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How does seeing effect aperture and focal length?

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#1 Whichwayisnorth

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Posted 02 May 2013 - 02:39 PM

Maybe I phrased that question wrong but hopefully you'll understand what I am trying to understand. ;)

I have two telescopes because while I am imaging I like the idea of having another telescope that I can look through.

On my left I have a 8" SCT. 203mm aperture, 2032mm F/L F/10. Resolution (Dawes)0.57 arc seconds. Secondary Mirror Obstruction 2.7 in (69 mm) Secondary Mirror obstruction by Diameter 34% Secondary Mirror Obstruction by Area 11%

In this OTA I use a Televue Everbrite dielectric diagonal and a Explore Scientific 82 degree 14mm eyepiece. So I have a magnification of about 145x which is fine for my typical back yard seeing conditions. I can usually catch Jupiter before it sets and then I switch over to Saturn. Saturn being in opposition a few days ago this was my main target. I found that both Jupiter and Saturn were reasonably clear. The bands on Jupiter were distinct and contrasty. I really enjoyed the view. Jupiters moons were nice and tight and well focused. Saturns rings had nice sharp edges and Saturn's moons were nice and tight and well in focus as well. The more I looked at Saturn the better it seemed. I tweaked the focus a few times but settled at one position and looked at it for a few minutes in my observing chair very pleased.

On my right I have a 14" SCT OTA with a native FL of F/8 2845mm Resolving power of .325 arcseconds. Secondary obstruction by area 13.8% Obstruction size=5.1" Obstruction by diameter=36.43%.

In this telescope I have a 20mm Televue 82degree Nagler Type 5. With this eyepiece I am at 142.25X magnification. I am using the blue Meade 99% reflective 2" diagonal . (I have also used the TV Everbrite and didn't notice a difference as it pertains to this question)

Both Jupiter and Saturn are significantly more washed out, less contrasty and the moons are not tight and focused (bubbly). I have checked and re-checked collimation and it seems to be really really close.

So my questions for those of you who probably already know where I am going with this;

1) Is it thermal equilibrium? I have waited at least 3 hours after sundown but I am not using active cooling.
2) Collimation? I checked it at 300X but no higher. I could not find any flaws. Also checked it with my camera.
3) The bigger the scope the more seeing impacts the view?
4) Central obstruction too big to expect more?

I would have thought going in that the extra light gathering ability would overcome the problems with seeing and central obstruction when compared to a much smaller SCT.

So what should I expect? Before I dig deeper I hope to get some feedback so I know more about this. I am supposed to go out to my clubs dark sky site on the 10th. I am hopeful for clear and steady skies so I can enlist the help of other club members in checking things out. I would like to learn more about what my realistic expectations should be.

If you are still with me after that text attack, thanks!

#2 brianb11213

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Posted 02 May 2013 - 04:12 PM

Whilst larger scopes do suffer more from bad seeing, the big issue here is temperature equalisation. I do not think that my 11" SCT would ever come to temperature equilibrium without the use of active cooling. A 14" SCT I'd expect to be worse.

The difference in central obstruction is too small to have any practical effect.

It is also possible that the "worse" scope has rougher optics but really you need to get the temperature effects controlled before you start blaming the optics (if they're reasonably well collimated).

#3 Whichwayisnorth

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Posted 02 May 2013 - 04:42 PM

Whilst larger scopes do suffer more from bad seeing, the big issue here is temperature equalisation. I do not think that my 11" SCT would ever come to temperature equilibrium without the use of active cooling. A 14" SCT I'd expect to be worse.

The difference in central obstruction is too small to have any practical effect.

It is also possible that the "worse" scope has rougher optics but really you need to get the temperature effects controlled before you start blaming the optics (if they're reasonably well collimated).


I just placed an order for an SCT cooler. If the item is in stock hopefully I'll have it before my next session.

This is why I posted this looking for feedback. I don't want to "blame" the optics without understanding more on what can cause issues.

#4 dscarpa

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Posted 02 May 2013 - 05:32 PM

It looks to me like my C9.25's issue with being sensitive to seeing as opposed to my IM-715 mak and WO ZS-110 has as much to do with CO size as aperture. It will be interesting to compare it with the 11" Teeter STS newt that's on order. As to CO size it's 19% for the STS, 25% with the mak and a 36% on the SCT. On some nights of fair seeing using higher power in the C9.25 improved it's performance on Mars, Jupiter and Saturn. They were not as bright so there was less light to blur the image. This doesn't work for lunar. David

#5 Whichwayisnorth

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Posted 03 May 2013 - 12:55 AM

Yah it must have been cooldown. As soon as the sun hit the mountain to my west I pulled the cap off the eye piece socket and put a sock over it and let the air get in/out. It is almost 11pm now so it has been cooling for several hours now. I just went out and put an eyepiece in there. Saturn looked really good and the moons are nice and tight.

I am looking forward to the SCT cooler that I ordered.

#6 leviathan

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Posted 03 May 2013 - 04:35 PM

This is 100% thermal equilibrium. My 8" SCT show today approximate the same picture of Jupiter at low elevation as 100ED (it was not cooled for a long time). After several hours outside it showed much more details on Saturn than 4" refractor.

I think 14" needs active cooling. SCT cooler (if you are talking about Lymax) will only speed up a bit the thermal equilibrium, but not significantly. You need a fans at back + on sides near main mirror to blow surface heat.

#7 Kraus

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Posted 06 May 2013 - 08:38 AM

Hmmm...

I'm sure you're like thousands who take the scope from a comfortable 72 degree house to 90 degrees or to 40 degrees ouside. Your scope suffers an instant twenty to thirty degree temperature change. You're torturing it. And then you wonder why you have seeing or cooling down problems.

Go with a permanent outside mount.

Your scope will thank you.

#8 Asbytec

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Posted 06 May 2013 - 04:53 PM

3) The bigger the scope the more seeing impacts the view?

4) Central obstruction too big to expect more?


On #3, there is a thread exploring that in general observing forum, if you're interested.
http://www.cloudynig...5822268/page...

On #4, there are tons of threads discussing that. Give a minutes, one will spring up. :)






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