Three Telescopes in a Soupy Sky

[John Donne]

October 18 2021
Oakwood, Illinois

I deployed 3 refractor telescopes this evening.
Two skywatcher ED telescopes and one Orion ED telescope.

The Central Illinois sky was mediocre.
This Sky condition presented the12.5 days old moon with a 2 degree extended bright halo and the stars as moderately scintillating points of light.

I have a 72mm f5.8 ED Skywatcher refractor, a 100mm f9 ED Skywatcher refractor, and a 110mm f6 ED Orion refractor set up in the side yard.

Views of Jupiter:

72mm f5.8 mounting my 6mm KK ortho and TV 2X barlow at 140X.
I was surprised that this little telescope in this sky would sustain this level of magnification.
Very clear equatorial banding with transient southern hemisphere banding. Northern Hemisphere less defined.
Some faint color is also revealed.
Jupiter's Moons reveal crisp.

110 f5.8 ED Orion mounted with 4mm KK Ortho at 160X.

At this magnification I could not get a crisp focus.
Dropped in my 6mm KK for 110X. This improved the view some but still not as crisp as the 72 f5.8.
It is interesting and surprising to me that the 110 f6 views were inferior at this relative magnification. Was this serendipity or is their another reason for this ?

100 f9 ED Skywatcher mounted with 6mm KK and then 7mm Pentax.
or here is a best res image


Jupiter was sharp as the 72ED f5.8 but the color reveal in the 72ED f5.8 was not there in the 100mm f9.

The moon

I then mounted my Nikon D810 in both the 72 f5.8 and the 100 f9.

I do not know how to calculate the magnification of a camera mounted directly to the diagonal to the diagonal so I am not sure what the magnification is for the imaging.

The 72 mm telescope of course had a much smaller image but it was crisp and clear.
The 100 mm F9 scaled wonderfully with 2X and 3X Barlow. I have posted the 2X image here.
viewtopic.php?style=1&f=67&t=21237

The 72mm f5.8 was tricky for the camera. It took me some fiddling to get the correct spacing to focus. This scope has very short focuser movement.

[helicon]

Great report and congrats Mark on winning the VROD for the day!

[John Donne]

Thank you Michael.
It was an interesting compare.

[John Baars]

Congratulations on the VROD!
Thanks for your comparison.
Have you figured out why there was a rather big difference?

Maybe it is my lack of knowledge of the language but I haven't found out what you exactly mean with this quote:

the color reveal in the 72ED f5.8 was not there in the 100mm f9.

Was there too much ( false?) color seen in the 72ED, but was the image in the 100 f9 right?
Or was the image in the 100f9 too pale and the image in the 72ED right?

[Makuser]

Hi Mark. A very nice trio of refractor telescopes used in your session. And despite the soupy sky you had some great views of Jupiter and the Moon. Thanks for your excellent report Mark and congratulations on receiving the TSS VROD Award today.

[Juno16]

Great report Mark!

Man, the Skywatcher 72 sounds like winner!

A fine and very sharp lunar image. Congratulation on the VROD!

[John Donne]

Congratulations on the VROD!
Thanks for your comparison.
Have you figured out why there was a rather big difference?

Maybe it is my lack of knowledge of the language but I haven't found out what you exactly mean with this quote:

the color reveal in the 72ED f5.8 was not there in the 100mm f9.

Was there too much ( false?) color seen in the 72ED, but was the image in the 100 f9 right?
Or was the image in the 100f9 too pale and the image in the 72ED right?

Yes John and thank you for your question.
I apologize for that confusing sentence.

The 100mm f9 did not show as much planetary color as the 72mm f5.8. I think it is just less light coming to my eye with the f9, but the other quality of the view was equal.

There was no false color in either of these two scopes. The view was quit good on that point. And the equatorials were well defined in both scopes.

I was surprised that the Orion110 f6 did not out perform the others.

[Bigzmey]

Congrats on VROD Mark! Sounds like you made the most out of mediocre conditions Mark.

I was surprised that the Orion110 f6 did not out perform the others.

Larger aperture should deliver better resolution, but only if seeing is good enough. 80mm scope has resolving power of ~1.4 arc*sec, if seeing is worse then that, larger scope will not provide any better resolution, but could be in fact counterproductive since it will produce brighter planetary image with extra glare washing off the details.

[WilliamPaolini]

Hi John. Some interesting results you got there. If my summary is correct, basically the 72mm and 100mm both had crisp views while the 110mm did not.

