2-inch EyePiece advice

[iPonchco]

So I bought myself a Celestron Luminos 2.5x 2-inch barlow.

I think it's awesome, but I think it's time to upgrade my 10 and 20 mm 1.25" plossi that came with my dob.

 

I want a 2".

 

Any thoughts or recommendations?

[Louis D]

So, rather than upgrade your 1.25" eyepieces to BST Starguiders or similar for high power usage, you bought a 2" 2.5x Barlow despite having no 2" eyepieces, correct?  You do know the main reason to buy 2" eyepieces is to get wider fields of view than is possible with 1.25" eyepieces, right?  Putting them into a 2.5x Barlow completely negates that advantage because it puts you back in the realm of what is possible with 1.25" eyepieces.

For instance, let's say you buy a 40mm Pentax XW to get to a widest possible field of view.  Good, I highly recommend doing something similar.  It will allow you to take in much wider swaths of the sky than is possible in a 1.25" 32mm Plossl.

Now, you put the 40mm Pentax in your new 2.5x Barlow, and you end up with a 16mm, 70 degree eyepiece that is massive in length, girth, and weight, not to mention cost.  Why not just buy a 16mm Explore Scientific 68° Series or OVL Nirvana-ES UWA-82º Ultrawide eyepiece in a 1.25" fitting?  It would be so much lighter and more compact.  Unless you need the long eye relief due to strong astigmatism in your observing eye (which you did not mention), I can't think of a good reason.

Most people just buy 1.25 2x Barlows because they're cheaper, lighter, and more compact than 2" Barlows.  That, and they tend only Barlow their 1.25" eyepieces to get to very high powers.  What was your reasoning for buying a 2" Barlow?

[iPonchco]

Thanks for replying and your feed back. 

Learning process and here is my thought process.  I figure since 2" EP do have a wider field of you then I'd want to get to having everything 2".  Perhaps I should have purchased the EP 1st then a barlow. I do wear glasses, so that and a wider FOV was my thought.

I'm still learning, so I wonder if I should return or keep going with my thought.  

[Louis D]

37 minutes ago, iPonchco said:

Thanks for replying and your feed back. 

Learning process and here is my thought process.  I figure since 2" EP do have a wider field of you then I'd want to get to having everything 2".  Perhaps I should have purchased the EP 1st then a barlow. I do wear glasses, so that and a wider FOV was my thought.

I'm still learning, so I wonder if I should return or keep going with my thought.  

2" eyepieces enable a wider true field of view, not necessarily a wider apparent field view.  It's all dependent on the size of the field stop inside the eyepiece that determines the true field of view.  A wider apparent field of view at a certain magnification requires a wider field stop.  At a certain point, that wider field stop may not fit inside a 1.25" barrel.  That is when the eyepiece design must be moved to a 2" barrel.  At some point, even a 2" barrel isn't big enough and a 3" barrel is needed.  Of course, this requires a bigger focuser and secondary mirror which decreases contrast in Newtonian designs.  3" eyepieces are fine in refractors, but you need a 3" diagonal.

Here's a diagram Tom Dey on CN put together to illustrate the field stop limitation of each barrel size.  0.965" was the standard for entry level scopes 50 years ago instead of 1.25".

The takeaway is that 50 degree AFOV eyepieces are limited by field stop at 32mm in a 1.25" barrel, 68 degree eyepieces stop at 24mm, 82 degree eyepieces stop at 16mm to 18mm, and 100 degree stop at 13mm.  In a 2" barrel, they stop at 56mm, 41mm, 31mm, and 21mm to 25mm, respectively.

[iPonchco]

 

My 8" dob can use both 2" and 1.25", so I thought getting a 2" barrel and 2" barlow would be best and make it easier changing from viewing and connecting my dslr. 

I can't say I understand the chart, but I appreciate the info.

[John]

Most often folks end up with one or two 2 inch eyepieces in their sets to deliver low power / widest angle views with scopes that take that fitting. In medium to shorter eyepiece focal lengths the 2 inch fitting does not provide any benefits so eyepieces in those focal lengths are in the 1.25 inch fitting. This is broadly what the chart that @Louis D illustrates.

