Effective aperture loss with 2" diagonal - are 2" EP's worth it?

[parallaxerr]

I'm in the process of selecting a range of new EP's as an upgrade to my existing Baader Hyperion MKIII Zoom. I recently experimented with a handful of Celestron & SW Plossls that were supplied as standard with a few of my previous scopes, which highlighted that the Zoom was performing really rather poorly in my ST120 @ F5.

The zoom displays really poor performance off-axis showing significant astigmatism (seagulls & elongation changing by 90° in and out of focus) in comparison with the Plossls, even at the lower AFOV settings. Worse still, it's only roughly the centre 40% or so that is on axis so my usable FOV is very small. The Plossls @ 10, 15, 25 & 32mm all show much sharper stars, closer to the edge of the FOV. I also tested my Altair LER EP's at 5,6 & 9mm, these too have much better edge performance @55° AFOV.

In the search for new EP's I have obviously had to consider 1.25"/2" versions, but there's potentially a problem with 2" EP's. With a 2" diagonal fitted, the focuser draw tube clips the light cone, reducing the effective aperture of the scope. This is a well documented problem with the ST120 standard focuser, mine has a Moonlite fitted, but the problem remains.

I tested effective aperture using the green laser method with a 5mm EP (1mm exit pupil) in both 1.25" & 2" diagonals. The 2" diagonal has a light path 32mm longer than to the 1.25", with the focal plane moving by that amount, measured from the focuser housing to the end of the draw tube. With the 1.25" diagonal I measured effective aperture as 116mm, with the 2" it measured 105mm.

So, I have it in mind to stick with 1.25" EP's so that I'm not wasting available aperture as my main interest is DSO & nebulae. I'd like to collect a set of the same brand over time, but often the longer FL's step up to 2". As such I am considering the ES82°'s up to 14mm in 1.25", then the ES68°'s from 16-24mm in 1.25" (I am aware there may be some overlap so will cherry pick between the two lines). Crunching the numbers suggests I won't want to go much longer than 24mm anyway, which is readily available in 1.25" format & my 32mm Plossl works well for opening up the exit pupil under dark skies, it just doesn't have the FOV of a 2".

However, I still feel I am being limited in my optios, so, the question is this. Should I suffer the loss in aperture to use wider 2" EP's? And, if so, am I likely to see any negative effect of the clipping, such as vignetting, or will I simply lose some image brightness? It's a bitter pill to swallow knowing that your 120mm is being clipped to 105mm!

One final consideration is this, the zoom comes to focus with only 4mm of in-travel remaining on the focuser using the 2" diagonal, so potentially, another brand with a different focal point may not come to focus at all.

Thanks for reading & I welcome any advice.

 

[Moonshane]

Is it the draw tube or the diagonal that vignettes the field?

If the former maybe albeit a little drastic could you cut a short section off the ota tube?

 

[parallaxerr]

It's the draw tube clipping the light cone as it's moved in to accomodate the longer light path of the 2" diagonal, shortening the OTA would make it worse.

[Stu]

6 minutes ago, parallaxerr said:

It's the draw tube clipping the light cone as it's moved in to accomodate the longer light path of the 2" diagonal, shortening the OTA would make it worse.

In that case, shortening the drawer tube is the only answer surely? That way it won't clip the light path but you may need an extension tube if using a 1.25" diagonal?

[Moonshane]

If you cut off just enough so the drawtube needs to come out just past the point where it no longer clips the cone, surely that would work? It would be a fine balance though to get It right.

[parallaxerr]

Just now, Moonshane said:

If you cut off just enough so the drawtube needs to come out just past the point where it no longer clips the cone, surely that would work? It would be a fine balance though to get It right.

Ah I see what you mean, of course the draw tube would need to be moved out to accomodate the shorter OTA. Bit of a drastic option though.

[Moonshane]

My logic Is that the larger aperture of the focuser housing might just help enough.

Or sell the moonlite to fund a larger focuser?

[parallaxerr]

2 minutes ago, Stu said:

In that case, shortening the drawer tube is the only answer surely? That way it won't clip the light path but you may need an extension tube if using a 1.25" diagonal?

Yes I actually considered this and I could get the current draw tube shortened by a machinist I know, but would lose the end stop, potentially allowing it to fall out. Perhaps a new stop could be fashioned.

I also contacted Moonlite about a shorter tube but the only option is considerably shorter...and expensive.

 

[parallaxerr]

4 minutes ago, Moonshane said:

My logic Is that the larger aperture of the focuser housing might just help enough.

Or sell the moonlite to fund a larger focuser?

