46 replies to this topic

[REC]

Last night we had pretty decent sky, clear and cold. The transparency was excellent and the seeing condition was good. I took my 10" Dob out 3 hours to cool off and ran the fan. Then an hour before dark I took out my C102 f/1000mm 9.8. I like to look at Jupiter just after the sun sets before it gets to bright. Easily saw two bands and all 4 moons.

 

So now dark out, as good as it's going to be for a 1st. Qtr. moon I started out looking at some bright star clusters first before viewing the bright moon with both scopes. Then grabbed my nebula filter to gawk at M42 with the Dob and a 28mm SWA EP. I could see all 4 of the Trap stars and the nebula pretty much filled the FOV.

 

Ok, time for the moon and Jupiter. I had a 8-24mm zoom in the C102 with a Baader M&S filter and went for Jupiter first. Using a zoom to start out with gives me an idea how much power I can use to get clean views of the planet depending on the seeing conditions. I was getting nice sharp pin points of the moons in the lower end of the zoom's range from about 80-100x. I could see 3-4 bands in and out with 2 always there. So not great seeing, but maybe good at 3/5 I would estimate.

 

So now I'm over to the 10" Dob f/1250mm and I throw in my 13mmT6i for 96x. Jupiter's disc was ok, but I don't think the moons are that sharp? I next try my 8.8mm for 138x as usually that's ok when the seeing is just good. I check out the moon and see pretty good detail in the craters and mountains. I was not getting and wavy air that I usually see when the seeing is poor. I go back to Jupiter and try for best focus on the moons and when I get them sharp I just wait for the detail on the planet to come and go. I kept fiddling with the focuser and I just can't get those moons pin point. I go back over the the refractor to compare, yep the moons are sharp pin point there. I would describe the moons in the Dob as "bloated".

 

So I ask myself this simple question....why is my 4" $60 Achro outperforming the $500 10" Dob on Jupiter? The scope was cooled off enough I thought and the collimation looked good before I took it out, so I just don't get it. BTW, I use a Hotech laser for the collimation. It fits very snugly in the 2" focuser, so no slop there. If I get the center beam on the primary target when adjusting the secondary screws and then use the primary screws to center the laser dot on the laser bulls eye target, I should be good to go?

 

Anyway, this is not the first time this has happened to me, so I wonder if I am missing something or it's just the limitation of the scope, which is a Apetura model. BTW, the DSO's seem to be ok most of the time and 10 inches of light gathering shows it over the C102.

 

Thanks for any input...Bob

[Brian Carter]

Unless your mirror is absolute and total trash, the reason is cool down, collimation, or seeing. Sounds like you took care of cool down and Collimation so... Seeing.

Big scopes are able to resolve more, so they are subjectively more affected by poor seeing. I noticed this last night. I set up my 15" next to so nice Apo refractors, and stars looked better in their scopes. Seeing was exceptionally bad for me last night, stars were bloated little orbs over 150x, but they looked fine in the refractor. Peering at the moon, I didn't see that shimmering you describe, but defocusing a star I could see very very fast upper air movement.

Now I know my mirror is not trash, it's actually quite good and I've seen pinpoint stars at 300x in it and views of Jupiter that look like photographs. Last night, I couldn't even see the GRS. So on nights of bad seeing, a small scope will be subjectively better because it isn't big enough to resolve the poor seeing, at least not as well as a larger scope. That's normal.

I beat the pants of the refractors on M13 though, and the Leo triplet, and M104, and M51... Shall I continue the list?

Edited by Brian Carter, 29 March 2015 - 09:25 AM.

[csrlice12]

Was at the clubs public viewing night and the seeing was  terrible...luckily M42 was there.  Jupiter was two bars and four dots at 12.5mm and worse at higher mag with the 102XLT.  Was a good crowd there though, beautiful day out.

[Jon Isaacs]

 

So now I'm over to the 10" Dob f/1250mm and I throw in my 13mmT6i for 96x. Jupiter's disc was ok, but I don't think the moons are that sharp? I next try my 8.8mm for 138x as usually that's ok when the seeing is just good. I check out the moon and see pretty good detail in the craters and mountains. I was not getting and wavy air that I usually see when the seeing is poor. I go back to Jupiter and try for best focus on the moons and when I get them sharp I just wait for the detail on the planet to come and go. I kept fiddling with the focuser and I just can't get those moons pin point. I go back over the the refractor to compare, yep the moons are sharp pin point there. I would describe the moons in the Dob as "bloated".

 

This is how I see it:

 

The question is not whether the moons are sharp but rather how much detail is seen on the planet itself.  

