70mm F6 doublet airy disk and rings question

[notFritzArgelander]

It occurs to me that there might be a risk in revising the existing baffling scheme to get rid of vignetting. Some economical designs deliberately introduce baffles that vignette the objective to conceal optical defects in the objectives. So I'd advise, as a caution, preserving the existing baffles for reuse in case you don't like the outcome. Flocking or repainting the focus tube can be done without risk.

[realflow100]

Without the baffles at all. the image seems about the same sharpness and airy disks and diffraction rings look the same. just very low contrast as well as more potential for glare and flares to appear. sorta equivalent to raising the black point too high in an image editing software (Making blacks become a shade of grey. and mid tones more washed out)?

[j.gardavsky]

Hello realflow,

I have been following your tests and efforts with a great interest.
You have alreedy been given an excellent advice by not_Fritz and John, and here are just some notes from my similar efforts in past.

1. The scattering of light in the OTA can be checked, and a better baffling or flocking may become necessary. The free aperture of the focuser plays also a role.
viewtopic.php?p=166675#p166675

2. Quite a lot of scattering emerges often on the walls around the zenith diagonal mirror, and there is an easy way to reduce it,
viewtopic.php?f=65&t=20119&p=166673#p166673

3. Testing the Airy diffraction pattern (star test) is important, and it makes sense especially when done through the bandpass filters, as you eliminate the chromaticity blur.
The first diffraction ring should be very faint on the wavelength of the Fraunhofer spectral line e (545nm), it should be brighter on H-Alpha, and even more brighter on H-Gamma. I have been making these tests with the bandpass filters Baader Solar Continuum (10nm bandpass around 540nm), and with the 470nm bandpass color filter. The H-Beta and H-Alpha filters are an alternative.
On the H-Alpha and H-Beta you will see brighter 1st diffraction rings coming from the zonal error (chromatic spherical aberration) of the refractor optics.

Regarding the effect of the aberrations on the Airy pattern, there is a helpful look up graphics,

from Professor Gross' lecture DA COS 12, IAP University in Jena.

4. The Cassini Division on Saturn is a very important test, both on the telescope optics and on the eyepieces, revealing the differences among the eyepieces.
Years ago, the old discontinued Leitz Periplan GF 25x (f=10mm) has offered at the magnification of 75x on my 6" F/5 achro the best visiblity and contrast of the Cassini Division, showing black arc, and not just grey.

5. Generally, the premium microscope eyepieces, barlowed if necessary, are very well suited for testing the telescopes.
And even better suited are the apochromatic planar corrected microscope objectives mounted in the focuser, when the tests are done through a projective eyepiece on a camera sensor, or directly on the camera sensor)*.
)* This applies to the "finite corrected microscope eyeieces.

Best,
JG

[realflow100]

my camera and scope combination gives me about 2.11 arc seconds per pixel
with a 70mm F6 aperture. 420mm focal length achromat telescope
and canon SL1/100D 15MP camera
What should the expected star size/image be like?
So far my stars seem to be very much around 4 to 5 pixels on the very smallest dimmest stars I could find in my polaris photo. or wider and very "round" looking. like a lot of anti-aliasing applied to the stars. they don't look super crisp or blocky.
even the slightest bit brighter stars are at least 6 to 8 pixels across. and still very round looking.
i'm not sure if its my scopes optics. or the atmosphere/seeing. or something else.
double double is detectably split with an eyepiece visually.

[notFritzArgelander]

my camera and scope combination gives me about 2.11 arc seconds per pixel
with a 70mm F6 aperture. 420mm focal length achromat telescope
and canon SL1/100D 15MP camera
What should the expected star size/image be like?
So far my stars seem to be very much around 4 to 5 pixels on the very smallest dimmest stars I could find in my polaris photo. or wider and very "round" looking. like a lot of anti-aliasing applied to the stars. they don't look super crisp or blocky.
even the slightest bit brighter stars are at least 6 to 8 pixels across. and still very round looking.
i'm not sure if its my scopes optics. or the atmosphere/seeing. or something else.
double double is detectably split with an eyepiece visually.

