The BST starguiders supertest!

[Ricochet]

On 12/01/2018 at 19:14, Louis D said:

Is the 25mm sharp to the edge?  I have the 25mm Meade HD-60, and it suffers from astigmatism starting 50% of the way out center to edge even at f/12.  It is so bad, I would never recommend it.  By contrast, the 4.5mm, 6.5mm, and 9mm versions are nearly completely sharp to the edge.

Having recently bought a couple of 25s to max out the FOV in my binoviewers I used a small gap in the clouds to try to investigate this.

Used natively in my f6 dob the 25mm shows significant astigmatism, becoming noticable perhaps 2/3 - 3/4 of the way to the edge and rapidly increasing. Stars become sharp T shapes with the "leg" of the T pointing towards the centre. The field however, is flat.

Screwing the nosepiece from my 2X AE barlow to the 25mm causes all hell to break loose. Astigmatism is completely swamped by field curvature so bad that I'm wondering if it really happened or if I dreamt it. Also, the barlow vignettes the edge of the field.

Using the 25mm in my Meade 2x TeleXtender turned it into a really nice eyepiece. Astigmatic Ts disappear and it is sharp across (almost all?) the field. No vignetting. 

Using the 25mm in the binoviewer with the AE barlow element screwed to the front giving 2-2.5x magnification produces similar views to with the TeleXtender, Nice, sharp views with a bit of vignetting at the field edge. My memory of lunar viewing with these says that the views go a little soft right at the edge before the vignetting so I'll have to test further when conditions are better as I didn't notice this on stars.

The 25s also show a touch of chromatic aberration at the edges, but this wasn't apparent on star fields and my memory of my old 8 and 12mm Starguiders says it's probably not so much a flaw specific to the 25mm Starguider. 

Having not used a 25mm Starguider before I was quite surprised at how far behind its performance was compared to the other focal lengths I've used (5, 8, 12). Looking at the design I notice that the 25mm is the only one in the range without a Smyth/barlow element in the nosepiece and suspect this is the reason for the performance drop. Had I bought a 25mm to use as a low power eyepiece I would be sending it back and I will exclude the 25mm from future recommendations to buy Starguiders. However, in the binoviewers with a barlow to clean up the optical issues they make a very nice, comfortable pair of eyepieces that I will be keeping.

[Louis D]

36 minutes ago, Ricochet said:

Using the 25mm in the binoviewer with the AE barlow element screwed to the front giving 2-2.5x magnification produces similar views to with the TeleXtender, Nice, sharp views with a bit of vignetting at the field edge. My memory of lunar viewing with these says that the views go a little soft right at the edge before the vignetting so I'll have to test further when conditions are better as I didn't notice this on stars.

Someone on CN uses the 25mm Meade HD-60s for a binoviewer as well.  I guess I could compare them in my binoviewer to my Vite 23mm 62 degree aspherics.  The Vite's fall apart at f/6 about 80% of the way to the edge and are quite good, light, and small in the binoviewer.

[Ricochet]

4 hours ago, Louis D said:

Someone on CN uses the 25mm Meade HD-60s for a binoviewer as well.  I guess I could compare them in my binoviewer to my Vite 23mm 62 degree aspherics.  The Vite's fall apart at f/6 about 80% of the way to the edge and are quite good, light, and small in the binoviewer.

I actually have a pair of aspherics as well and was quite impressed with the optical quality, especially considering the price, but I found a couple of issues for my use: 

1. The undercuts mean my (Starguider) binoviewer doesn't grip them properly, especially the left hand eyepiece clamp. 
2. I'm no good with eyepieces that I need to hover over and prefer eye cups I can make a light contact with. 
3. Due to 2. I found it quite difficult getting the IPD right and positioning my eyes over the right point, especially as, with the dob, altitude changes alter the angle that the eyepieces are at relative to the ground. With the Starguiders' eye cup design there is a natural "mechanical centering" of the eye to an eyepiece which makes setting the IPD very easy. 

[25585]

On 17/01/2018 at 16:38, Louis D said:

My problem is that the bridge of my nose is about 15mm high (or more).  Unless someone makes eyeglass frames with more flush mounted lenses (inset below the bridge) instead of being level with the top of the bridge, I'm stuck needing a rather large BVD based on frame availability.  Does anyone know of frames with small lenses that sit close to the eyeball?  Don't say contacts, the natural limit of this concept, since I can't stand wearing them.

Perhaps why Patrick Moore wore a monocle?

[25585]

None. 16mm is too small eye relief with a eye lens flush with the top. Recessed, effective distance is way too small.

[Louis D]

14 hours ago, 25585 said:

Perhaps why Patrick Moore wore a monocle?

I've definitely got the eyebrows for it. 

[Arcturus D]

I didn't reckon much to the BSTs in my F6 8" dobs, as most of them had strong coma.

[Louis D]

14 hours ago, Arcturus D said:

I didn't reckon much to the BSTs in my F6 8" dobs, as most of them had strong coma.

Are you sure it wasn't astigmatism?  I have never seen an eyepiece exhibit coma.

[Arcturus D]

26 minutes ago, Louis D said:

Are you sure it wasn't astigmatism?  I have never seen an eyepiece exhibit coma.

I am assuming its the coma from the scope, as F6 is nearing the fast side of things and these eyepieces are not corrected well. I have Meade and Televue eyepieces and some cheapy Plossls and none of these eyepieces display the problem that I experienced with the BST Starguiders. 

In the end it doesn't really matter what the aberration is called, the effect is too much for my liking, but other people may be more tolerant towards the experienced effect than me. ?

[Arcturus D]

39 minutes ago, Louis D said:

Are you sure it wasn't astigmatism?  I have never seen an eyepiece exhibit coma.

Here is another thread regarding the coma in BST Starguiders.

 

[John]

Some BST Starguiders / Explorers are better corrected than others I've noticed. The 18mm, 12mm and 8mm seem the best and I found they performed pretty well in my F/5.3 12" dobsonian. The 25mm, which I have now, is not so good and does show some edge astigmatism in my F/6.8, F/6.5 and F/5.3 scopes. Not awful even in the faster of those but quite noticable in the outer 20% of the field. My mainstream eyepieces are Ethos, Delos and Pentax XW's so I've some well corrected ones to compare the BST's with

 

[Louis D]

2 hours ago, Arcturus D said:

Here is another thread regarding the coma in BST Starguiders.

 

Try testing with a coma corrector to eliminate that variable.  Once the scope's coma is minimized, it's much easier to see what sort of aberrations the eyepiece has by itself.  I will admit that some higher end eyepieces deal with coma inherent in the system better than most lower end eyepieces.  However, I stand by my statement that no eyepiece I've ever used introduced coma into the image.  Astigmatism and chromatic aberrations, yes.  Coma, no.

17 hours ago, Arcturus D said:

I didn't reckon much to the BSTs in my F6 8" dobs, as most of them had strong coma.

Perhaps I took issue with your wording.  Had you said "as most of them don't deal well with the strong coma from my scope", I'd have probably agreed with you.  Until you correct out the coma in the scope, you can't really evaluate an eyepiece's edge correction or declare that it has strong coma of its own.  I even use a field flattener visually in my 72ED because a curved field makes it difficult to isolate whether the field curvature is coming from the scope or the eyepiece.