The barlow you have purchased can be used with either 1.25 inch or 2 inch eyepieces, the former requiring a suitable adapter.

 

[iPonchco]

I like the barlow I purchased and I took several pictures with my DSLR of the moon.  It had great detail and to my eyes I was very impressed. 

I am mainly interested in planetary... I was checking out Jupiter and Saturn these last couple of weeks and very much stoked.  

I want to get a 2" eyepiece, so that I can easily remove my dslr and pop in my EP without using any adapters.  To me it makes sense.

I have my eye on the Celestron Luminos 19mm.  Any suggestions I would consider of course.  

 

Thanks again!

 

 

[Zermelo]

18 hours ago, iPonchco said:

I can't say I understand the chart

Possibly there was a key or other info with Tom's original chart but I can't find the original article on CN - perhaps @Louis D can provide a link.

It looks like each continuous line represents an AFOV, with the dots representing actual eyepieces that can be bought. The colours correspond to specific designs from a manufacturer, with an accompanying label in the same colour (though not always adjacent to the line in question). In most cases there is just one range shown for each AFOV, but the 100° line is derived from both the TV Ethos and ES ranges. 

So for example the yellow line represents 50° AFOV eyepieces (in this case, the TV plossls). Increasing the focal length (follow the line from the bottom left, towards up/right) requires a progressively larger field stop within the EP. When you get to 32mm F/L you need a field stop of around 27mm, which is the largest that a 1.25" barrel can accommodate (hence the popularity of a 32mm plossl in 1.25" format). You can continue to increase the EP F/L in the 1.25" format, but you can't increase the field stop - the graph shows the yellow line moving horizontally now, to get to a 40mm version. So yes, you can use a 40mm, 50°, 1.25" eyepiece, which will give you a lower magnification than the 32mm but no advantage in field of view; and you will lose some of the light on the way (vignetting). [EDIT: see comment by Louis D below]. But if you then increase the barrel size to 2", the maximum field stop is around 46mm, which means you can get a 50° EP with a F/L as long as approx. 55mm before vignetting again occurs.

The other lines show more exotic designs with larger AFOV. They hit the barrel size limit sooner (i.e. the focal length is shorter than it would be for a design with smaller AFOV) but of course their larger AFOV gives a wider true field of view.

Louis will correct me if I've misunderstood.    

 

Edited August 12, 2021 by Zermelo
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[Louis D]

The original CN thread on eyepiece graphs.

Here's the original chart that started that thread which Tom then modified:

2 hours ago, Zermelo said:

So yes, you can use a 40mm, 50°, 1.25" eyepiece, which will give you a lower magnification than the 32mm but no advantage in field of view; and you will lose some of the light on the way (vignetting).

Explanation sounds correct except for this bit.  The amount of light integrated across the two fields of view (32mm and 40mm) should be the same since each has the same field stop (and likely field lens) size.  The 40mm simply compresses that light into a smaller apparent field of view yielding a larger exit pupil which can be useful with aggressive nebula filters such as an OIII filter.

 

14 hours ago, iPonchco said:

I want to get a 2" eyepiece, so that I can easily remove my dslr and pop in my EP without using any adapters.  To me it makes sense.

You can get low cost 2" to 1.25" adapters which you can leave permanently attached to your 1.25" eyepieces to avoid having to deal with an adapter on the fly.  You can even replace the thumbscrew with a grub/set screw to make it more flush fitting in the focuser and your eyepiece case.  If you do this, you don't need to limit yourself to 2" eyepieces.

Here's an image from CN showing exactly this with Tele Vue eyepieces (along the bottom edge of the image).

[iPonchco]

Alright peeps and again thank you.  Its apparent I didn't do enough research.  

I have a  8" Skywatcher dob and I'm digging it. Got myself a dslr too. I have until tomorrow to return my purchase and look for a 1.25 barlow.

 

I looked up the 16mm Explore Scientific 68° Series.  I'll aim for that later.