 

The sharpness of the moons depends on a number of factors but I would not expect to see sharp moons at 142x, that's nearly a 2mm exit pupil..  If you want the moons to be disks, it will take greater magnification and more than likely much better seeing.  I would want the seeing to support 300x or so to see clean moons in a 10 inch. 

 

In any event, the refractor cannot resolve the disk of the moons, the central spurious Airy disk is considerably larger in diameter than the moons so what you are looking at is the diffraction disk. The Newtonian is capable of resolving the disks of at least two of the moons because it has a much smaller, tighter Airy disk, the refractor's disk is 2.5x larger in diameter. So when you are observing the moons with the Newtonian, you are asking more of it, there is more too see, both in terms of the detail but also in terms of defects in the seeing.  This disk is tighter and the image brighter, both these bring out the defects due to seeing. 

 

In my experience, I rarely see Jupiter's moons as clean round disks in a larger scope but I also see much more planetary detail than I do in a 4 inch refractor.  

 

Jon

 

 

 

 

 

 

[REC]

Unless your mirror is absolute and total trash, the reason is cool down, collimation, or seeing. Sounds like you took care of cool down and Collimation so... Seeing.

Big scopes are able to resolve more, so they are subjectively more affected by poor seeing. I noticed this last night. I set up my 15" next to so nice Apo refractors, and stars looked better in their scopes. Seeing was exceptionally bad for me last night, stars were bloated little orbs over 150x, but they looked fine in the refractor. Peering at the moon, I didn't see that shimmering you describe, but defocusing a star I could see very very fast upper air movement.

Now I know my mirror is not trash, it's actually quite good and I've seen pinpoint stars at 300x in it and views of Jupiter that look like photographs. Last night, I couldn't even see the GRS. So on nights of bad seeing, a small scope will be subjectively better because it isn't big enough to resolve the poor seeing, at least not as well as a larger scope. That's normal.

I beat the pants of the refractors on M13 though, and the Leo triplet, and M104, and M51... Shall I continue the list?

Brian, well the seeing may be it as you point out. Actually you are not that far from where I live in NC, so I'll bet you had the same sky conditions there as me. So in general, the larger aperture Dob is going to be more sensitive than the smaller refractor when seeing is not that good? Well in that case, I'm glad I have the option of the refractor for those nights.

 

Jon, I wasn't trying to see round discs on the moons, just trying to get the pin point like a star, which I saw in the refractor. I always thought that when viewing Jupiter and Saturn I would just get the best focus on one of the moons and let that be the gauge for seeing the best detail of the planet.

 

I'll keep all this in mind on my next night out, so thanks all for your feedback:)

[gene 4181]

 Rec, try wrapping your dob tube in neopreme insulation. the blanket type, 5/8 thick. cut out around the focuser and finder bases. I leave about 8in. at the bottom uncovered around the mirror cell. the thermal's from our bodies will mess with the view.  where I was last night, the view of Jupiter was  average , but the view of the moon was poor. the seeing was coming and going.  I can see the effects of body thermals when cozying up to the scope to view.  I was able to see the grs and the indentation around it in the belt.  north and south polar caps  were there but not defined. I use the moon's of Jupiter to get best focus, I was getting faint diffraction rings around the moons, but they kept breaking , it wasn't a good night.

Edited by gene 4181, 29 March 2015 - 11:06 AM.

[Brian Carter]

Unless your mirror is absolute and total trash, the reason is cool down, collimation, or seeing. Sounds like you took care of cool down and Collimation so... Seeing.

Big scopes are able to resolve more, so they are subjectively more affected by poor seeing. I noticed this last night. I set up my 15" next to so nice Apo refractors, and stars looked better in their scopes. Seeing was exceptionally bad for me last night, stars were bloated little orbs over 150x, but they looked fine in the refractor. Peering at the moon, I didn't see that shimmering you describe, but defocusing a star I could see very very fast upper air movement.

Now I know my mirror is not trash, it's actually quite good and I've seen pinpoint stars at 300x in it and views of Jupiter that look like photographs. Last night, I couldn't even see the GRS. So on nights of bad seeing, a small scope will be subjectively better because it isn't big enough to resolve the poor seeing, at least not as well as a larger scope. That's normal.

I beat the pants of the refractors on M13 though, and the Leo triplet, and M104, and M51... Shall I continue the list?

Brian, well the seeing may be it as you point out. Actually you are not that far from where I live in NC, so I'll bet you had the same sky conditions there as me. So in general, the larger aperture Dob is going to be more sensitive than the smaller refractor when seeing is not that good? Well in that case, I'm glad I have the option of the refractor for those nights.

