FOV Measurements On A Nexstar 6SE
Posted 11 February 2010 - 06:51 PM
There have been numerous discussions on this forum about the attainable FOV of the Celestron Nexstar 6SE telescope with various eyepieces(EP). It is commonly believed and documented that the limiting factor is the baffle tube. Therefore, at some point in progressively longer EP focal lengths(FL), the FOV will not increase. This test is to determine the FL where that limitation commences with the Agena SWA EP set.
The equipment used in this test is a Celestron Nexstar 6SE(scope). The EPs are the Agena 2" SWA set in 26mm, 32mm, and 38mm FLs. All EPs are specified as having an apparent field of view(AFOV) of 70 degrees.
The EP was placed in the scope and aimed at a light post one block away. The scope was placed in the axis readout mode so that the azimuth(AZ) value could be determined. The scope is slewed at rate #3 so that the edge of the pole was aligned with the edge of the view in the EP. The AZ value was recorded. The scope slew was then continued until the same edge of the pole was aligned with the opposite side of the EP view. The AZ value was recorded again. The difference between these two numbers was calculated as the measured FOV. Also, the calculated FOV was obtained and compared to the measured value. The result is shown as the percent of the measured value compared to the calculated value.
The following is a table showing the results.
EP(mm) CaFOV(deg) MeFOV(deg) Ratio(%)
26 1.21 1.06 88
32 1.49 1.32 89
38 1.77 1.31 74
None of the EPs attained the calculated FOV. This could be caused by an incorrect published AFOV of the EP, an incorrect FL of the scope, or an error in the equivalent FL based on the fact that the test object was only 200 yards away. This changes the geometry of the scope internal parts from the design point for infinite objects. The later is likely the cause and not of consequence in this test. Note that the 26mm and 32mm EPs were of similar performance in that they both achieved about 89% of the calculated value. Note also that the 38mm EP performance was significantly less. Note in particular that the measured FOV was essentially the same as the 32mm. Therefore, the baffle tube limitation lies somewhere between 32mm and 38mm EPs within the Agena set. If the maximum FOV is desired using this set with the 6SE scope then the 32mm is the longest practical FL.
The table of results will not position it's columns correctly using tabs or spaces. The four columns of data correspond to the four headings respectively.
Posted 11 February 2010 - 07:30 PM
You've confirmed my recent suspicions, although I was planning a more elaborate test... on the sky. This would've been extremely difficult, (for me at least) so I'm glad and thankful for your efforts.
I have a University 70° 38mm, (which may be a clone to yours) and although I knowingly fool myself into thinking the wonderful views I'm seeing are indeed a glorious 1-3/4 degrees I've always suspected that my tube was stealing part of it. It was not wasted on me that the fov in my 12T4 and 8-Ethos had such extremely sharp edges by comparison.
Still, I would not trade this quality optical for anything. For $70, it is about the best deal I ever made.
Thanks for your work.
Posted 11 February 2010 - 07:39 PM
That's exactly the same procedure I used when testing my 8SE. I was able to get around 1.9° TFOV, while it looks like the maximum for the 6SE is about 1.3°.
I'm guessing that the reason the calculated TFOV was off was due to the location of the eyepiece. Since you were using 2" EPs, you must be using a 2" diagonal. This will put the focal plane at a different point than the design spec. When you move the mirror to change the focus to accommodate the new position, it also changes the focal length of the scope and therefore the magnification. I would think this is a small effect, but I've been told by others that it can be much larger than you'd expect. In any case, it's only off by about 10% except with the 38mm which is limited by the baffle tube.
Posted 12 February 2010 - 04:51 AM
I am glad I choose Hyperion 31mm over 36mm (after consulting you guys). I tested my eyepieces by daytime and I have excellent views with no vignetting at 31mm. I also used my Focal Reducer and so the central obstruction at 31mm and 24mm (and pinpoint at 17mm). Finally I noticed something I never did during the night. The field stops of 31 and 24mm where very sharp, buy the FOV of my Hyperion 17, 13, 8mm and Explore Scientific 14mm, there was a yellowish ring around the FOV. When I searched with my eye the field stop, the ring was vanishing and the field stop was sharp! It is no vignetting , neither dirt, neither something from the baffle tube (it would be obvious in 31 and maybe 24mm). Then what is it? Have you ever noticed something like that? Unfortunately, I didn’t check the Plossls and my Ortho (it might be a problem of SWA and UWA?).