I can understand why the 110mm at 160x might not have a very crisp view as it was operating at the most magnification and most magnification per inch aperture of all the scopes (0.44mm exit pupil) and also has the highest resolution of the the three. So those two factors can easily make it perform less well in a given seeing condition compared to the others. Another factor that could contribute is with the slightly larger aperture and glass mass, it may not have been fully acclimated to the outdoor temps yet so some residual thermals and/or residual Spherical Abberation in the unacclimated lens may have been at work.

As far as you getting more "color" detection on Jupiter with the 72mm vs. the 100mm on the surface seems a bit odd. But given that it was not giving a good view and the color you saw in Jupiter you describes as "faint", could very well be explained by it not giving as clear of a view possibly. Another factor that might have been at work is that since the exit pupil was brighter in the 100mm than in the 72mm at the magnifications you were using, and you characterized the color as being "faint", that the brighter regions on the planet thru the 100mm scope could have washed-out what little color there was to be seen. So just depending on factors like that, it is not always the case that the generalization that a larger exit pupil and brighter view will show colors best.

Btw, was the 100mm as crisp with either the 6mm or 7mm eyepiece?

If you repeat the experiment, give the 110mm more acclimation time. I would also encourage that if you are using a Barlow with one scope, that you use it with all the scopes choosing longer focal length eyepieces so the magnifications are leveled between them all.

Given that you are using 3 different apertures differences in the views should be expected. If you level the magnifications then they will all be operating at differing exit pupils so brightness and contrast will be different between the three and those factors themselves can lead to views looking more crisp or less crisp. If you level the exit pupils so they are all the same brightness, then the image scale is different and the ones with the larger image scale (and magnification) may exceed the seeing ceiling and not show as crisply. So it is a difficult thing to properly interpret the whys behind what one is seeing when using disparate apertures. I personally like to level the magnifications when using different aperture scopes. And when I do that I choose a magnification that is less than what the largest aperture telescope can easily achieve clearly so that way I am not nearing the limit of what the atmosphere will allow for all the scopes.

Just FYI, given your experiment here is how the exit pupil and resolution parameters fell out:

72mm = 1.60 arcsec resolution
100mm = 1.16 arcsec resolution
110mm = 1.05 arcsec resolution

110mm @ 160x = 0.44mm Exit Pupil
72mm @ 140x = 0.50mm Exit Pupil *crisp view*
100mm @ 150x = 0.67mm Exit Pupil *crisp view* ??
100mm @ 129x = 0.78mm Exit Pupil *crisp view*
110mm @ 110x = 1.03mm Exit Pupil

Note1 - depending on the seeing on a given evening, it is not unusual for a higher resolution scope to show poorly compared to a lower resolution scope when both are at the same magnification since the higher resolution scope will be able to detect impacts from seeing more readily since it is resolving smaller details. This is why I feel it is important to not use a magnification on any of the scopes that exceeds what the largest aperture can *easily* use for a crisp view (*easily* because I do not want them to be operating near the limit of what the current seeing will allow since seeing fluctuates, so want to stay well within the limit).

Note2 - my rationale on using a Barlow in all if needing it in one is that it better levels the optical train for all the scopes, and given that some of your scopes are relatively fast focal ratios, especially for an Abbe design, the central spots produced by the Abbe with Barlow will probably be tighter than the Abbes without Barlow and of course the off-axis will be better as well.

[John Donne]

@WilliamPaolini.

Thank you Bill for this thorough response.

The exit pupil thing is a detail I forgot about when I set this comparison up in my mind. You are spot on as you well know.
Other details you mention are equally valuable to me.
All three scopes are kept at outdoor ambient so that is probably not a factor in this case.

I will do this again...more carefully.
Thank you Bill !

[WilliamPaolini]

Have you done a daytime outdoor test with the 3 scopes? If not you should try that as may show interesting results. For that test make sure to point them at a target that is relatively close, say less than 25 meters or yards away as this preserves the contrast to what you normally expect when viewing naked eye. Further and the contrast is visibly reduced from the extra atmosphere. Point it at something familiar to you like flowers or tree bark or other natural objects that will have varying colors and surface features. Setting all scopes near 150x should be telling. If the 110 is still an under performer in that test then likely its collimation may be off, or it may not have a good figure on the lens. If it does come up short try masking it to 100mm so can directly compare to your 100 f/9, and then mask to 72mm so can directly compare to your 72 scope. Those tests may give you some better insight into what is going on with the 110mm if anything.

[John Donne]

@WilliamPaolini
I will definitely do this day time test Bill.
I will report back.
Thank you.