I am currently practicing digital photometry, but I find myself increasingly drawn to visual photometry.
Having reached a certain age, I have researched various resources and accounts regarding the exit pupil, and I believe a 4mm (or slightly larger) exit pupil is now my personal limit.
I have a question concerning binoculars for visual variable star estimates:
How do ‘entry-level’ binoculars, such as the Celestron SkyMaster 15x70 or the TS-Optics LE 15x70 (priced under $180 USD), perform for this specific task?
I have noted that the sharpness likely drops off beyond 50% of the field of view due to coma, chromatic aberration, etc. However, some users claim they remain ‘usable’ up to 75% of the field. Could anyone kindly confirm if this is realistic for accurate magnitude estimates?
Furthermore, my research suggests that many variable star observers use the alternating centering technique (moving the variable and then the comparison star to the center of the field) to mitigate poor off-axis image quality. Is this technique systematically used by most observers? It seems quite challenging to memorize the brightness of the variable accurately while slewing to center the comparison star.
For context, I use a DIY ‘Bino Body Mount’ (Astronomica design) which provides excellent stability between heartbeats !!
Binoculars, low-cost entry-level, evaluation for visual magnitude estimates is tough. I’m not sure I would only restrict my purchase purely on exit pupil diameter.
Some low-cost binoculars are junk, poor optics, poor build quality, poor glass, coma, etc.
I have always used 7x50s… they can’t be beat no matter the age of your eyes! Field of views are between 6.5-7.5 degrees. In my older age I have mostly used 9x63s, specifically the Orion mini-giant 9x63s made in Japan. They are very nice. Exit pupil is 7 but the FOV is about 5 deg. The only down-size is that not all the collected light makes it into your eyes at our age. Exit pupil larger than your eyes pupil is an image brightness issue, not a major problem esp.
Using my 9x63s I can monitor Mira down to minimum using a camera tripod mount. Brighter objects don’t need any support generally but you may always use the “Leaning on the lamp post”–Hermans Hermits , leaning over your vehicle, etc. methods. Some kind of support can be useful esp. when viewing up near the zenith… sounds like you have that part covered.
I’ve rarely used the centering of each object and going on memory to make estimates but prefer having variable and comparison in the same field of view when ever possible. That speaks for binoculars with a wider FOV than 15x70s.
If you are only considering the two brands mentioned… then the decision is yours. There are plenty of reviews on the web.
I currently have 8x32 binoculars (Fully Coated, BaK-4). The FOV is 7.4°, and they have a fairly large sharp field of view (I’d say about 75-80%).
The theoretical magnitude limit for these binoculars is 8.6 .
For those binoculars the measure correct limit = 8.6-2 = 6.6 (referencing Dr. Shaffer’s AAVSO Webinar and AAVSO manual), which is a bit tight.
I plan to test them soon on targets like Delta Cep (3.5–4.4) and W Cyg (5.1–6.8).
If I want to lower my measurement limit to roughly magnitude 8.0, I would need to increase the objective diameter to 64mm. Taking my 4mm pupil diameter into account, that suggests a magnification of 16x.
A common model like the Celestron SkyMaster 15x70 might therefore be suitable. The FOV is 4.4°, though I have some uncertainties regarding the off-axis image quality (any feedback?) and the potential “gymnastics” required to center each stars for comparison.
Since you aleady have the 8x32 and you want one of the 15x70s, pick one and go for it. The web is your friend as I have no experience with the models you mentioned.
Of course, your faint limit is somewhat determined by background sky brightness. I live in a mostly rural fairly dark sky area which helps getting faint estimates.
Use which ever bino is best for the brightness, comparison distances, etc. of the variable you are interested in estimating. Say Mira when bright use the 8x32s, when down near minimum use the 15x70s with stability support.
Have fun! Be advised that there will be “No milk today”, Hermans Hermits , if you spend all your milk money on new binoculars!
Any bins with an objective >50mm-ish will profit immensely from being mounted on a camera tripod. I used one of these for my first (schooldays) 10 x 50s then 20 x 70s. Most camera tripods will even allow you to rack out the axis to which the bins are attached (I used meccano) so they can ‘overhang’ which makes zenithal observing slightly more comfortable.
Interresting!
The issue with tripod-mounted setups, even with a fluid head,I think, is obviously the neck strain caused by observing at altitudes above 45°—especially once you reach a certain age!
To avoid this discomfort, a “Bino Body Mount” paired with a simple reclining chair seems better suited for “nomadic” observing compared to a parallelogram mount. This setup is very stable, except for one thing: heartbeats. I’m not sure yet if that slight vibration is a hindrance for magnitude estimation.
In fact, my main concern is whether the field quality of “standard” achromatic BaK-4 FMC binoculars (the affordable 15x70 type, like Orion in the US??) is adequate for variable star work. Also, out of curiosity: what percentage of “sharp” field is considered acceptable? For exemple, is 50% enough ? Google is stingy with information on this.
Some time ago I asked vereteran AAVSO observer Peter Williams (sadly, deceased earlier this year) about his extensive use of large binos (15x70 or 20x80, if I recall) for visual observing. He used an ordinary camera tripod with the legs shortened. In a seated position, he steadied the setup by supporting the tripod over his legs and held it in place. He sent me a photo which I thought looked a bit awkward but his large number of bino observations clearly demonstrate that this worked for him.
Thank you very much for this feedback.
I assume that the main advantage of Peter’s system, if I understood correctly, was that it prevented his knees or feet from being obstructed by or hitting the legs of a tripod conventionally set up on the ground!
I’ve never tried Peter’s setup but I assume that it allowed him to move the tripod + binos around while supporting the tripod on his legs for stability. Just my thoughts, but I can’t be sure. Requires experimentation.
If the binoculars only have 50% “sharp” FOV it would then depend on your usable FOV, right.
The 15x70s mentioned originally would have a 2-deg. usable FOV. Is that sufficient for your observing program stars? If you have the ability of centering comp, contering var, centering comp as was mentioned and get good visual/binocular estimates compared to others then OK!
Even cheap binos should have better usable FOVs that 50%.
I have a pair of Celstron 7x50 Cometrons but hang if I have been able to find them. They date from the 1990s I think. The objectives and eyepieces have a reddish tint, ruby coating, according to the google. They work(ed) just fine.
OK, you’re confirming the obvious. So, since centering is recommended by the manual, the concern over the quality of the usable field of view (FOV) becomes pretty secondary.
Except that if the sharp FOV is wide, you don’t need to center things strictly perfectly (putting aside other precautions needed due to eye imperfections).
I would have loved to hear some feedback on the centering + brightness memory method, because it’s a mental exercise that, I imagine, gets trickier as soon as a Comp star leaves the FOV relative to the target—which is more likely to happen with 15x70 binoculars than with 7x50s or 8x32s.
Anyway, that being said, as a first step, it’s time for me to get started with naked-eye observing using my 8x32s
Exit pupil larger than your eyes pupil is an image brightness issue, not a major problem esp.
, leaning over your vehicle, etc. methods. Some kind of support can be useful esp. when viewing up near the zenith… sounds like you have that part covered.
, if you spend all your milk money on new binoculars!