Yes it would, if the draw tube didn't extend past it. The moonlite is a bit overkill on an ST120, but I already had it. Not sure I want to spend even more on a 3" version, but will research the cost.

Edit: A quick search suggests a 3" is out of the question, due to cost...cough, cough!!!

[Stu]

12 minutes ago, Moonshane said:

My logic Is that the larger aperture of the focuser housing might just help enough.

Or sell the moonlite to fund a larger focuser?

I don't see how that works Shane? Surely the relationship between diagonal and drawer tube is fixed, so shortening the OTA doesn't move the drawer tube further out relative to the light path?

Moonlite might do a shorter drawer tube which would actually move it out of the light path i.e. 2" diagonal is in the same place and there is less length of drawe tube going inside the scope? Or am I missing something?

EDIT Posts crossed  

[Stu]

12 minutes ago, parallaxerr said:

Yes I actually considered this and I could get the current draw tube shortened by a machinist I know, but would lose the end stop, potentially allowing it to fall out. Perhaps a new stop could be fashioned.

I also contacted Moonlite about a shorter tube but the only option is considerably shorter...and expensive.

 

Could the machinist not put a couple of threaded holes in suitable places for short bolts to be placed to act as an end stop?

[D4N]

Whether the restriction actually applies to a 2" ep is another matter entirely, I know the focuser position for my 2" aspheric is fully out but for my 1.25" ep's it is much further in.  I would assume this has to do with the 2"-1.25" adaptor and the position of the lenses in the ep.

 

I'm surprised the Baader MkIII zoom is performing so poorly, I find it gives good stars in all of my scopes.

[parallaxerr]

4 minutes ago, D4N said:

Whether the restriction actually applies to a 2" ep is another matter entirely

I'm surprised the Baader MkIII zoom is performing so poorly, I find it gives good stars in all of my scopes.

Good point Dan, I hadn't considered the adapter, which will of course add to the problem. it could be that a 2" EP comes to focus further out than the Zoom when fitted with the adapter.

The zoom seems OK in my other scopes at F5.9, 7.5 & 11. Perhaps F5 is just pushing it too much.

[parallaxerr]

23 minutes ago, Stu said:

Could the machinist not put a couple of threaded holes in suitable places for short bolts to be placed to act as an end stop?

That was my thinking. I'm very reluctant to go down this route though, I'd be very unhappy if I ended up with chuck marks on the draw tube from shortening.

I suppose what it boils down to is this...I'm calculating my DSO EP FL requirements based on exit pupils that I know work for me. Matching, avaialble 1.25" EP's can provide these exit pupils up to 2.7° TFOV, is this enough!? Am I going to miss out if I don't have an EP capable of 4° TFOV in my collection?

[D4N]

I normally use the 2" fitting on my Baader MkIII zoom as well, maybe there is something to that?

My scopes are f4.9, f5, f6.7 (also reduced to f4) and f11.6, granted the latter isn't much of a test but the first three are fairly fast and I haven't had any problems with it.  A little coma on the reflectors but that is nothing to do with the EP.

[Stu]

3 minutes ago, D4N said:

I normally use the 2" fitting on my Baader MkIII zoom as well, maybe there is something to that?

My scopes are f4.9, f5, f6.7 (also reduced to f4) and f11.6, granted the latter isn't much of a test but the first three are fairly fast and I haven't had any problems with it.  A little coma on the reflectors but that is nothing to do with the EP.

The Baader hyperions tend to have a poor reputation in fast scopes. I've not used them but have read enough to know that it seems to be a real issue.

[Stu]

9 minutes ago, parallaxerr said:

That was my thinking. I'm very reluctant to go down this route though, I'd be very unhappy if I ended up with chuck marks on the draw tube from shortening.

I suppose what it boils down to is this...I'm calculating my DSO EP FL requirements based on exit pupils that I know work for me. Matching, avaialble 1.25" EP's can provide these exit pupils up to 2.7° TFOV, is this enough!? Am I going to miss out if I don't have an EP capable of 4° TFOV in my collection?

A 3.5 or 4 degree fov is lovely for objects like the Veil or North America Nebula, or perhaps M31 all from a dark site. Outside that most objects will fit in the smaller fov.

[cloudsweeper]

As an ST120 user, I'm finding this all very interesting, and also a cause of some concern  re effective reduction of aperture, since I mostly use a 2" diagonal with it.

For convenience, my EPs down to 10mm are 2", or dual-fit, or have semi-permanently fitted twist-lock adapters so I can just swap them over with ease.

So I'm missing out on aperture inside this range then?

