How do Dobs/Newtonians fare in singapore light polluted skies

[akdwivedi]

Hi skywatchers,

I have been primarily using SCTs (previously C8s and now a C6) and refractors (Orion 120ST, SW 80ED, Stellarvue 102ed, SW120ED) for visuals from my patio. So far the SCTs and refractors are doing fine as I mostly view planets and luna. I tried some DSOs and star clusters but the view wasnt so good, due to bright skies and bad seeing.

I wanted to check with the community about the scopes used for visual astronomy from Singapore. I dont drive, so going to darker skies across the border is out of question.

Will a larger dob/newtonian - say something like 10"-14" f/4-5 work better. Which dobs do you use, which eyepieces do you use. looking forward to your advice and tips.

thanks

[joachim.ong]

I have a C8 and a 10 inch self-built dob, have briefly used an 8 inch dob for a while, also had the chance to compare the 10 inch side by side to a 12.5 inch in SG.
I am a firm believer that "aperture is king" and would say the difference in aperture is as expected. Mainly small, noticeable increments but don't expect a miracle; dim objects just appear less dim. You probably get the same feel in going from your 120mm refractors to the C8.
For me stars are nicer and more colourful in the dob for some reason, maybe because it is possible to use larger exit pupils, or maybe because SCTs have a spherical primary mirror. But I think this preference is quite personal.
The one thing I can recommend for light polluted skies is a proper UHC/OIII type filter if you have yet to try one. That said, I'd pick an 80mm in dark skies over any scope in Singapore.

[akdwivedi]

Hi Joachim

thanks for your feedback. I do think the aperture of a 10-12" dob might give better views than refractors/SCT (which I have). However, I was concerned about heavy light pollution in singapore skies (specially my place is surrounded by these light emitting buildings all around).

Fpr planetary and lunar, SCT certainly gave me better views though I must say 120ED/80ED do have more sharpness/resolution.

I was looking at 10"-12" skywatcher flextube prices are decent. and skywatcher claims 94% reflectivity on their mirrors. I think the flextube also allows the tubes to be adjusted a little shorter so that a binoviewer may be used. But then I watched Ed's youtube channel where he said a 8" gets twice as much used as a 10" and a 12" gets used even lesser.. :-).. choices/decisions..

[joachim.ong]

Making sure stray lights don't enter the scope should be the top priority. If that means you need to walk a little, a lighter scope might be better. Get a trolley too, if you have space at home you can leave the dob on it all ready to go.

[rlow]

I have used those telescopes (or similar) that you mentioned. For newtonians, I have used from 4 to 15 inch and bigger. So I share what I see in the scope performance etc.

There is a big jump from C6 to 10-12 inch dob, both in size/weight as well as performance. A 10-12 inch newtonian will show brighter and better details on planets and brighter open/globular clusters than C6 and other smaller 4 to 5 inch refractors. This is so, as long as the newtonian is properly cooled, well collimated, light-baffled, and most importantly have good quality optics.

Good quality optics refer to one which has lesser optical aberrations. Mirror reflectivity is important but may not be that critical. I've seen SkyWatcher newtonians which have good mirrors, and some not so. Either try your luck on mass-produced optics, or pay more for premium optics.

Eyepieces of higher quality may perform slightly better but not as significant compared to upgrading to a bigger aperture telescope with good quality optics.

Unlike on planets, the bigger newtonian show more but may not perform in the same significant way on fainter DSOs like nebulae and galaxies under our urban night sky,. This is due to the limiting factor, which is the light-polluted sky. If you don't drive but can move the scope to observe DSOs from the outskirts of Singapore under darker skies viewed towards the sea, you may get better DSO views compared to the views from your patio.

Viewing under truly darkest skies away from cities' light domes is significantly better for all DSOs. For planets and moon, observing under our urban night sky is already optimum.

The SkyWatcher 10 to 12 inch flextube has a relatively massive and heavier base. The SW 10 inch base is 12.0 kg, while the SW 12 inch base is 17.4 kg..If you intend to primarily observe planets from your patio, then these size/weight portability issue may be less of a concern, compared to moving it to a darker observing site. The original base can be replaced with a lighter compact "home-made" rocker box/base with better movement.

[akdwivedi]

thank you Richard.. I guess my next step is to post a WTB ad in classified section. One more question the skywatcher sells newts under different names.. Would it be correct to say that the quattro is meant for AP and not that much benefit for visual.

[rlow]

You are most welcome. I also learnt from asking around.

The SW Quattro is an imaging newt which is designed more for imaging with full frame camera, CCDs etc. if you compare the 10" SW Quattro to the 10" SW Flextube, there are fundamental differences.

Mounting is different, Flextube comes on a dobsonian (AZ) mount, or with the Synscan Goto dob. While the Quattro is an OTA only, meant to mount on a separate GEM or AZ/GOTO mount.

Quattro has oversized secondary mirror with 35% central obstruction (CO) for better illumination of full-frame sensor while the Flextube with 25% CO which gives better contrast for visual use. I prefer CO below 20% for visual use, and my 15" f/4.5 and 22" f/4.1 dobs are designed with 17% and 18% CO respectively. FYI, a 22" f/3 dob may have 25% CO.

Also the Quattro has f/4 mirror which typically requires a coma corrector, while the Flextube is using f/4.7 mirror, so you can perhaps get by without a coma corrector. Using dob without coma corrector has huge benefits like cost, brighter image, less balancing issue. The coma corrector like Paracorr 2 is used on newtonian with focal ratio below f/6 to give coma-free sharper image. Paracorr cost >500 USD. I have used 10" f/4.7 and f/5 newt without coma-corrector for general viewing. For critical high-power planetary viewing, I would still prefer using the 10" f/4.7 with coma corrector.

The f/4 mirror also requires to hold more critical collimation than a f/4.7 mirror. Two points here for faster mirror, it is a bit harder to collimate faster scopes, and more critical that the scope hold collimation when slew across the sky. In some cases, faster scopes may flex a bit etc and lose that critical collimation.

The f/4 scope also require better corrected eyepieces (eg, Ethos, Naglers etc) than a f/5 scope. Especially for widefield and ultrawidefield eyepieces, the lesser quality eyepieces may not perform as well, showing aberrations towards the edge of field.

F/4 mirror is much harder to figure to high quality than a f/4.7 mirror. So for mass-produced mirrors, you may possibly get a better quality (higher strehl ratio) f/4.7 mirror than a f/4 mirror, if all other factors being equal.

If you have intent to do imaging, the Quattro can be used for both imaging and visual, just note that visual use may be somewhat compromised compared to the Flextube. There are more differences but i only highlighted some key points above. Those who start in imaging usually go for a sturdy mount (higher payload) first, and also start with smaller, easier-to-use scope like a 3-5" apo refractor rather than 10" f/4 newtonian which requires regular critical collimation, thermal stability and a more sturdy mount than the small apo which hardly need collimation and is a smaller load on a mount.

Please note that these are general comments based on the various newtonians I have used, and apply to visual use of newt dobs in general They are not specific on the actual perfomance of either SW Quattro or SW Flextube. Also note that mass-produced telescopes sometimes vary in quality and performance.