You asked a simple question about using a barlow. What follows is about setting an eyepiece strategy for your ongoing purchases. If this is not of interest, just skip it.
I am going to outline my eyepiece strategy. After accumulating over 20 eyepieces I have focused in on this approach. I have used this for my Orion XT8 8”/203 mm Dob, 1200 mm FL, as the example. But the same strategy applies to my 80 mm F5 refractor and my 127 mm F15 Mak. If your FL is different you can redo the calculations for your scope.
- One or two low power wide view eyepieces
- One or two midrange eyepieces
- Two to four high power eyepieces that include a plan for the top target magnification for that scope
- Or, Zoom plus Barlow to cover mid range and high power
- Planning to use a barlow can save you money
Selecting an Eyepiece - Orion telescope
This is a very general discussion of eyepieces and why there are a variety of designs
Useful Formula -
Highest target top magnification = aperture in mm X 2 So, for my 8" Dob, 203 mm aperture that would be 406X under exceptional good conditions. For my 80 mm this would be 160X. For my 127 mm this would be 254X. This is not a limit it is just a target to establish a range of magnifications.
Power or Magnification = Focal Length Scope / Focal Length eyepiece
True Field of view = Apparent Field of View eyepiece (AFOV) / magnification of eyepiece (This is a simplified formula that will provide a very close approximation under most circumstances)
Lowest power – often a spec provided by your telescope maker. But it can be whatever you like. I would recommend something with an exit pupil of no more than 7 mm. Exit Pupil = Focal length of eyepiece / Focal ratio of scope. If your scope only takes 1.25” eyepieces, your lowest power widest view would be a 32 mm Plossl (50 degree AFOV) to define your lowest power. If you have a 2” focuser you can likely go lower and wider.
Now divide the mag range into three parts. Using my Orion XT8i as the example, Orion recommends lowest useful as 29X, highest useful as 300X and highest theoretical as 406X. So I will work out the eyepiece range somewhere between 29X and 300X For this scope, that beaks down to up to under 100X for low power. 100X to 200X for medium power. Over 200X as high power.
Here is a sample range, for illustration purposes based on a 1200 mm FL. Remember the formula is: Power or Magnification = Focal Length Scope / Focal Length eyepiece
31X = 38 mm
60X = 20 mm
100X = 12 mm
120X = 10 mm
150X = 8 mm
171X = 7 mm
200X = 6 mm (can be achieved with a 12 mm and 2X barlow)
The above magnifications work almost every night at my site, under my observing conditions. The magnifications below work when the atmospheric conditions are very good. See the reference section for information on “seeing”.
240X = 5 mm (can be achieved with 10 mm + 2X barlow)
266X = 4.5 mm
300X = 4 mm (can be achieved with 8 mm + 2X barlow)
Over 300X – I usually barlow one of my other eyepieces as I can rarely use this much mag
You don’t have to have all of these magnifications, but the more you have the better you can match to your target and conditions. Note that the mm gaps get smaller as the power goes up.
Here is how I have addressed this range for my Orion XT8i. I use many of the same eyepieces in my other two scopes.
38 mm 2” - I have the Agena Astro SWA, same as the Orion Q70 series. I use the 38 mm 70 degree AFOV in my XT8i. Happy with it but it does show edge distortion in my F5.9 scope. I consider it acceptable for the price but some would not. https://agenaastro.c.../agena_swa.html
20 mm 2” - I also have the Meade 20 mn 82 degree as my second 2” low power wide view that performs well for me in my F5.9 scope.
For my Single FL 1.25” eyepieces I have the Meade and ES 82s. Like them very much
Meade 82 degree
Explore Scientific 68 degree and 82 degree line. I have two of the 82 degree and really like them. Many reports compare these favorably with the premium eyepieces.
TIGHT BUDGET? – Get a zoom and a barlow An 8-24 mm zoom eyepiece plus a barlow somewhere between 2X and 3X will give most people the full typical mag range of their scope $100. For your scope, if you get the zoom I would not get a barlow unless you need to push higher than the zoom will go.