Jon, I wasn't trying to see round discs on the moons, just trying to get the pin point like a star, which I saw in the refractor. I always thought that when viewing Jupiter and Saturn I would just get the best focus on one of the moons and let that be the gauge for seeing the best detail of the planet.

I'll keep all this in mind on my next night out, so thanks all for your feedback:)

It is important to note that you're not actually seeing more detail in the refractor, at least you shouldn't be able to if the issue is just seeing. You're just able to resolve poor seeing better in the bigger scope so it looks worse.

Let's put it this way: if the seeing blurs anything greater than 1" (actually this would be a good night for me), this is below the resolution limit of a 100mm scope, so essentially the views are perfect. But a 10" can resolve detail down to .5", so the views will be blurred, you will see the poor seeing.

Last night, the seeing was not 1", it was atrocious. The refractors were definitely affected and I could see it, I could just see it better in te 15". This meant that the views looked worse in the larger scope, but the amount of detail was actually not worse at all.

So if you're talkingn about the aesthetics, yeah, be glad you have the refractor option. This is totally valid and a good reason to have a smaller scope. A pretty view is one reason why all of us like looking through a scope. But the bigger scope will always show more detail, even in bad seeing. Despite the bad seeing last night, there were frequent seconds where it would settle down and I would get a quick glimpse of better detail that the refractors couldn't see. That's the advantage of aperture in bad seeing.

[Jon Isaacs]

 

Jon, I wasn't trying to see round discs on the moons, just trying to get the pin point like a star, which I saw in the refractor. I always thought that when viewing Jupiter and Saturn I would just get the best focus on one of the moons and let that be the gauge for seeing the best detail of the planet.

 

It's worth spending a moment and thinking about this:

 

This is about expectations... 

 

You shouldn't expect to see the moons as pinpoints a 10 inch scope because to a 10 inch scope, they are round disks.  To see them as round disks, you will need the seeing to support that sort of resolution and you will need to use sufficient magnification to resolve that level of detail.   

 

The reason they look like points in the refractor is that the refractor does not have the capability to resolve the disks.. when you see them as pinpoints, what you are seeing is the Airy Disk... To see the Airy disk in the Newtonian, you would need at least 250x.. Probably more.  But if the seeing were sufficient to see resolve the Airy disk of a star, it would be sufficient to see the disk of a moon of Jupiter as a disk...

 

The refractor draws it's images with larger pixels..

 

jon 

 

 

[sealcove]

I did a similar comparison last week between my 10" Apertura dob and a new to me Explore Scientific 5" achromatic.  I didn't do anything exhaustive or even consider what magnifications I was at while viewing.  I simply set out both scopes about 90 minutes in advance (the dob lives in an unheated garage) and did some observing using both scopes using the same three EP's (ES 8.8, 14, and 24).   It probably doesn't matter, but for the 10" I use a Glatter laser and tublug to collimate, and I had also recently pulled the scope apart, improved the center mark on the primary and made sure the secondary is optically centered (using the grid hologram attachment with the laser). I also had read that the primary mirror clips on these GSO scopes tend to come too tight, so I loosened each until you could just get a business card between the clip and mirror (a recommendation I have read here).   

 

The targets were primarily Jupiter, the crescent moon & M81/82:

 

For me, I was equally thrilled with the views from each scope.  With Jupiter, using the 10" I was picking up more disturbances from either seeing or tube currents, but overall the image was excellent and I could see more detail with the 10".  The Refractor did give you a sense of a slightly more crisp image at times, but the reflector with the same EP was a considerably higher magnification (for example, with the 14mm 59X for the 5" and 89X for the 10"). It was pretty much the same story looking at the moon, though I did find a need for a lunar filter with the 10" and I did not with 5" (that seems to make sense to me).

 

With M81 & M82 it was no contest, the 10" produced a much more impressive image of the two galaxies with considerably more detail to my eye.  That said, the 5" refractor has a wider field of view with the same EP, so that was a plus and I was still getting some nice views of the galaxies with it.  

 

I came away from the session feeling happy with my pair of scopes.  The intent of the 5" was to have something that was a bit more compact to travel with (or to haul to harder to get to viewing spots), but that still has reasonable light gathering ability.  The only problem with my comparison is that I now realize that my 5" collimation was not spot on, so when I try it again I may have a different impression (it wasn't that far out, so probably not). 

[REC]

 

 

Jon, I wasn't trying to see round discs on the moons, just trying to get the pin point like a star, which I saw in the refractor. I always thought that when viewing Jupiter and Saturn I would just get the best focus on one of the moons and let that be the gauge for seeing the best detail of the planet.

