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

[parallaxerr]

2 minutes ago, John said:

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

 

If using a 2" diagonal with 1.25" adapter yes, but not when using a 1.25" diagonal which, in my case, has a light path shorter by 32mm.

[Stu]

4 minutes ago, John said:

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

 

John, the OP is saying that the vignetting does not happen when using a 1.25" diagonal as the drawer tube is positioned further out vs using a 2" diagonal which is when the problem occurs.

[John]

2 minutes ago, parallaxerr said:

If using a 2" diagonal with 1.25" adapter yes, but not when using a 1.25" diagonal which, in my case, has a light path shorter by 32mm.

Thanks. I see the issue now.  I tend to agree that cutting a piece off the drawtube is the only solution then. The trick is to work out how much needs to come off

 

5 minutes ago, Stu said:

John, the OP is saying that the vignetting does not happen when using a 1.25" diagonal as the drawer tube is positioned further out vs using a 2" diagonal which is when the problem occurs.

Thanks for the clarification Stu.

[Timebandit]

 

The decision of the posts seem to be for you to cut the drawtube?

If you need to borrow an angle grinder then let me know?

[StarryEyed]

The problem here is that the effect of this is unknown. Will the result be better or worse. Stopping down a lens typically improves the image in an achromatic telescope. This is not a relativly simple thing as moving an eyepiece further into the optical path to reach focus. There will be an effect on the scopes optical configuration. Unless the original focuser did not also protude into the light cone there is no way of knowing what might happen. Something that needs further investigation I would suggest.

[cloudsweeper]

This has been most informative.  But for me, the drop in aperture and thus in exit pupil when using a 2 inch diagonal is not much of a price to pay for still having widefield views.  I don't fancy doing a mod, and will still go for a better focuser.  

I hope Jon also reaches a decision he is happy with.  I would recommend the 2 inch EPs option!

Doug.

[Paz]

7 hours ago, parallaxerr said:

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!

I've been patiently stalking the second hand market for SLV's and so far only have the 5, 4, and 2.5mm ones - these are short but they cover the bases down to an exit pupil of 0.5mm in the ST120 and ST80. The 20, 15, and 10mm's are on my hit list.

The ones I have got I think are good and have made me aim for more of a set. I can't comment on their quality compared to everything else out there as I don't feel qualified and haven't done anything I would call scientific to evaluate them but they are easy to use, robust, reasonably small, reasonably light, and give very good clarity.

I opted for only a 50 degree field of view just to keep to small eyepieces with long enough eye relief. If I went for a bigger afov then I would need an eyepiece with more size and weight. If I wanted more afov wthout more size and weight I would have to trade off eye relief.

I've got 82 and 72 degree eyepieces also but they are bigger/heavier and only come out with the big dob - but this is just my own personal preference.

[Peter Drew]

I think it would and I also have my suspicions regarding the CA mitigation. 

[Saganite]

18 hours ago, Moonshane said:

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.

Apart from always wanting a long focus refractor, the very fact that Dave had professionally shortened the 5" f15 to accommodate binoviewers was perfect for me, which is why I did not think twice about buying it, and any resale value is irrelevant, as it won't be going anywhere during my lifetime.

[spaceboy]

This is an interesting read. It all makes sense how it could cause vignetting but I'm not so sure it would effectively reduce aperture ??. There are after all some 1.25" eyepieces that eat up inward focus (6mm BGO springs to mind) and the draw tube would end up in very much a similar place even with a 1.25" diagonal. Using filters can also require a tad more inward focus, which many a person do use with fast achros to combat CA. I understand the logic behind it all but you would think if it is common knowledge that this is happening, it is an easy enough fix at Synta to avoid any repercussions. You can't after all advertise a 120mm scope as 120mm if your supplied '2" Crayford focuser' then clips the aperture with a 2" diagonal.

I have though read a few threads now where people feel the need to shorten their draw tubes for one reason or another so it does make you think that there is something gone wrong somewhere? This is not just related to refractors for BV use but also Newtonians where people have found the draw tube extrudes into the light path. I assume this is why SW do the low profile focuser as an aftermarket part. Not so the case with the refractor offering as the SW piano black DS focuser draw tube is even longer than the stock focusers . I wonder if similar issues plague the longer focal length refractors ? I know there were some claims made that the internal baffles of some synta refractors cut in to the light cone effectively reducing aperture. Personally I feel this was more the case of someone having a bad day during assemble and didn't put the baffle far enough down the OTA but there has been mention a couple times of this issue both sides of the pond.