THE ZOOM EYEPIECE INSTEAD OF SINGLE FOCAL LENGTH EYEPIECES – This is my favorite eyepiece.
The zoom is single eyepiece that effectively replaces a range of eyepieces. Works like the zoom lens on a camera. For example an 8 mm to 24 mm zoom would overlap 4 eyepieces above and provide every magnification between them. If I include a 2X barlow for use with the zoom it will provide every magnification from 50X to 300X without gaps, my primary planning range.
The zoom sounds great, but there is a trade-off. The field of view of the zoom runs from a narrower AFOV at the 24 mm range to a wider FOV at the 8 mm range. So, like any approach, the zoom is a compromise. I find that compromise quite acceptable when weighed against the benefits listed below, I prefer the zoom.
I use the Baader Hyperion Zoom. 8-24 mm as my only eyepiece in the midrange and I barlow it to extend it to the high range for the XT8. But as I have a low power wide view 20 mm eyepiece I tend to use the zoom mostly in the 18 mm to 8 mm range for the midrange.
Lower cost zoom – Celestron 8-24 – This was my first zoom. Works well at the price and a good way to test your interest in zooms. $66
Higher priced Zoom – Baader Hyperion 8-24 mm – My main eyepiece in my Orion XT8i – $290
- I never expected the zoom eyepiece to become my primary eyepiece, but it has.
- With a zoom, the eyepiece seems to disappear as you just move in and out at will, no swapping, no thinking about eyepiece changes
- The Celestron 8-24 zoom is good and comparable to my Plossl eyepieces
- The Baader Hyperion is great and comparable to my Explore Scientific eyepieces
- Watching doubles split as I rotate the barrel is wonderful
- One filter serves over a wide range of magnifications, no screwing and unscrewing to try other eyepieces
- Moving smoothly between small changes in magnification helps when seeing is not the best
- I am always working at the optimum magnification for this target.
- Sharing the view with others is easier, especially in my manual tracking Dob - I hand it over at low mag so it stays in the view longer. They zoom back in to whatever magnification works best for them.
- My eyepiece case has been greatly simplified
- Kids love the zoom
When I observe, 90% of the time, in all of my scopes, I use one or two low power, then the zoom for the midrange. Then I barlow the zoom for the high range if I need it, and that is all I use. I have single FL eyepieces in my kit, but they are rarely used.
Barlow Lens - A barlow is an intermediate optical device that goes between the eyepiece and the objective lens or primary mirror. It effectively gives each eyepiece two magnifications, one with and one without the barlow. A 2X barlow is used in the example below. For visual use, barlows ranging from 1.5X to 3X are common. So, if you have 3 eyepieces properly spaced and add a barlow, you now have 6 magnifications. You can read more about barlows at this link: http://www.telescope...rlows/99807.uts
What is SEEING and why it can be bad. How it will limit how high you can go.
This is not a problem with your telescope or your eyepiece.
Lower Cost Single FL eyepieces – for your Medium to high power magnifications.
AT Paradigm line 60 degree AFOV. I don’t have these but I read so many good reports about them, especially in scopes of F5 or higher focal ratios. The Agena Astro Dual ED are the same eyepieces under a different label.
Zoom eyepiece review – Includes the Celestron zoom –
This is an older review and the tech specs may not match the current Celestron zoom, but I find the comments nicely reflect the performance of the Celestron zoom I own –
Baader Hyperion Mark IV Zoom review - The current model -
A very good discussion, by Al Nagler, about eyepieces and magnification for those who want to go a little deeper.
Eyepiece Designs - This is the one I turn to when I am trying to understand or explain the differences between the various designs. There are many different designs, Many are named
for their original designer, such as Huyghens, Ramsden, Kellner, Plossl, Konig, Erfle, Branden and Nagler.
Discussion about Paradigm eyepieces
Meade 82 vs. ES 82