Posted 12 February 2010 - 06:31 AM
Posted 12 February 2010 - 08:25 AM
Posted 12 February 2010 - 09:37 AM
Now I think of it, probably it has to do with my pupil size. Since it was day and bright, my pupils should be 1-1.5mm. The center of the FOV hits the ''pale spot'' and gives sharp image, but the rest hits lateral regions of the retina, where color isn't perceived. When I move my eyeball, the pale spot looks there and the yellow tint goes away, because I can see true colors again.
I've not noticed anything like that. My only wide fields are the Agena EPs and an Orion Q70. The Orion is the 38mm and the same as the Agena. I tried a Celestron FR/FF but took it off due to the central darkening effect. If you want a wider field you can hit the limit with a 32mm 70 deg EP and eliminate the FR. If you want a flatter field then it's of use but a 26mm might be your longest EP that's acceptable. Even then I could see the effect.
I hit the limit with my aspheric, I got the FR just to play and maybe later for some astrophoto. But I use another 0.5 FR (for EPs 0.75x) with my zoom and my reticle ortho a lot, to fit more stars into these specific EP's. I use it also with 14mm ES 82 to downgrade the magnification.
Posted 12 February 2010 - 09:51 AM
The error in the measured FOV is likely due to the equivalent FL of the scope at 200 yards with 2" optics. I took the data for the 26mm EP and calculated the effective scope FL. It turns out to be 1717mm or 14% longer than specified. That sounds reasonable in that the target moves closer to the front then the focal point moves further out the back. Using the two FLs, 1500 and 1717, and the standard lens equation, I get a target distance of 330 yards. That is further than I estimated but later today I'll pace it off just for fun.
It would be interesting to do the same test with the 26mm EP using a target a mile or two away. I'm sort of boxed in here so can't see horizontally much further than my light pole. If I have a chance to do a "show-n-tell" to the ham radio club at our weekly picnic I can get a shot across a lake. With our nasty weather the picnics have all been indoors at a restaurant. We're getting snow now in the Florida panhandle but just drizzle here in St. Petersburg.
Posted 12 February 2010 - 11:10 PM
TFOV = 15.04*T*Cos(delta)
where delta is the star's declination, Cos is the Cosine function, and T is the measured drift time interval. If the time is measured in minutes, the field will be in minutes of arc, and if the time is in seconds, the field will be in seconds of arc. For example, if a star has a declination of 27.0 degrees, and a measured drift time of 2.50 minutes, the true field of view is then 33.5 arc minutes. For stars within 3 degrees of the celestial equator, the Cosine function can be approximated to 1, and the formula becomes TFOV = 15.04*T. Alternatively, a near-equatorial timing in minutes can also be divided by 3.989 to get the true field in degrees. Some useful stars for this kind of measurement are: Zeta Aquarii, Delta Ceti, 10 Tauri, Delta Orionis, Alpha Sextantis, Zeta Virginis, Nu Aquilae, ect. Generally, you should use a stopwatch that is accurate to a tenth of a second for these drift timings, and take an average of several timings to reduce the effects of timing errors. For scopes on an altazimuth mount, it is probably best to use stars only near the meridian.
Also, for true field of view calculations, the best formula to use is the Field Stop Method, as it can be somewhat more accurate that the old TFOV = AFOV/magnification formula. The Eyepiece Field Stop method involves measuring the physical diameter of the Field Stop at the front of the eyepiece. The field stop is usually a ring or narrow baffle located just in front of the front "field" lens of the eyepiece. In some more complex wide-field designs, the field stop may be inside the front field lens between the elements, and in some less-expensive eyepieces, the field stop is the inside of the eyepiece barrel itself. The field for a given eyepiece is given by:
TFOV = (180/Pi)*EFSD/TFL
where EFSD is the eyepiece field stop diameter and TFL is the telescope's focal length. The "180/Pi" out front is just the number of degrees in a radian (about 57.296). For example, if the eyepiece has a field stop diameter of 25.40mm (1 inch), and the telescope focal length is 1410mm, the true field of view would be 1.032 degrees. However, remember that the focal length for the SCT will be slightly variable, so you many not get the same figure as you have measured using the star drift method. Clear skies to you.