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Old 14-07-2021, 04:53 PM
MichaelWB (Michael)
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C6 Question

Hi All.

I have a Celestron C6 which I bought 9 months ago to get back into astronomy after many years away, and I have been quite happy with it. Recently my neighbour bought a 10” GSO dob, so I took my C6 over to his place to compare scopes and eyepieces on Jupiter and Saturn that are nicely placed from his front verandah.

As both have a 1500mm f.l. It seemed like a good way to compare his standard plossls with the Meade SWA’s I got with my scope. He had seen Omega Centauri through his GSO with my 16mm Meade when he first picked it up and was very impressed, but we had never compared eyepieces to see if he might want to upgrade his.

Anyway, the upshot of this isn’t about the eyepieces in the end. It was the fact that his scope gave noticeably crisper images than mine no matter which eyepiece we used. Now, I’m wondering if this is because of the difference in diameter, after all, his mirror has nearly 3 times the light gathering of mine, or might there be something more to it. Both scopes seem to be collimated well.

As I said, I am still happy with the scope, it’s the perfect grab and go, but this discovery has taken the shine off it a bit.

Does anyone have any ideas or suggestions, or is it just the nature of the beast?

Thanks, Michael
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Old 14-07-2021, 06:30 PM
Dave882 (David)
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Hi Michael
As they say- “aperture is king!” - well at least that’s true for visual astronomy. All things being equal, a 10” scope will always bring brighter and better resolution viewing than a 6” scope.
But, they also say “the best scope is the one you use” and I reckon there’s plenty of big scopes tucked in the back of garages that hardly see the sky anymore because they’re too hard to move/transport.
You can still have plenty of fun with a c6!!
Clear skys!
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Old 14-07-2021, 06:55 PM
astro744
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Could be the 10” was at thermal equilibrium and the 6” not since you said images in the larger scope were sharp. Your ‘scope should also be capable of sharp images but compared to the 10” will be noticeably dimmer (although ample brightness at say 150x for planets).
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Old 16-07-2021, 01:00 PM
MichaelWB (Michael)
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Thanks for the feedback guys. We are going to have another night soon and I will have another go at it, with what has been said in mind. Also to do the original eyepiece comparison. And yes, I am still very happy the C6. As you say, the best 'scope is the one you use, I can take it out and be observing in 2 or 3 minutes.
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Old 16-07-2021, 01:05 PM
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Tulloch (Andrew)
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Aperture rules, especially when looking at detail on the planets.

Just go out and buy a 16" Dob - then you can show him who has the better scope
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Old 16-07-2021, 01:50 PM
N1 (Mirko)
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The C6's image should be crisp up to 150x at the very least. If not, there's an issue with it. Thermal or collimation related, bad sample less likely. Up to that power, its aperture disadvantage is irrelevant.

Edit: I'll qualify that by saying it should be sharp. Crispness may be a function of brightness too, which the C6 does have a bit less of. But for its form factor, it's hard to beat.

Last edited by N1; 16-07-2021 at 02:05 PM.
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Old 16-07-2021, 03:30 PM
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The C6 has a central obstruction of 37% by diameter while the Dob is about 25%. A larger CO means less contrast plus a less stable view in so-so seeing. Unless the seeing is really good I'd expect the Dob to be crisper than the SCT.

Thermal issues might have been affecting the C6. Have you considered insulating it?

I'd also expect that the brighter images in the Dob might give the impression of being sharper simply because they're easier on the eye.
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Old 16-07-2021, 03:45 PM
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possible easy solution

Hello,

On one of your comments I noticed you said you could be setup and using the scope within a few minutes.

If the scope is stored at outside temperatures, like in a garage, you'd be fine. If you're taking the scope from inside to out, it will take time for a closed scope with 6" of glass to equalise to the ambient temperature.

Leave it outside under an eve or similar, wait till the temperature has dropped less than 2degrees in one hour.

Then try the optics, look at near zenith objects, that will reduce the amount of atmospheric distortion the optics have to deal with.


