Eyepiece for 650mm f/5 reflector recommendations please

[Karen Johnson]

I am about to purchase my first telescope, SkyWatcher Explorer 130P DS, mainly for astrophotography but also viewing.  It comes with a 28mm eyepiece.  I’ve been trying to find out what size eyepieces I need for viewing the moon, other planets and star fields.  There is loads of info out there if you understand it but I am a little lost.  Any 130 P DS users out there that could share their recommendations?

[Louis D]

To nicely frame the moon, you'll want somewhere around a 1 degree true field of view eyepiece since the moon is 1/2 degree across.  I would probably recommend a BST Starguider for your rather demanding on eyepieces f/5 reflector.  Since these eyepieces are 60 degrees apparent field of view, this equates to needing 60x in power (60degrees/60x=1 degree).  Given your scope's 650mm focal length, this would equate to 650mm/60x=10.8mm.  The closest available focal length would be 12mm for 650mm/12mm=54x.  This would also be a useful power for looking at many open clusters and larger nebula as well as for solar viewing with a full aperture solar filter.

Now, to get better, close-up views, I would recommend jumping up to the 5mm Starguider for 650mm/5mm=130x.  You would still have a very reasonable 1mm exit pupil (eyepiece focal length/scope focal ratio=exit pupil).  Exit pupil is how big a circle of light is coming out of the eyepiece and into your eye.  Below about 0.7mm and floaters can start to be an issue.  This power would also be useful for looking at brighter nebula, planetary nebula, and planets themselves.

[Karen Johnson]

2 hours ago, Louis D said:

To nicely frame the moon, you'll want somewhere around a 1 degree true field of view eyepiece since the moon is 1/2 degree across.  I would probably recommend a BST Starguider for your rather demanding on eyepieces f/5 reflector.  Since these eyepieces are 60 degrees apparent field of view, this equates to needing 60x in power (60degrees/60x=1 degree).  Given your scope's 650mm focal length, this would equate to 650mm/60x=10.8mm.  The closest available focal length would be 12mm for 650mm/12mm=54x.  This would also be a useful power for looking at many open clusters and larger nebula as well as for solar viewing with a full aperture solar filter.

Now, to get better, close-up views, I would recommend jumping up to the 5mm Starguider for 650mm/5mm=130x.  You would still have a very reasonable 1mm exit pupil (eyepiece focal length/scope focal ratio=exit pupil).  Exit pupil is how big a circle of light is coming out of the eyepiece and into your eye.  Below about 0.7mm and floaters can start to be an issue.  This power would also be useful for looking at brighter nebula, planetary nebula, and planets themselves.

Thanks Louis, that’s very helpful, I did find some older threads eventually after much searching and these eyepieces were recommended by others so I shall take your advice.  Is there any benefit to me buying a Barlow 2x to go with these do you think?

[Louis D]

1 hour ago, Karen Johnson said:

Thanks Louis, that’s very helpful, I did find some older threads eventually after much searching and these eyepieces were recommended by others so I shall take your advice.  Is there any benefit to me buying a Barlow 2x to go with these do you think?

I tend to avoid Barlows because they're a pain to keep putting in and pulling out of the focuser.  Once in, I tend to leave them in.  That said, if you can locate a quality Barlow, they're fine.  I tend to buy vintage, Japanese made Barlows myself.  Chinese made stuff tends to be all over the place quality-wise.

A Barlow would not be useful with the 5mm because 2.5mm would be much too high a power for that scope on most nights.  That, and a 0.5mm exit pupil is verging on being a pain to use due to eye floaters.

The Barlow would make the 12mm a 6mm effectively, which is quite close to the 5mm.  So, again, not a big gain.  I would probably spend the money on the 8mm BST SG to split the difference in power.

Thus, you'd have 28mm, 12mm, 8mm, and 5mm eyepieces for 23x, 54x, 81x, and 130x respectively.  That scope's short focal length doesn't lend itself well to high powers.  In my experience, 30x per inch is usually doable with fast reflectors which equates to ~150x for your scope.  This would equate to a 4mm to 4.5mm eyepiece, but it would not be that big of a jump over 130x visually.

You can see below in my comparison image of AT Paradigm (BST Starguider) and Meade HD-60 apparent fields of view how the image scale changes at different focal lengths:

Edited November 3, 2020 by Louis D

[Zermelo]

I have an F/5 reflector and three of the BST Starguiders, and they perform very well for the money. Many people on here have recommended them.

Your other option is to consider a zoom eyepiece. They will obviously span a range of focal lengths and can be very useful, for example when identifying and splitting double stars. Because of the compromises involved in design they won't be quite as sharp as a fixed-focus eyepiece, and the apparent field of view will generally be narrower. But you can get very decent ones like this for not much more than a single BST. 

If you did want to consider a barlow (they can be useful to fill in "gaps" in the range, with an existing EP), you again have a whole range of quality and cost to choose from. You can pay well into three figures, but it probably makes sense to align the quality and spend to match the eyepieces. To complement the BSTs (or that zoom), these two barlows have been recommended previously (I have the first on order):

https://www.firstlightoptics.com/barlow-eyepieces/baader-classic-q-225x-barlow.html

https://www.rothervalleyoptics.co.uk/revelation-astro-25x-barlow-lens-125.html