I'm interested in a wide view eyepiece for my 150/1200mm reflector, I have the 10 & 25mm that came with the scope and recently picked up the 8-24mm baader zoom (awesome btw). Now I'd like something with a wider view, would a 30 or 40mm vixen npl be a good addition? The wider eyepieces seem well priced so spending a bit more is an option if it will afford a real keeper? Thoughts?
There are a few things about eyepieces that you need to understand in order to make a sensible purchase. Eyepieces are more complicated than just their focal length. Focal length is only the start of a very long story.
First, wide field. This is very involved. You may have noticed that there is a thing called Apparent Field Of View (AFOV), and that it is described as an angular value, such as 52deg, 68deg, 83deg, etc. It describes the apparent angular width of the field that is visible through the eyepiece.
Now, there is a catch. For a given AFOV value, there is a maximum focal length that will allow for this value for a given barrel size. Barrel size is the 0.96", 1.25" or 2" eyepiece barrels typically used with telescopes, with the latter two being most common. Best way to describe how significant this is, is with an example. Take two straws, both the same length, but one is wider in diameter than the other. When you look through each straw, you will notice that the larger one will show a wider true field than the smaller one. Now, say you put an eyepiece into each straw, no matter how long you make the focal length of the eyepiece, the smaller diameter straw will never show as much of a true field as the wider straw. This is the same with telescope eyepiece.
When you look at a line of eyepieces within a specific AFOV, you will notice a few things. We'll take as an example the 68deg AFOV. You will notice that in the 1.25" barrel size, the longest focal length you will find in this barrel size is 24mm. You just cannot get a 1.25" barrel eyepiece with a longer focal length than 24mm that gives a 68deg AFOV. The 1.25" barrel just won't show more of the sky.Yes, you can get a 40mm 1.25" eyepiece, but it won't show you any more of the sky than a 24mm 68deg eyepiece. You will only get a smaller, much smaller, AFOV, closer to 35deg actually. Now, if we use a 2" barrel eyepiece, the longest focal length that will give a 68deg AFOV is 42mm.
If we stick with 1.25" eyepieces, there is a maximum focal length for a given AFOV. 52deg AFOV, the max fl is 32mm. 68deg is 24mm. 83deg is 17mm. 100deg is 13mm. While all these numbers may seem totally unrelated, what all these eyepiece values have in common is that they all show exactly the same maximum amount of they sky. That's it. The 1.25" barrel just won't allow more of the sky to be seen.
And conversely, there is a maximum focal length to AFOV relationship with 2" barrel size.
There are other elements that make for a better choice in eyepiece for your scope, but to note these down now would make for a ridiculously long post. I think you have enough to consider here now! If you would like more info, just ask. Eyepieces are a very vexed proposition that is shrouded in a lot of misunderstanding.
I note your scope is f/8, which is a very forgiving focal ratio - almost any eyepiece will work well with it - a 30-40mm Plossl will be fine.
The field of view of an eyepiece is limited by the diameter of the eyepiece field stop (a black ring inside the barrel) - or possibly the barrel itself if there is no field stop (as is the case with some eyepieces). For the widest field of view in a 1.25" eyepiece, this will be close to 30mm diameter. If you go shopping for low power eyepieces turn them around take a look at the telescope end to see how big the field stop is.
Some examples below, these are (left to right):
Four 1.25" barrels: Vixen LVW 5mm, 8mm, 13mm, 22mm
Then in the 2" barrels: LVW30mm, TMB Paragon 40mm, Vixen LV50mm. Note that the 40mm has a wider true field of view (no field stop !) than the 50mm.
Now I'm betting your scope has a 1.25" focusser. So as you can see in my example the LVW22mm has a field stop pretty much as big as it can be in a 1.25" barrel; this eyepiece is a 65 degree AFOV.
But... as Mental points out the maximum focal length in a 1.25" barrel depends on the design of the eyepiece - in particular the apparent (angular) field of view (AFOV). Referring to this table: http://www.twcac.org/Tutorials/eyepiece_table.htm For a budget Plossl, the AFOV is ~ 35-40 degrees, the premium Meade and Televue Plossls offer 40-50 degrees.
I'd take that bet with some confidence. Most 6"/f8 Newtonians sold these days come standard with a 2" focuser.
Cheers,
John B
While I dont know for sure I think my focuser will only take 1.25 eyepieces. I unscrewed the eyepiece holder and the barrel only measures 42mm inside dia.
Looking at the other 150mm in the same brand (Skywatcher) the 750mm version looks to have a different focuser and its listed on the web site as taking 2" or 1.25. Is upgrading my focuser to a 2" even worth considering?
If your focused has an adaptor that allows 2 different eyepiece sizes, then yes it will take both 1.25" and 2" eyepieces. If it has just a single tube it is a 1.25" only.
While I dont know for sure I think my focuser will only take 1.25 eyepieces. I unscrewed the eyepiece holder and the barrel only measures 42mm inside dia.
Looking at the other 150mm in the same brand (Skywatcher) the 750mm version looks to have a different focuser and its listed on the web site as taking 2" or 1.25. Is upgrading my focuser to a 2" even worth considering?
Hi,
I guess this must be the telescope you bought then:-
I wouldn't worry about upgrading the focuser at the moment. I think that would be burning money on a scope you will probably want to upgrade in the next 12 to 18 months if you stay with the hobby. There are a few reasons for this which will become apparent to you as you learn more about the hobby over the coming months.