I think that there might be vibration in the mount that is smearing your star images. Photographically your images indicate (2 arc sec per pixel, 4 pixels across) an 8 arc sec circle of confusion. The visual splitting of epsilon Lyrae would not be possible. Since you've reported that, I'd think about the mount.....

[realflow100]

Was on a fairly rock solid sturdy tripod untracked. pointing at polaris. I shot a bunch of very short exposures and stacked them in DSS
so its a photo made up of about 9 minutes worth of 10 second long frames
stars at the edges would have some field rotation. but when stacked they should be fine
the stars in the very center should be as sharp as possible.
There wasn't any trailing visible in any of the images
stars looked fine in individual frames.
I threw out the worst 1% outlier frames which had very clear obvious shake and blur.
the rest had stars indistinguishable from each other. just very slight differences from atmosphere/turbulence.

here is 2 images of very short 1/4 shutter speed photos showing the smallest detectable stars in the image. at iso 12800
theres a lot of noise but they are both actually stars.
Theres a lot of atmosphere turbulence in the first image
the first image is a star off to the right of the frame. and the second is close to center of the frame
very faint stars. 100% scale crop. no rescaling

[mikemarotta]

ive very faintly seen andromeda naked eye in averted vision 1 or 2 times. but it took over 20 minutes of looking in that area back and forth over and over before I finally confirmed it. and I had to look quite far away from it before it was even detectable for brief moments

Yes, I find the same sort of thing that I can locate a faint object naked eye and then have a hard time with the instrument. Similarly with binocular versus telescope: see it in the bino and search and search in the telescope.

Never seen orion nebula once though naked eye. not even a tiny hint of it.

I think that you have to be in really good skies to see the Orion Nebula (Messier 42) naked eye as anything other than maybe some haze around a star and then only because we know what we are looking at. It was never recorded in ancient times. '

Realize that Messier 7 is called the Ptolemy Cluster because it is recorded as a "cloud" in the Almagest of 100 AD. Similarly, about 900 AD the Persian astronomer Abd al-Rahman al-Sufi put what we call the Andromeda Galaxy on his map, again, as a "permanent cloud." When Galileo was first using his telescopes, Messier 44 "The Beehive" was called "The Manger" (Praesepe) and was only seven stars, again, naked eye, of course, though also noted as a cloud by Ptolemy. Galileo found it to be a cluster of 30. (Wikipedia says 40.)

Anyway, the Orion Nebula is definitely a sight for some kind of instrument, not naked eye.

its very hard to see orion nebula even in my telescope with a UHC filter. just a faint fuzzy featurless glowy fuzz with some stars in and around it. usually i can't even detect the wings in the nebula. and the dust piller in the core is also very hard to notice. ... I need at least 40x with my telescope before it even begins to show the wings or dust piller.

That's pretty much what it looks like to me, even in an 8-inch Schmidt-Cassegrain. "... just a faint fuzzy featurless glowy fuzz with some stars in and around it." I think that you might be able to catch The Trapezium. It took me several nights over several months to understand what I was looking at. Then, I saw it. Anyway, those other stunning details only come out in photographs and television shows. You do see more if you know more about what you are looking at and you seem to have a good understanding of the nebula area.

I can only detect those things in averted vision for brief fleeting moments over a long observing session.

Yes, it is clear that you spend a lot of hard-earned time at the eyepiece. I admire your tenacity. I have less patience.

I also tried splitting polaris but wasn't able to see its companion star visually. Too dim I think. could be light pollution. it was detected in a photograph though.

It is not so much the light pollution (though that is always a problem) as it is the aperture of your telescope. 70mm is not enough. I have noted it easily with an 8-inch Schmidt-Cassegrain that I borrowed from the local club, but not my 4-inch (102mm) refractor, at 105X and 185X. I did note it when observing a 4.5-inch (115 mm) refractor, though. So it is a matter of how much light you gather, not what magnification you use. At least, that's my experience.

Again, thanks for your reports.

I got a new mount for my 70mm and did some observing last week. More on that later.

Best Regards (and clear skies!)

Mike M.

[realflow100]

I can see Trapezium at somewhat high magnifications (looks like 4 stars. not very distinct but there)
though theres no detail in the nebula there. still just a faint fuzzy glow.