whats better in poor observing conditions - 16" or 8"?

[MikeMS]

Okay, I never fully understood the implications of having too large a telescope under poor conditions until now, but I am coming around to understanding this.

I have an old C8 orange tube OTA on a CG5 mount and it gives a lovely image, especially on planets and the moon.

I have access to a 16" meade SCT in an observatory in Riyadh where the skies are light polluted, sandy and the air temperature very high. The only good thing is no humidity.

Now the 16" needs some TLC, the mirror flop takes the star out of a 40mm eyepiece, the finderscope is broken, the dec motor is not very good and it is polar mounted.

The polar mount makes the eyepiece difficult to reach.

Now, I do not think that even with all the above working it will give as pleasing a view on bright objects as the 8" SCT. Is this true? Whilst the larger optics will give a larger image, it is also increasing the imperfections in the image.

My thinking is it should be alt az mounted, the polar mounting is never going to be used for astro imaging, so why not have the eyepiece at a sensible position. If a tripod with castors is made, it could be mobile and taken to a better site. But it is always going to be very heavy to move. 

Is there a question? Given the poor skies, is it better to stay with the 8" scope?

Kind regards,

Mike.

[Schorhr]

  I am by far no optic expert but I have recently read an article (german, rohr.aiax.de/2003-4_gesamt.pdf) that  relativized  the misconceptions regarding larger aperture and seeing effects.

To boil it down, the effects of seeing are more often an actual problem in large cheap quality optics;  

 (Seeing influences will be visible more with a mediocre mirror that barely joins the light in the focal point then on a good mirror)

If seeing is an issue on a larger telescope, you can always stop it down to a smaller aperture;

But the larger aperture of course allays has benefits on good conditions, and with a smaller instrument there's no trick to fix the missing aperture :-) 

[Moonshane]

ultimately, a 16" scope assuming it is optically good and collimated / in tune with the environment will in theory provide a brighter image with more resolution / detail. however, I tend to agree that a smaller aperture does sometimes work better in poor conditions.

in your shoes, I'd take the 8" along and use that and if the view stands higher magnification than usual, start ramping the 16" up until the view dissolves.

[astronymonkey]

I think the only thing you can do is give it a go. I have both a 16inch and 8inch dob and it can be frustrating when you go to the effort of getting the 16inch out and the sky is too poor for it to be worth it - but then when the sky plays ball the 16 simply leaves the 8 for dead, absolutely no comparison. When the sky looks iffy I tend to get the 8inch out and if planets are looking good then ill only get the 16 out if I think there is enough clarity to warrant the bigger scope and the 8 inch only takes 2 mins to set up. For deep sky though I only bother if its nice and clear and for me its the 16 every time unless im in a rush ie to catch a setting object when I get in from work.

[LukeSkywatcher]

Its my understanding that atmospheric conditions are not "magnified" by aperture but rather by increasing the magnification of the EP in any size scope.

I could be wrong.

[ronin]

The 8".

It is easier to carry back inside then the 16".

[Peter Drew]

I have the reverse situation, an observatory mounted 16" SCT in the UK and a balcony 8" SCT in Tenerife. The clear skies, better transparency and steady images in Tenerife allow me, on the average, to use higher magnifications on planets and the Moon than with the 16" in the UK. The 16" is equatorially mounted and I find little difficulty in accessing the eyepiece other than low northern directions. The 16" is certainly more stable in the alt-azimuth configuration and seems to require less accurate setting up for goto purposes. An 8" telescope has a theoretical resolution threshold of 0.7 arc seconds, better than most nights seeing allows.   

[MikeMS]

All.

Thank you for the replies.

I have given a side by side test in some regard, I carried my 8" up to flights of stairs to put it outside the observatory with the 16" in it and even just places on an uneven floor got Albireo in the 32mm eyepiece and it was great. We only managed to get one star in the 16" in the time there, the mirror flop lost it again.

I did not mention the observatory door can be only opened when the dome slot is over it, so the dome has to be electronically moved each time.

The latitude here is 24 degrees and this gives the polar mounted scope more tilt than the UK and whilst a big fork mount, I am sure this tilt does affect the movement. The alt az seems so much better.

And maybe I under played the poor skies, with a struggle you can only just see Albireo when it is over head with the naked eye.

I will offer to have a pedestal with castors made to use it in alt az mode so it can go away from the city and try and cure the other issues. Like a C14 I restored here, then take it to the desert for some use. The milky way is clearly visible 30 kms in the other direction from my home, unlike the 30 km to the scopes current location.

Kind regards,

Mike.