[StarryEyed]

Yes the OP says

"With the 1.25" diagonal I measured effective aperture as 116mm, with the 2" it measured 105mm." With his moonlight focuser.

I wonder about the stock focuser?

 

[parallaxerr]

17 minutes ago, StarryEyed said:

Yes the OP says

"With the 1.25" diagonal I measured effective aperture as 116mm, with the 2" it measured 105mm." With his moonlight focuser.

I wonder about the stock focuser?

 

Exactly the same. Both draw tubes measure the same length unfortunately.

I was hoping it was the forward baffle in the standard focusers draw tube that was causing the issue, but it remains with the moonlite, which isn't baffled.

[parallaxerr]

I think I've come to a decision, well, with regards to barrel size anyway.

I'm going to stick with 1.25", ensuring any clipping is kept to a minimum as I can still get up to 2.7° TFOV, which is plenty. If I really want to go wider, it would only likely be worth it under very dark skies when away from home (not very often), which would help counter the effect of the reduced effective aperture anyway.

Now it's down to eyepiece choice...and I'm torn. I have the money waiting but just can't decide - ES68/82° combination vs Vixen SLV.

An odd choice you may think, due to the different AFOV's, but at F5 where they get all their use, it may be that the SLV's perform better across the field and they have a really good range of FL's that fit in with my exit pupil requirements. Some report the ES's soften towards the edges @ F5, so the extra AFOV wouldn't count for much.

What to do?

[Moonshane]

My recommendation for widest 1.25" field would always be a 24mm Panoptic. Expensive even used though. Other than this if your skies allow a largish exit pupil then a 32mm plossl would be a good bet. 

[parallaxerr]

Yes I have a Celestron 32mm plossl and I like it a lot, although my skies don't make the most of it (overly bright sky). I'm looking forward to trying it under darker skies.

I made a mental note earlier to try it with my Baader semi-apo next time I'm out, which may control the LP sufficiently to improve contrast and make use of the exit pupil.

[Moonshane]

You won't get a bigger field in 1.25"

[parallaxerr]

I know

[StarryEyed]

Edge sharpness bugs me and I didn't realise this until I looked through my 17mm Hyperion. So I would pick the SLV. 

[parallaxerr]

Just now, StarryEyed said:

Edge sharpness bugs me and I didn't realise this until I looked through my 17mm Hyperion. So I would pick the SLV. 

My thoughts exactly. All this research and discussion makes me think I'll end up looking for off-axis performance and if it's not there, I'll be dissapointed. Reports suggest the SLV's are good at F5.

[John]

I like sharp edges and wide views too. Thats why I bought Tele Vue back in the late 1980's and still do. They are good at that

I did try a few wide but less sharp eyepieces before I decided on TV which put up good views when I had F/10 scopes but the astigmatism when the focal ratio dropped much below F/8 became distractingly obvious. I know that this does not bother everyone, but it does me.

[parallaxerr]

It bothers me too John. Unfortunately TV's are out of reach for the forseeable future. I read your review on the SLV's which seemed promising and if I can't get a well corrected wider field within budget, then I'd prefer a good 50°.

[John]

1 hour ago, parallaxerr said:

It bothers me too John. Unfortunately TV's are out of reach for the forseeable future. I read your review on the SLV's which seemed promising and if I can't get a well corrected wider field within budget, then I'd prefer a good 50°.

The SLV's are very nice, and sharp, eyepieces. I tend to agree with Al Naglers motto "Show no field unless it's sharp".

 

[StarryEyed]

I do like Televue. The day I dropped my 32mm plossl and broke it I didn't even get angry, just very sad which surprised me and made me realise how good it was, I replaced it with the Hyperion and have decided to pass that on and get another TV 32mm or 25mm plossl anything shorter I have Vixens LV's so I can wear my glasses. Both TV and vixen get my vote even with the smaller FOV and both are great companies in their own right.