Review: 10" f/5 Guan Sheng Dobsonian

[iceman (Mike)]

Hi all.

Darren Wong (dhumpie) has kindly submitted a review of the 10" f/5 Guan Sheng Dobsonian.

If you'd like to read the review, go to the IceInSpace Reviews page, or directly by clicking on the link below:

10" f/5 Guan Sheng Dobsonian

Thanks to Darren for submitting the review! Much appreciated.

If you'd like to submit a review, how-to or DIY article, or any other content for the site, please contact me.

[ausastronomer (John Bambury)]

Darren/Mike,

Just to correct something in the technical specs at the beginning of the review. Darren has listed the Central obstruction as 23%. It is actually 25.2% having a 63mm minor axis secondary and a 250mm primary.

Nice review BTW

CS-John B

[Starkler (Geoff)]

Originally Posted by ausastronomer

Darren/Mike,

Just to correct something in the technical specs at the beginning of the review. Darren has listed the Central obstruction as 23%. It is actually 25.2% having a 63mm minor axis secondary and a 250mm primary.

I was also going to say something about that and offer a further correction as well
The mirror does have a 63mm minor axis but the plastic holder which the secondary slips into adds additional thickness of the order of about 3mm, making the total central obstruction of the order of 26.4%.

[iceman (Mike)]

I was also going to point out that it's extremely unlikely that Darren saw the Encke division in a 10" scope. The Encke division is only 325km wide, well outside the resolution of a 10" scope at 0.35" wide.

It would really require a much bigger scope with very high magnification and incredible seeing. It would also be best seen when the rings were at their full tilt, like last opposition.

Not saying he didn't see it, just that it's unlikely and didn't want newbies to get the impression that a 10" GSO dob will let them see the encke gap every night.

[janoskiss (Steve H)]

According to the results published in Donald Bruns' article "Resolution Realities" on p. 69-71 of AS&T, Encke would be just barely discernable in a 10" in perfect seeing. It is not visible in a 6" but very distinct in a 16".

[ausastronomer (John Bambury)]

Mike,

Also a good point.

As you mention the Encke Division is only 325km wide and requires an actual resolution of about .35" which is the theoretical limit of resolution for a 12.5" telescope. Its worth noting that sometimes a telescope will outperform its "theoretical limit" of resolution, depending on the target. The Encke division can be seen in a 10" telescope, which has a theoretical resoltion of about .46" and has indeed been seen in smaller scopes because you are not actually resolving the Encke Division itself but the contrast difference from light to dark. That having been said the chances of seeing it in a 10" telescope are pretty slim and conditions need to be absolutely perfect, in addition everything else needs to be perfect including collimation and high transmission high contrast eyepieces. You won't see it in a 10" telescope with a budget plossl conbined with a budget barlow. I could not see it at Lostock on the Thursday night when we had a great image at 455X in my 10" with "A" grade optical components. I could easily see the "Encke Minima" that night but not the division. I have seen it twice in my 10" under perfect conditions. Once at home and on another occasion when I observed very early am midweek from Crackneck Lookout at Bateau Bay observing across the Ocean. Seeing was 10/10 that morning. On both occasions I used a 5mm UO HD ortho for 250X. I have also observed it once in a 16" newtonian and once in my old 8" newtonian. That's 4 times seen for me in 4,000,000,000,000,000,000 attempts, albeit 99% of those attempts have been in scopes of 10" and smaller. The chances of seeing it 1st time out with your new 10" scope ? About 1 in 1,000. It is certainly a reasonably doable thing in scopes 16" and above when conditions are ideal, its just something I have never chased. If conditions are perfect tonight it may be worth the chase in Hector at Kulnurra.

CS-John B

[ausastronomer (John Bambury)]

Originally Posted by janoskiss

It is not visible in a 6" but very distinct in a 16".

Steve,

I can appreciate you are quoting from the article but don't ever kid yourself that it's an "easy" target in any amateur telescope. Rarely does the seeing get sub arc second let alone sub .5"

CS-John B

[janoskiss (Steve H)]

To elaborate for those who don't have access to the above cited AS&T article: It was about a series of qualitative experiments conducted under idealised controlled conditions. Cassini and Encke were imaged from a high resolution photo placed some distance from the telescope. So effectively it was done under simulated space-telescope-like conditions. The image recorded with the 10" shows just a hint of Encke, the 6" did not show a trace of it, and the 16" showed it clearly.

[astro_south (Andrew)]

As someone who was there, I can confirm it was infact the Encke Minima we were observing - just a typo from Darren I'm sure.

Originally Posted by ausastronomer

You won't see it in a 10" telescope with a budget plossl conbined with a budget barlow.

John - we were using 13mm T6 Nagler and a 8mm and 6mm Radian with shorty plus barlows (just for the record )

Originally Posted by ausastronomer

The chances of seeing it 1st time out with your new 10" scope ? About 1 in 1,000.

John - It isn't the first time he has used a GS 10" scope, or for that matter larger scopes. We collimated his scope when he arrived so it was performing great. Darren is a very skilled observer (certainly not new to this game and has done amazing things with his 80mm ST) and I can assure you he is the "1 in 1000" <- realising that your probability includes the influence of sky conditions

[ausastronomer (John Bambury)]

Originally Posted by astro_south

Darren is a very skilled observer (certainly not new to this game and has done amazing things with his 80mm ST) and I can assure you he is the "1 in 1000" <- realising that your probability includes the influence of sky conditions

Andrew/Darren,

I wasn't questioning Darren's skill as an observer in any way, in fact his sketching is superb, as good as I have seen if you leave O'Meara out of the equation. This by necessity means his observing skills and eyesight are equally as good. My probability of 1 in 1,000 solely related to the likelihood that conditions would be favourable enough to see the Encke Division on the night that the scope happened to receive 1st light.

Apologies if my post came across the wrong way.

CS-John

[astro_south (Andrew)]

No worries John, and no offence taken from my point of view. Just my INTP personality making sure the facts are correct I realised your probability included sky conditions, and you are right that we were lucky the sky was well above average.

[stringscope (Ian)]

I would just like to thank Darren for making the effort to write a review on the 10" GSO Dob. I know from personal experience writing these reviews takes quite a bit effort and time.

I found Darren's review interesting, informative and if I was in the market for such a scope, would be most useful to me.

WRT the secondary size issue, I note Darren in his review specificaly gives an approximate obstruction figue only - (quote) "central obstruction : 23% (approximately)" (end quote). It might be worthwhile checking to see if the latest 10" GSO dobs are being fitted with a different sized secondary.

Regards to all

[dhumpie]

Hey guys,

Thanks for coming to my defence Andrew And yes Andrew was right, it was Encke minima we saw that brilliant night at Parkinson's and not the division. Typo As for the central obstruction, I did a rough estimation using a ruler. Sorry for the confusion.

Darren

[Miaplacidus (Brian)]

Nice review, Darren. Thanx.