Steve
The dob with the open tube will cool a lot quicker, fan or no fan. The central obstruction of 37% is a big issue as mentioned, you have a 4" lens after that which is getting smallish.

Last edited by mura_gadi; 16-07-2021 at 03:58 PM.
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Old 18-07-2021, 03:07 AM
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Quote:
Originally Posted by MichaelWB View Post
Hi All.

I have a Celestron C6 which I bought 9 months ago to get back into astronomy after many years away, and I have been quite happy with it. Recently my neighbour bought a 10” GSO dob, so I took my C6 over to his place to compare scopes and eyepieces on Jupiter and Saturn that are nicely placed from his front verandah.

As both have a 1500mm f.l. It seemed like a good way to compare his standard plossls with the Meade SWA’s I got with my scope. He had seen Omega Centauri through his GSO with my 16mm Meade when he first picked it up and was very impressed, but we had never compared eyepieces to see if he might want to upgrade his.

Anyway, the upshot of this isn’t about the eyepieces in the end. It was the fact that his scope gave noticeably crisper images than mine no matter which eyepiece we used. Now, I’m wondering if this is because of the difference in diameter, after all, his mirror has nearly 3 times the light gathering of mine, or might there be something more to it. Both scopes seem to be collimated well.

As I said, I am still happy with the scope, it’s the perfect grab and go, but this discovery has taken the shine off it a bit.

Does anyone have any ideas or suggestions, or is it just the nature of the beast?

Thanks, Michael
One difference is the respective resolutions of the instruments.
The Dawes Limit for resolution on double stars is 114mm/Diameter in mm
That puts the closest double star resolution (which also relates to small details on Moon and planets) as 0.75" on the C6 and 0.45" on the 10"
Ultimately, if both scopes are collimated (you have checked yours, right?) and cooled, the 10" will always win in a 'revealing small details' contest.
No matter how world class a 6" refractor is, a good 16" will always blow away the 6" in planetary details.
I've often said that if you are really into lunar and planetary observing, you don't buy a refractor--you buy a large reflector.
Here is a site with images of the Moon taken through, mostly, an 18" scope:
http://higginsandsons.com/astro/
Resolution like that is simply not possible with refractors of amateur size.
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Old 18-07-2021, 08:30 AM
astro744
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The OP has a 6” SCT not a 6” refractor and his neighbour has a stock GSO 10” Newtonian, not a ‘good’ 16” or 18” Newtonian. (Nothing to say the SCT or GSO aren’t ‘good’).

A more interesting comparison would be a 6” f10 SCT vs a 6” f5 Newtonian vs a 6” f8 Newtonian. All have the same aperture but the SCT is a closed tube. The 6” f10 SCT has 37% obstruction (by diameter which refers to contrast), 6” f5 Newt has 30% and 6” f8 Newt has 20% obstruction.

Anything over 25% obstruction by diameter has detrimental effects on contrast when viewing visually and it is noticeable. However the effects of thermal instability is just as detrimental.

If the OP ever gets a chance I would recommend he compares a 4 or 5” APO refractor with a 6” SCT and see what appears ‘crisper’ to him. The views in the smaller refractor are stunning but to get the same magnification the view will be dimmer. Is there any more detail visible in the smaller aperture? The rules for determining resolution would say no. Is the view ‘crisper’; often it appears so.

Now to get the same magnification in all ‘scopes mentioned one would have to use different focal length eyepieces of equal quality on axis if tracking or across the more of the field if not tracking.
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Old 18-07-2021, 08:44 AM
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I only used the 6" refractor and 16" reflector as examples of scopes that could be compared. I guess I was unclear in using examples of expensive scopes to show the effects of aperture on resolution.

In the Dawes Limit example, I used the C6 and 10" newt.
I was merely pointing out that it doesn't matter how expensive the 6" scope is, a larger scope will resolve smaller details.

Now, if talking about the optics in the typical SCT versus the typical 10" Chinese newtonian, you could be comparing 1/2 wave optics with 1/6 wave optics. Or comparing a well cooled scope with one not yet cooled. Or an out-of-collimation scope with one well collimated (why I asked about collimation). There are other possibilities.

