dark site trip (whenever that may be!)

Obviously the Pan is straightforward but only gives a 3.2mm exit pupil so a 32mm or even 40mm Plossl may be a better bet; 5.4mm for the latter.

Cue a Stu special experiment to fit a 1.25" filter to a 2" eyepiece.....

Well, using a T2 to 1.25" adaptor the wrong way round, screwed into the base of a 2" eyepiece holder, I managed to get this setup so the filter was positioned close to the fieldstop. I'll obviously have to post a picture to show this as I'm sure that explanation is not enough! This obviously creates a smaller field stop and reduces the field of view, as expected, but would still be comparable with using a 40mm plossl.

As I was packing away, I had another thought, and screwed the T2 to 1.25" barrel (and filter) into the 2" barrel of my diagonal (usefully enough a Baader which is threaded for T2). I use an extension tube to reach focus, and was wondering if my experiment would put the 1.25" filter in a position where the light path was narrow enough to avoid vignetting.

Sure enough, a quick test indoors showed the full field stop of the 40mm was visible. I seem to have found a way of using the smaller filter on 2" eyepieces without losing fov.....

Always conscious of the 'no such thing as after lunch' saying, am I missing anything? Where is the flaw in my plan?

If you stayed awake this long, you are obviously immune to my insomnia treatment program

[John]

I think your logic is OK Stu. As far as I can see, as it comes into the diagonal barrel, the light cone from the objective will have narrowed to a diameter that is less than the aperture of the 1.25 filter so you should not be clipping the light cone at that point.

The only thing that I have noticed is that filters placed further away from the field lens of the eyepiece can be prone to picking up a few more reflections. I don't know why that is but I have noticed it when I've had a 2" filter on the end of the diagonal barrel rather than on the eyepiece barrel

 

 

[YKSE]

Stu, I think it sound correct

putting the 1.25" in the focus plane, you'll get TFOV of about 27mm field stop; using T2 to 1.25" adapter to 2" barrel is quite close to focus plane, therefore you'll get quite hard vignetting, but I'd still expect the TFOV is still slightly wider than true 1.25" 40mm plossl (maybe you can verify it in some way)? Using the filter in front of the diagonal, it becomes so far away from focus plane(field stop), it should behave like 37mm baffel diameter in C8, i.e.e there is vignetting for sure, but it's so gradually, that our eyes just don't really see it unless you look for it very carefully.

[Pig]

I think you should stop being a tight wad and get yourself a 2”  filter ?

[Stu]

Just now, Pig said:

I think you should stop being a tight wad and get ourself a 2”  filter ?

Nice one Shaun  

Finances say I should sell this one but I shall hang onto it if at all possible, it's a good spec!

[Pig]

I guess so ? its always worth hanging onto something if it works better than expectations ..... your still a tight wad though ?

[Stu]

30 minutes ago, Pig said:

if it works better than expectations .....

Well so far it has just made my lounge and kitchen look pretty and red, very Christmassy. Have yet to see anything astronomical through it!!

[Pig]

I have never had much sucess with filters despite many trials ... maybe they just need lots of aperture ?

[GavStar]

I was distinctly unimpressed with filters for several years - planetary filters didn’t make any material difference to me and I wasn’t very happy with the DGM NBP filter despite many people liking it. I guess it shows everyone’s eyes are different.

However, earlier this year I got an oiii filter and this has ‘blown’ me away. Lovely views of the Veil and then really surprisingly great extension of the m42 nebula from my light polluted back garden. Best I’ve seen of m42 and most reports say a uhc is better than oiii on m42 - not in my case.

[Stu]

19 minutes ago, Pig said:

I have never had much sucess with filters despite may trials ... maybe they just need lots of aperture ?

Not aperture, just dark sky, ironically!

Veil and NAN with OIII, dark sky and only 4" are very nice. Still need to be properly dark adapted though, they are not magic  

[Pig]

19 minutes ago, GavStar said:

I was distinctly unimpressed with filters for several years - planetary filters didn’t make any material difference to me and I wasn’t very happy with the DGM NBP filter despite many people liking it. I guess it shows everyone’s eyes are different.

However, earlier this year I got an oiii filter and this has ‘blown’ me away. Lovely views of the Veil and then really surprisingly great extension of the m42 nebula from my light polluted back garden. Best I’ve seen of m42 and most reports say a uhc is better than oiii on m42 - not in my case.

What scope are you using Gav?

[GavStar]

For m42 it was a 130mm frac, for the Veil a 100mm frac so not much aperture.

[Pig]

Sounds good .... thank you ?

[Piero]

For those large extended targets it's also important to keep the exit pupil low. I use 4mm-6mm with my NPB or OIII filters when I want to catch as much nebula as possible. Dark skies, eye adaptation are even more important. A small aperture will give you a small image scale, which, for these large targets, is not necessarily bad. I had some really lovely views of the NAN and Veil nebulae with my TV60 on the Dolomites + OIII and UHC filters a few years ago. 

[John]

I've never found colour filters a lot of use in all honesty. I've tried a number of them on the planets but ended up preferring unfiltered views. Similarly with the quite expensive Tele Vue Planetary Filter - the bubble gum pink tint did nothing for me and while the main features of Jupiter did gain a little more contrast, this was at the expense of the harder to see features I felt.

For quite a while I just had one filter - an Astronomik O-III and that did me quite nicely. I have the Lumicon O-III (one of the good ones), the DGM NBP and the Astronomik H-B now (latter is 1.25") but the O-III is still the most used by some margin.

As Stu says, none are really "magic bullets" but the O-III gets closest I think and with small scopes enables quite nice views of some objects when, filterless, there is practically nothing to see.

 

 

[Stu]

2 hours ago, Piero said:

it's also important to keep the exit pupil low.

Agreed Piero. That's exactly why I was messing around with the 2" 40mm eyepiece in the first place. I'm looking forward to giving it a go with the OIII and UHC on some decent targets this winter. We are hoping to arrange a dark site trip before Christmas if the weather will cooperate....

[Piero]

57 minutes ago, Stu said:

Agreed Piero. That's exactly why I was messing around with the 2" 40mm eyepiece in the first place. I'm looking forward to giving it a go with the OIII and UHC on some decent targets this winter. We are hoping to arrange a dark site trip before Christmas if the weather will cooperate....

Sounds great Stu! How dark is this site? Any mag measurements? 

[Stu]

48 minutes ago, Piero said:

Sounds great Stu! How dark is this site? Any mag measurements? 

Last time we went (actually our first visit!) we measured 21.3 I think. It's pretty good, if we can get a night of good transparency.