Observing planets with large aperture scopes

Canopus Lim · Post by **Canopus Lim** » Fri Oct 20, 2006 9:32 am

Since it is hazy these few weeks and not getting any better, I will just talk briefly on my experience of using my 12.5 inch 'Obby' on the planets.

Since majority of the scopes used in Singapore are less or equal to 8 inch in aperture, the 12.5 inch scope is considered LARGE by Singapore standard. Actually a 12.5 inch is the smallest of the large aperture scopes so that is why I call my 12.5 inch "Obby" (Baby Obsession/ Obsession Baby).

I have brought out (use) Obby at least 20 times this year; I think it is considered a lot. Whenever there is clear enough sky to see the planets or stars I will bring Obby out. Mostly I observe at home but if there is real clear skies, I will go to East Coast park (the end of cyling track..beside National Sailing and SAFRA) to observe the star clusters in the south. I would say that location has become worse because of many streets lights recently installed but it is still the best for southern and eastern skies due to no obstruction and less lights.

With my experience of 20 times bringing out Obby, I would think that seeing and collimation plays a HUGE part in looking at planets especially for bigger aperture scopes. The reason is that the big aperture scopes has very high resolving power and hence the 'defect' of the skies can be seen clearly. If the seeing is not good, the planet looks unfocused and features unsharp but when the seeing is good, it looks photographic (highly detailed, colourful and sharp)! Also, with poor collimation, planets will look wash out but seeing I feel has even greater impact than less than collimation. About seeing... it varies EVERY SECOND. It is noticeable. To see high detail and sharpness, the average seeing has to be good and that limits the maximum detail that can be seen. However, to get the best out of scope, I think one has to be patient. I have observed Jupiter/ Moon for more than an hours time at the eyepiece and it pays for being patient because the seeing varies every second. Whenever there is suddenly better seeing, lots of details can be seen for a few seconds..then it goes back to the 'average' seeing (images not that sharp as those brief moments of brilliance). Do not expect sharp and detailed images through out.

That is why people with small scope think that it beats a big aperture scope in less than perfect seeing. That is FALSE. Small scopes aesthetically looks better because of its poorer resolution and hence inability to see the seeing defects clearly and on the average the images look sharper. However, it offers much less detail than a big aperture scope if one patiently wait for the atmosphere to allow those details to be seen in the big aperture scope. In short, if one wants an aesthetic looking planet, it will be better to stop down the big aperture scope or use a small scope. If one wants the maximum detail, colour and sharpness, I would recommend a big aperture scope on condition that the observer is patient. It is impossible to see lots of detail in a big aperture scope if you look for only 5min unless somehow the atmosphere is so coorperative at the moment you look at it.

About turning on the fan... I would usually do that to quickly acclimatize the mirror. I think the advantage we have in Singapore is the lack of thermal gradient and hence telescopes just cool much faster and we have less one gremlin to fight with. The 3 gremlins are Seeing, Collimation and Thermal managment. Collimation of newtonian scopes without doubt is VERY IMPORTANT. For those who have large aperture scopes or intend to build one, I suggest you buy the BARLOWED laser collimator to collimate the primary. It is very accurate for the primary and is not affected by mechanical tolerances of the laser collimator and focuser (which is a real pain).

Lastly, I have read that observing planets at twilight (at dusk) is very good. I have tried it and it is really very good. Also, it would be best to observe planets near zenith but not at zenith as it would be hard to hand track at zenith. Hope these information is useful for the dob community here.

Usual magnifications I use for Jupiter:
220x (average to good seeing), 300x (very good seeing)

Usual magnifications I use for Moon:
220x (for bad seeing), 300x (average to good seeing), 440x (for good seeing), 600x (have not got an opportunity to use that effectively)

elton · Post by **elton** » Fri Oct 20, 2006 2:37 pm

Agree. So much so that I don't observe planets <45 deg (at least 60 deg for critical viewing) from the horizon or when the seeing is below average.

Also need to distinguish between poor seeing and poor thermals around you. After a hot day, all the buildings, roads and pavements continue to convect heat into the night. Viewing planets over bodies of water tend to give steadier seeing, eg. from Changi/East coast as they rise and from Lim Chu Kang as they set.

Canopus Lim · Post by **Canopus Lim** » Fri Oct 20, 2006 3:17 pm

Yup, but quite hard to avoid buildings. I would also prefer to put the scope on grass as there will be less heat dissipitated from the grass than on the pavement.

Do you know how to differentiate local seeing from the surrounding buildings etc and atmospheric seeing?

I only know that tube currents and a telescope that have not reached ambient temperature will have 'lightning' and 'lines' kind of effect on a defocus star; like the star is having a lightning storm. I have seen that on my C5 when I immediately use it and did not give it time to cool down.

Canopus Lim · Post by **Canopus Lim** » Fri Oct 20, 2006 4:17 pm

Article on Seeing

Quoted from the website " At these times, transparency can be reduced, but seeing can be excellent. It is also my experience that strong winds are often associated with poor seeing. "

I agree with the statement. Good seeing goes with poor transparency (more clouds) and bad seeing usually goes with good transparency (few clouds). Also, when observing, if the clouds are moving very fast it means strong winds and the atmospheric seeing tends to be bad. It would be good to observe when the clouds are hardly moving as the atmospheric seeing tends to be better.

elton · Post by **elton** » Fri Oct 20, 2006 8:30 pm

generally if the air is still the seeing is good because singapore does not have high altitude winds like jetstreams. if the air is still, your scope has cooled for half an hour and the image still looks like it is swimming in muck, then quite likely the the area around you is convecting a lot. Teleport cools so quickly that most times it is hard to recognise temperature equalisation issues

Canopus Lim · Post by **Canopus Lim** » Sat Oct 21, 2006 12:45 am

Thanks Elton. Hmm.. so I think it would be good to go to the open field in beside Bedok South secondary as it is quite far from buildings (less convection heat), on a field and not many street lights to interfere; the only problem is the cars moving in and out of the place.