But, even if the 6" SCT were the best one ever made and the 10" dob an average GSO, the 10" aperture will still resolve smaller details on the moon and planets.
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Old 22-07-2021, 09:13 PM
MichaelWB (Michael)
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Hi everybody.

I am surprised and pleased with the number of responses from the IIS clan that my query generated.

There seem to be a number of votes for thermal equilibrium and relative apertures, and collimation gets an honourable mention too. I also like Morton’s thoughts on central obstruction.

The ‘scopes were both outside for about 45 minutes before we noticed the differences. The dob is definitely well collimated according to my laser collimator, though I can’t be sure about the C6, but out of focus stars are circular, and I believe that is a good sign. I haven’t yet got into the finer details of SCT collimation. During the observing session we were able to see the Cassini division and were able to see an eclipse/transit (not sure of the correct nomenclature) on Jupiter's surface by one of its moons with the C6 (one of several astronomical firsts for me with this ‘scope).

Andrew, you might be pleased to know that after comparing ‘scopes we walked back to my place and dragged out my recently completed 14” f7.2 (I picked up the mirror and diagonal on ISS classifieds without any real idea of its quality or provenance, but I got lucky). we had some great views of Saturn, got a good look at the eclipse, but especially seeing detail in Jupiter’s cloud bands for the first time. Yes, it didn’t take long for me to get aperture fever.

My neighbour is quite taken with the C6 and wants one for himself. He regularly works in the country and has access to pristine rural skies, but the dob is a bit too big to cart around and set up after a day's work. He also loves the slow motion controls on the alt/az mount vs. the nudge, nudge of the dob.

Anyway, thanks for the feedback.

Michael
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Old 22-07-2021, 10:11 PM
MichaelWB (Michael)
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BTW, can anyone give me step by step instruction on how to add a photo to posts. I see I am not the only one who can't figure it out, because I occasionally see this mentioned when I look through the classifieds. Please keep it simple and don't assume that I understand computer jargon. Maybe I should direct this to the moderators, but I'll see how it goes for now.

Thanks, Michael.
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Old 22-07-2021, 10:59 PM
raymo
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When you have completed your text in your post, scroll down the page and
you will see a little box containing the words Manage Attachments.
Click on the that box and a window will open. You will see on the left side a column of Choose Files. Click on the top one and you will get shown all the locations on your PC where there are files that you can download. Use the slider to find the location you want, such as Desktop. Click on the location you want[such as desktop].
Scroll down in the location and click on the file[image] you want. At lower right you will see box containing Upload. Click Upload and wait a few seconds,
then the details of the downloaded image will appear in blue at the bottom of the page.
You can repeat this by selecting the next Choose File down the column if you want to post several images.
The max file size is 200kb. If your image is 2Mb or less then IIS will
automatically downsize it.
When you have finished with your images a list of them will be shown near
the Manage Attachments box. You can click on any of them to view them
and decide whether they are okay for posting. This is worth doing, as some
images don't take kindly to downsizing.
raymo
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Old 23-07-2021, 10:57 PM
MichaelWB (Michael)
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Thanks Raymo.

I'm pretty sure I've got it. I tried a dummy run and previewed the image as you suggested. You were right, not all images are suitable. I will take another photo later and try again. I was trying the "insert image" icon and couldn't figure it out. Now I know how.

Thanks again.

Michael
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Old 24-07-2021, 10:25 AM
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Michael,

Just to reiterate what has already been said well... it's part expectations and part familiarity/understanding your scope.

The C6 is "only" 150mm aperture, and since resolution is proportional to aperture, that will limit the extent of accessible fine features, but also keep in perspective that the atmosphere will mostly limit even a small scope such as a C6 most of the time!

Good nights do come and go though, and a bigger scope will certainly show you more on a better night. But also don't be discouraged by theoretical limitations. Ask Morton about my C6 he may not recall a night at a star party looking at Saturn through my C6, at about 400x IIRC. Large CO and all. Of course, a bigger scope next to me would have shown more under the same conditions. That's just the laws of physics

The things to maximise your C6 have already been touched on... ensure thermal equilibrium, whether that's by letting your scope cool outside before use or by using an insulating jacket.