 

It's worth spending a moment and thinking about this:

 

This is about expectations... 

 

You shouldn't expect to see the moons as pinpoints a 10 inch scope because to a 10 inch scope, they are round disks.  To see them as round disks, you will need the seeing to support that sort of resolution and you will need to use sufficient magnification to resolve that level of detail.   

 

The reason they look like points in the refractor is that the refractor does not have the capability to resolve the disks.. when you see them as pinpoints, what you are seeing is the Airy Disk... To see the Airy disk in the Newtonian, you would need at least 250x.. Probably more.  But if the seeing were sufficient to see resolve the Airy disk of a star, it would be sufficient to see the disk of a moon of Jupiter as a disk...

 

The refractor draws it's images with larger pixels..

 

jon 

 

That's interesting, never would have thought the moons would show up at disc's. I have heard this reported and in photo's before, but thought that was just visible through much larger scopes than mine. Makes sense about the smaller scope not being able to see discs and just star like points of light.

[REC]

 

 

Unless your mirror is absolute and total trash, the reason is cool down, collimation, or seeing. Sounds like you took care of cool down and Collimation so... Seeing.

Big scopes are able to resolve more, so they are subjectively more affected by poor seeing. I noticed this last night. I set up my 15" next to so nice Apo refractors, and stars looked better in their scopes. Seeing was exceptionally bad for me last night, stars were bloated little orbs over 150x, but they looked fine in the refractor. Peering at the moon, I didn't see that shimmering you describe, but defocusing a star I could see very very fast upper air movement.

Now I know my mirror is not trash, it's actually quite good and I've seen pinpoint stars at 300x in it and views of Jupiter that look like photographs. Last night, I couldn't even see the GRS. So on nights of bad seeing, a small scope will be subjectively better because it isn't big enough to resolve the poor seeing, at least not as well as a larger scope. That's normal.

I beat the pants of the refractors on M13 though, and the Leo triplet, and M104, and M51... Shall I continue the list?

Brian, well the seeing may be it as you point out. Actually you are not that far from where I live in NC, so I'll bet you had the same sky conditions there as me. So in general, the larger aperture Dob is going to be more sensitive than the smaller refractor when seeing is not that good? Well in that case, I'm glad I have the option of the refractor for those nights.

Jon, I wasn't trying to see round discs on the moons, just trying to get the pin point like a star, which I saw in the refractor. I always thought that when viewing Jupiter and Saturn I would just get the best focus on one of the moons and let that be the gauge for seeing the best detail of the planet.

I'll keep all this in mind on my next night out, so thanks all for your feedback:)

It is important to note that you're not actually seeing more detail in the refractor, at least you shouldn't be able to if the issue is just seeing. You're just able to resolve poor seeing better in the bigger scope so it looks worse.

Let's put it this way: if the seeing blurs anything greater than 1" (actually this would be a good night for me), this is below the resolution limit of a 100mm scope, so essentially the views are perfect. But a 10" can resolve detail down to .5", so the views will be blurred, you will see the poor seeing.

Last night, the seeing was not 1", it was atrocious. The refractors were definitely affected and I could see it, I could just see it better in te 15". This meant that the views looked worse in the larger scope, but the amount of detail was actually not worse at all.

So if you're talkingn about the aesthetics, yeah, be glad you have the refractor option. This is totally valid and a good reason to have a smaller scope. A pretty view is one reason why all of us like looking through a scope. But the bigger scope will always show more detail, even in bad seeing. Despite the bad seeing last night, there were frequent seconds where it would settle down and I would get a quick glimpse of better detail that the refractors couldn't see. That's the advantage of aperture in bad seeing.

 

Interesting perspective about seeing in one size scope vs. a larger one. BTW, on my Clear Sky Chart for last night, it was solid dark blue for transparency but a light blue for seeing which meant average I think. Would that kind of seeing also effect stars as well as planets? I remember looking at the 4 Trap stars in M42 and they where not that pin point as well. I guess that's why I couldn't see the "F" star in the 10"?

[Brian Carter]

My seeing was also light blue, that is definitely below average.

Seeing affects everything, smears each point of light over a wider area. For stars this means that instead of points they are orbs. And faint stars smear to the point where they can disappear.

[areyoukiddingme]

I agree that seeing sounds like the stronger culprit, but could collimation be shifting as the scope is being moved around? 

[rowdy388]

Just curious what collimation tools are you using and are you confident it is
correct? You seem to have lots of very nice playthings in your collection.
Dave Y

[beatlejuice]

I agree that seeing sounds like the stronger culprit, but could collimation be shifting as the scope is being moved around? 

