Why no BST StarGuiders in longer focal lengths?

[RobH2020]

Hi all,

first time poster here at SGL, only really been getting into astronomy as of 2020. 

I've been looking around for my first eyepieces to upgrade the stock ones that came with my Sky-Watcher 6" dobsonian - which are 10mm and 25mm Kellners.

I've decided on the BST StarGuiders as my first ones, as they're in my price range and seem to be very well regarded on here and Cloudy Nights. Just £41 delivered through Alan at Skys the Limit.

However I've just realised they only go up to 25mm focal length. 

Do we know why that's the case? Would longer focal lengths have no increase in FOV due to the size of the barrel or something?

If not, can anyone recommend a similar eyepiece (spec and price) in a longer focal length, such as 30/32mm?

Cheers!

Rob

 

 

[RT65CB-SWL]

Sorry Rob, unable to answer you question; but just dropped by to say: "Hi @RobH2020 and welcome to SGL." 

Edited February 20, 2020 by Philip R

[rwilkey]

Hi Rob, and welcome to the Lounge.  Alan is a great guy to deal with, when I first started buying from him he used to send me a little pack of sweets with my order.  I have also met him a couple of times in London.  On your question, I am not sure why the StarGuiders are not made in longer focal lengths except to say they would need better glass and be better corrected.  As an alternative I would recommend the Explore Scientific 52° LER Series 30mm, a bit more expensive but you can guarantee well-corrected and comfortable to use (long eye relief).  Found here: https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-52-ler-series-eyepieces.html

[John]

Because they have a 60 degree apparent field of view, the field stop (the ring inside the eyepiece that defines the field of view) would need to be larger than the inside diameter of the 1.25 inch barrel to maintain that beyond 25mm focal length. In other words, longer focal length BST Starguiders would need to be 2 inch format eyepieces. There are other eyepieces that already occupy that niche such as the Panaviews and the Aero ED's.

A 30mm in the 1.25 inch fitting, such as the Vixen NPL 30mm for example, would have an apparent field of view of around 50 degrees (limited by the inside barrel diameter again) so it would not show much, if any, more sky than a 25mm with a 60 degree apparent field of view. Hope that makes sense !

 

[RobH2020]

Thanks guys!

That explains it John,  I was wondering if it was the field stop.

Rwilkey, thanks for that ES recommendation. That 30mm 52deg eyepiece seems to have a very similar field of view (in terms of what you can actually see in eyepiece) to the 25mm 60deg StarGuider, though at a different magnification of course. That's what I see by plugging the scope and eyepieces into one of those online equipment checkers anyway. There's a 40mm in that series but it's a 2" barrel.

So my lesson here seems to be there are no 1.25" barrel eyepieces in the 60-ish-degrees range over about 25mm. I need to ramp up to a 2" barrel, and probably pay a little more. Does that sound like I've understood?

On another note - could someone tell me what ED stands for? I see it used in the StarGuiders, and in John's note above about the Aeros. I've tried Googling it but it's a tricky one to pin down!

[John]

ED = Extra low Dispersion glass. One element or more of this more expensive glass. It's also used in ED doublet refractors and triplet apochromat refractors. You will see much discussion of the various ED glass types when refractors are discussed !

Sounds like you have got the relationship between barrel size and max field of view size correct as well

If you want wide fields and long eyepiece focal lengths then the 2 inch format is the way to go.

 

 

[rwilkey]

1 hour ago, RobH2020 said:

So my lesson here seems to be there are no 1.25" barrel eyepieces in the 60-ish-degrees range over about 25mm. I need to ramp up to a 2" barrel, and probably pay a little more. Does that sound like I've understood?

Hi Rob, that just about sums it up, what scope do you have and does it have a 2" focuser?  Yes, they get more expensive and a 2" would get you more of the sky.  If your telescope focal ratio is slower than f/6 then the 32mm Panaview 70° would be a good choice as John suggested above.  On-axis this is a stunning ep and was my first upgrade, never looked back!

[RobH2020]

1 minute ago, rwilkey said:

Hi Rob, that just about sums it up, what scope do you have and does it have a 2" focuser?  Yes, they get more expensive and a 2" would get you more of the sky.  If your telescope focal ratio is slower than f/6 then the 32mm Panaview 70° would be a good choice as John suggested above.  On-axis this is a stunning ep and was my first upgrade, never looked back!

Thanks John. I've an F/8 150mm dobsonian - a Skywatcher Classic 150p. Yep it has a 2" focusser! 

[joe aguiar]

Another not expensive one could be the celestron omni 32mm plossl.  It's not expensive has a 52 degree fov but as John said its almost the same thing as the 25 at 60 

A 32mm will have abit lower power of course but fov will be near the same.

It's kinda  like people who have a 32mm plossl and thought about a 40mm 1.25 ep which has a 44 degree fov, in this case most people just stay with the 32mm plossl ep

Joejaguar 

[joe aguiar]

If u want lower wider views u need to go to a low power 2 inch ep

Does your scope have a 2 inch focuser?

If So u can get one low power 2 inch ep

Joejaguar 

[RobH2020]

47 minutes ago, joe aguiar said:

If u want lower wider views u need to go to a low power 2 inch ep

Does your scope have a 2 inch focuser?

If So u can get one low power 2 inch ep

Joejaguar 

Hi Joejaguar,

yes my scope has a 2" focusser so can definitely be in the market for a 2" wide-field (e.g. 60deg) 32mm-ish eyepiece.

Do let me know if you know of any worth having in a similar price range to the BST StarGuiders, i.e. around £40-50.

Cheers!

Rob

[rwilkey]

1 hour ago, RobH2020 said:

I've an F/8 150mm dobsonian - a Skywatcher Classic 150p. Yep it has a 2" focusser! 

Hi Rob, most ep's will perform well in your scope (f/8) and 2" ep's will be no exception

[John]

13 minutes ago, RobH2020 said:

Hi Joejaguar,

yes my scope has a 2" focusser so can definitely be in the market for a 2" wide-field (e.g. 60deg) 32mm-ish eyepiece.

Do let me know if you know of any worth having in a similar price range to the BST StarGuiders, i.e. around £40-50.

Cheers!

Rob

Keep an eye on the used equipment market (ie: classifieds here and the UK Astro Buy & Sell website). I picked up a Skywatcher Aero ED 30mm for £50 delivered last year and thats a decent low power eyepiece with a 70 degree field of view.

 

[banjaxed]

GSO do a 30 mm 68 degree 2” eyepiece for just over £50.00. Might be worth a look.

[Raph-in-the-sky]

1 hour ago, RobH2020 said:

Thanks John. I've an F/8 150mm dobsonian - a Skywatcher Classic 150p. Yep it has a 2" focusser! 

I very recently got a used 32mm SW panaview (70° AFOV) for £50. In my f4.7 dob it's far from perfect edge to edge but it gives very pleasing wide field views. With your telescope it should be excellent.

[RobH2020]

Thanks for the suggestions everyone! A good range of prices for me to consider / look out for on the used markets.

[rwilkey]

2 hours ago, Raph-in-the-sky said:

I very recently got a used 32mm SW panaview (70° AFOV) for £50. In my f4.7 dob it's far from perfect edge to edge but it gives very pleasing wide field views. With your telescope it should be excellent.

That's right Raph, you get seagulling at the edges, but on-axis it is brilliant.