This means you need to stick with 1.25" eyepieces. This is a bit limiting by todays standards but not the end of the world. 25 years ago 1.25" focusers were standard fare on both large and small scopes, but not today. It's a bit like saying an EH Holden (1964) was a great car. Well in 1964 it was, by todays standards and compared to modern cars its not something I would get around in. Alex has already given you sound advice regarding the AFOV. With a 1.25" focuser you are limited to an eyepiece having a product of its focal length and AFOV limited to about 1650. What I mean by this is you multiply the eyepiece focal length in mm by its AFOV in degrees and the product needs to be under 1650, otherwise you do not gain any True Field of View. For instance with a 24mm Televue Panoptic having an AFOV of 68 degrees and a focal length of 24mm its product is 1632, which as Alex correctly pointed out, is about maximum for a 1.25" eyepiece. If you went with a Plossl type eyepiece having a 52 deg AFOV you can go to a maximum focal length of about 32mm. On that basis I would be looking at some second hand eyepieces like the Vixen 22mm LVW, Televue 24mm Panoptic or a 30mm plossl type like the Orion Ultrascopic or Celestron Ultima.
And it does only come with a 1.25" focuser (sorry wavy) . Most of the 1/2 decent 6" scopes these days all have a 2" focusers
Yes that is my scope although it cost $200 less than the marked down price in that link
Quote:
Originally Posted by ausastronomer
This means you need to stick with 1.25" eyepieces. This is a bit limiting by todays standards but not the end of the world. 25 years ago 1.25" focusers were standard fare on both large and small scopes, but not today. It's a bit like saying an EH Holden (1964) was a great car. Well in 1964 it was, by todays standards and compared to modern cars its not something I would get around in. Alex has already given you sound advice regarding the AFOV. With a 1.25" focuser you are limited to an eyepiece having a product of its focal length and AFOV limited to about 1650. What I mean by this is you multiply the eyepiece focal length in mm by its AFOV in degrees and the product needs to be under 1650, otherwise you do not gain any True Field of View. For instance with a 24mm Televue Panoptic having an AFOV of 68 degrees and a focal length of 24mm its product is 1632, which as Alex correctly pointed out, is about maximum for a 1.25" eyepiece. If you went with a Plossl type eyepiece having a 52 deg AFOV you can go to a maximum focal length of about 32mm. On that basis I would be looking at some second hand eyepieces like the Vixen 22mm LVW, Televue 24mm Panoptic or a 30mm plossl type like the Orion Ultrascopic or Celestron Ultima.
Cheers,
John B
Ok I hope some of this is sinking in. Crunching the eyepiece numbers and my scope specs into the calculator linked above gives an arc minute value, is a lower or higher value giving a wider view? A couple I crunched..
Firstly thanks to all who have posted help here, its apprecated.
Yes that is my scope although it cost $200 less than the marked down price in that link
Ok I hope some of this is sinking in. Crunching the eyepiece numbers and my scope specs into the calculator linked above gives an arc minute value, is a lower or higher value giving a wider view? A couple I crunched..
Or am I getting lost the more I dig deeper into all this? Haha.
Indio
Yes that's correct. The only thing I would suggest is that you avoid any eyepiece with a focal length longer than 35mm. The 40mm 40 deg AFOV eyepiece whilst it maximises TFOV is like looking through a drinking straw because of the narrow AFOV.
An excellent choice for your scope would be a 35mm Orion Ultrascopic. These have a 49 deg AFOV and in your telescope would give a TFOV of 85.8 arcmin which maximises TFOV without being too restrictive in AFOV. Further it gives an exit pupil in your scope of about 4.3mm. Whilst a 2mm exit pupil is considered the optimum for most general purpose observing a ~4mm exit pupil is considered to be about ideal for popping dim threshold targets due to a an ideal compromise between image brightness and contrast. These eyepieces are no longer made and haven't been made for over 5 years. However, they are considered one of the benchmark eyepieces in regards to ~50 deg AFOV eyepieces. In your F8 scope they will give tack sharp images right across the entire TFOV. The sharpness, contrast and light throughput is excellent, as is comfort, eye relief and general ergonomics. They are a very high quality Japanese made eyepiece. These eyepieces were sold for a long period and come up on the 2nd hand marked reasonably frequently in the USA less frequently in Australia but they do come up in Icetrade classifieds. I also suggest you look here under "eyepieces"
The almost exact same eyepiece to the Orion Ultrascopic (all made by the same Japanese Optical factory to almost identical specifications) was also sold as:-
Celestron Ultima
Parks Gold Series
Antares Elite
If you can find any of those other eyepieces in 35mm (30mm would also be good) you can buy them with confidence as a high quality Japanese made eyepiece that will work exceptionally well in your scope. You should get one 2nd hand for <$100. The last one I saw sold on Cloudy Nights classified for $US60.
It wouldn't hurt to send him an email or phone him and see what he has in stock, what the price is, and what the shipping cost is. Glenn Speers and his wife Sherri run Antares Optical (Sky Instruments) in Canada.
Actually thinking a bit more about this Glenn Speers at Antares Optical in Canada may still have some stock of the Antares Elite Plossls...(snip)...
Cheers,
John B
I've been in touch with Glenn, no stock of the 30 or 35mm. Thanks for your help anyway, I have a bit better understanding of what in looking for now.