 

Sure could,  enough so that the view of fine detail could be compromised a little.   Best to check collimation if you plan on viewing a planet for a while.

 

Eric

[orion61]

 

 

So now I'm over to the 10" Dob f/1250mm and I throw in my 13mmT6i for 96x. Jupiter's disc was ok, but I don't think the moons are that sharp? I next try my 8.8mm for 138x as usually that's ok when the seeing is just good. I check out the moon and see pretty good detail in the craters and mountains. I was not getting and wavy air that I usually see when the seeing is poor. I go back to Jupiter and try for best focus on the moons and when I get them sharp I just wait for the detail on the planet to come and go. I kept fiddling with the focuser and I just can't get those moons pin point. I go back over the the refractor to compare, yep the moons are sharp pin point there. I would describe the moons in the Dob as "bloated".

 

This is how I see it:

 

The question is not whether the moons are sharp but rather how much detail is seen on the planet itself.  

 

The sharpness of the moons depends on a number of factors but I would not expect to see sharp moons at 142x, that's nearly a 2mm exit pupil..  If you want the moons to be disks, it will take greater magnification and more than likely much better seeing.  I would want the seeing to support 300x or so to see clean moons in a 10 inch. 

 

In any event, the refractor cannot resolve the disk of the moons, the central spurious Airy disk is considerably larger in diameter than the moons so what you are looking at is the diffraction disk. The Newtonian is capable of resolving the disks of at least two of the moons because it has a much smaller, tighter Airy disk, the refractor's disk is 2.5x larger in diameter. So when you are observing the moons with the Newtonian, you are asking more of it, there is more too see, both in terms of the detail but also in terms of defects in the seeing.  This disk is tighter and the image brighter, both these bring out the defects due to seeing. 

 

In my experience, I rarely see Jupiter's moons as clean round disks in a larger scope but I also see much more planetary detail than I do in a 4 inch refractor.  

 

Jon

 

What Jon said +1

[Starman1]

I have to add this to the conversation:

 

While seeing is suspect, and I believe that was the bulk of your problem, your method of collimating the primary mirror is also destined to be inaccurate,

and the collimation of the primary mirror has tighter error tolerances than the secondary.  So your collimation might have been out a bit.

Whereas your secondary collimation, using the simple beam of the outbound laser, is accurate enough, your primary collimation, using the return of that

beam, is not.  To collimate using the return beam would require an outbound accuracy you are unlikely to be able to acquire.

So you should learn to use the barlowed laser collimation technique for your primary mirror, which is inherently significantly more accurate, or use a cheshire to collimate the primary.

 

And if you collimate during darkness, the barlowed laser technique will get you there.  It is way more sensitive and accurate than merely seeing the return of the beam on the 45 degree window.

 

Here are some links that explain the process better.  Note that these were from people seeking better collimation than the return of the simple beam laser:

http://www.cameracon...collimation.pdf

http://www.obsession...er/collimation/

 

If you don't have a barlow, you can certainly get a used one.  If your Hotech is used with the 2" adapter, I'd get a 2" barlow to use with it.

Or you could use a Blug or TuBlug from Howie Glatter, both of which work with the HoTech laser.

[REC]

Wow...thanks for all the great reply's!  I'll have consider all that's said:)

 

Bob

[Abhat]

I have been doing similar comparisons over last few days comparing Jupiter in my Z8 Vs C102. While C102 seems to show high contrast views and appear sharper, Z8 gives significantly brighter views with less contrast.  My Z8 is well collimated and views are as sharp as one can get in F/6 reflector.

 

My personal experience is significantly brighter views in Z8 completely destroy the C102's more high contrast views. GRS appears as a nice bulge in C102. In Z8 it appears distinct and clean. Same is true with Cassini division on Saturn. In fact my wife refuses to take a look at Jupiter or Saturn unless in her words the "Heavy weight Champ" is out.

 

For shadow transits and quick lazy looks I still prefer C102 due to its quick set up and cool down.

[Pinbout]

Hey bob if you come up for neaf, after a rack of lamb diner, i could test your scopes for you...

Edited by Pinbout, 29 March 2015 - 06:53 PM.

[Brian Carter]

Hey bob if you come up for neaf, after a rack of lamb diner, i could test your scopes for you...

Mmmm lamb and scope testing, such a wonderful combo.

[star drop]

I suspect seeing was the problem.

[Herr Ointment]

Or lack of lamb.

[REC]

Hey bob if you come up for neaf, after a rack of lamb diner, i could test your scopes for you...

Thanks for the Dinner Invite:)

[REC]

I suspect seeing was the problem.

From what I am reading here, sound like it. I'll give it another shot this week if we get better seeing.