The standard focuser is not great (of course) - it's a bit slack (although I've adjusted the worst of that out), and jerks a bit at the point of best focus.  For these reasons, I think I'll go for a DS Crayford.  Do you knowledgeable folk think that would have any effect on the reduced aperture??

Doug.

[Paz]

I went through this line of thought about my st120 last year. In the end I decided to stick to 1.25" eyepieces as the philosophy of the st120 in my collection of scopes is that it is a small and simple set up and I didn't want to start doing surgery on the scope. I've got cheap plossls at 32mm and 25mm and I'm collecting vixen slv's for shorter focal lengths, in order to have easy going eye relief, easy maintenance of balance, and physically small eyepieces that I can just put in my pockets so I don't have to take out a big case of stuff for an observing session.

[parallaxerr]

57 minutes ago, cloudsweeper said:

As an ST120 user, I'm finding this all very interesting, and also a cause of some concern  re effective reduction of aperture, since I mostly use a 2" diagonal with it.

For convenience, my EPs down to 10mm are 2", or dual-fit, or have semi-permanently fitted twist-lock adapters so I can just swap them over with ease.

So I'm missing out on aperture inside this range then?

The standard focuser is not great (of course) - it's a bit slack (although I've adjusted the worst of that out), and jerks a bit at the point of best focus.  For these reasons, I think I'll go for a DS Crayford.  Do you knowledgeable folk think that would have any effect on the reduced aperture??

Doug.

Possibly you're only getting 100-110mm of effective aperture. One point Dan raised above is important and that's where you EP's come to focus wrt draw tube travel. If you have less that 55-60mm (rough guesstimate) of draw tube showing when you're focused, the light cone is probably being clipped. It's been talked about on CN a fair bit and some say Sky-Watcher do it purposely, to cut down on CA.

The only way to check if a new focuser will be the same, I suppose, is to compare the length of the draw tubes, or go for a 2.5" or 3" focuser.

20 minutes ago, Paz said:

I went through this line of thought about my st120 last year. In the end I decided to stick to 1.25" eyepieces as the philosophy of the st120 in my collection of scopes is that it is a small and simple set up and I didn't want to start doing surgery on the scope. I've got cheap plossls at 32mm and 25mm and I'm collecting vixen slv's for shorter focal lengths, in order to have easy going eye relief, easy maintenance of balance, and physically small eyepieces that I can just put in my pockets so I don't have to take out a big case of stuff for an observing session.

Your post rings many bells Paz. I want to keep things simple too and am aiming for 3-4 very carefully selected EP's based on my exit pupil requirements. I have been eyeing up the SLV's too as they fit my FL requirements perfectly in the 15, 20 & 25mm's. Have you tried one in the ST yet? My only concern is the FOV, but, given that I'm used to only 40-50% decent on-axis performance with the zoom (so probably only 20-30°AFOV) then 50° of sharp field would be great!

[StarryEyed]

Its truly not worth the risk of messing something up. Use it the best way you can and if it still bugs you sell it on unmodified and move on. If you start cutting things up nobody will want it afterwards.

[Moonshane]

Opinions will always differ on this sort of thing. It would not particularly worry me to do it or to buy a scope that has been modified although it does have an effect on the price.  Personally a shortened tube  would be attractive as I use binoviewers.

[Timebandit]

Hi . I would personally make the best of the scope as it is , and experiment with the" add on's" instead. Cutting down a scope is one big decision ,as there is no going back. I would think it would effect the later selling of the scope and therefore limit the market and the price you are asking.

The manufacturer makes the scope at the dimensions they do for a reason. And that is because the scope will give the best performance within reason and appeals to the majority of the possible customers in that market. A "customised scope" to the degree of cutting and altering the dimensions is bound to limit the second hand market. And if it goes belly up and the cutting goes wrong the scope may just be good for the bin or parts.  Leave it in one original piece IMO

[Peter Drew]

It depends a lot on what brand of telescope you are modifying. A mod to a Skywatcher to improve a function would be seen as a good thing, the same mod, if needed, to a Takahashi would be seen as damaging it!. If I had a Skywatcher that was being vignetted due to the length of the drawtube then I would have no hesitation in shortening it. What I would do, is to insert a 2" diagonal with the longest focal length eyepiece that I was going to use and focus it with an inch of inward travel to spare and then mark the drawtube. then do it again with the shortest focus eyepiece. I would then remove the whole focuser and cut off any internal excess. To revert to 1.25" format would simply need an extension piece.   

[John]

If the vignetting is due to the forward end of the drawtube won't that affect both 1.25" and 2" eyepieces ?