Collimation of an SCT is fundamental to getting the best out of it, and isn't hard once you've been practicing - certainly easier than a newtonian! Starting with a defocused star looking like a donut... the secondary shadow (centre of the donut) should be centered. Then increase the magnification with a shorter focal length eyepiece. Then progressively start shrinking the donut, collapsing towards focus. Stop regularly and watch... as you get closer to focus you should start seeing rings around a central bright spot. Adjust the collimation screws appropriately to ensure the rings are concentric about the bright spot.

As you get close to focus, the rings should collapse equally in all directions into the focussed star, and at this point you will probably see the energy ripples of the Airy disk. Note that that as you approach good collimation, the adjustments needed on the screws are tiny, and your ability discern will be affected by the atmospheric turbulence (seeing)... if the seeing is not good, the rings will be jumping around all over the place. Try again another night!

Oh, and if you're unsure which screw to turn... the screws are 120 degrees apart...what I do is to stick my finger in front of the scope in the orientation of one of the screws to figure out which is which (this is the beauty of a small scope, it's all within arms reach!). What you're looking for is the direction in which the rings are offset. Once you've found the orientation that matches, adjust the screw a tiny amount. If it makes the offset worse, turn the screw the other way.

Keep in mind that you may need to adjust more than one screw to get the rings concentric...and after _any_ adjustment of the a collimation screw, always centre the star again in the FOV before making a subsequent adjustments. With your final collimation checks/adjustments, you want to be aiming for 300-400x magnification.

It may be hard to visualise, and it's certainly hard to describe, but hands-on I hope it will make more sense.


Quote:
Originally Posted by MichaelWB View Post
Hi everybody.

I am surprised and pleased with the number of responses from the IIS clan that my query generated.

There seem to be a number of votes for thermal equilibrium and relative apertures, and collimation gets an honourable mention too. I also like Morton’s thoughts on central obstruction.

The ‘scopes were both outside for about 45 minutes before we noticed the differences. The dob is definitely well collimated according to my laser collimator, though I can’t be sure about the C6, but out of focus stars are circular, and I believe that is a good sign. I haven’t yet got into the finer details of SCT collimation. During the observing session we were able to see the Cassini division and were able to see an eclipse/transit (not sure of the correct nomenclature) on Jupiter's surface by one of its moons with the C6 (one of several astronomical firsts for me with this ‘scope).

Andrew, you might be pleased to know that after comparing ‘scopes we walked back to my place and dragged out my recently completed 14” f7.2 (I picked up the mirror and diagonal on ISS classifieds without any real idea of its quality or provenance, but I got lucky). we had some great views of Saturn, got a good look at the eclipse, but especially seeing detail in Jupiter’s cloud bands for the first time. Yes, it didn’t take long for me to get aperture fever.

My neighbour is quite taken with the C6 and wants one for himself. He regularly works in the country and has access to pristine rural skies, but the dob is a bit too big to cart around and set up after a day's work. He also loves the slow motion controls on the alt/az mount vs. the nudge, nudge of the dob.

Anyway, thanks for the feedback.

Michael
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Old 28-07-2021, 06:02 PM
MichaelWB (Michael)
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This is the 14" f7.2 I built recently. It seems pretty good so far. It's a bit of a beast to move, over 60kg, buy the wheels make it manageable. It is not too hard to disassemble and reassemble, but it's not something I would contemplate doing on a regular basis.
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Old 28-07-2021, 06:08 PM
MichaelWB (Michael)
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Re all your advice on the C6. I will take a serious look at the collimation as soon a I get some clear skies, but it's been pretty average here on the Central Coast for some weeks now.

Once again, thanks to everyone for your advice.

Michael.
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Old 28-07-2021, 07:56 PM
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Michael, I forgot to mention that I have found that the images seem to withstand downsizing better if you downsize them yourself before downloading them to IIS.
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