View Full Version here: : 12" dob users - high mag
10-06-2011, 11:50 PM
Can anyone enlighten me as to the mag limit under suburban skies? I'd have thought no more than around 250x (my scope is f/5), but reading around the net suggests otherwise, and I've got my eye on a 5mm...
10-06-2011, 11:55 PM
That 5mm will spend a LOT of time in its nice warm case.
I find that 9mm is it most of the time for decent views, but now have a 7mm for when the seeing is better than average.
12" f/5 also.
11-06-2011, 12:00 AM
Just as I thought.:lol: I use a 10mm most of the time when viewing from my yard. Might as well save my hard-earned for a nice Pentax XW10. Thanks mate.
11-06-2011, 12:29 AM
Absolutely agree with Jason.
Even here in my dark skies I have used my 6mm twice in the years I have had it. Both times for about 1 minute. Waste of time.
I also have a Very expensive Pentax XW7 which I have only had the opportunity to use about 5 times in 5 years. The seeing needs to be excellent and that is rare.
But my 9mm is quite happy to give pretty good views, so that is as far as I push it.
All this in a GS12" Dob :thumbsup:
11-06-2011, 12:48 AM
I am quite happy with my 8mm Baader with my 12" DOB. Never felt the need to purchase any higher mag lense
11-06-2011, 12:51 AM
I would think that you could push magnification a bit higher (300x) but as has been mentioned the available nights may be few.
Fit and finish of the mirror would also be a factor...
Having said that, I spend most of my time around 200X for the things I want to see...
11-06-2011, 09:39 AM
Another thing to be aware of is mirror cooling.
These 12" f/5 Chinese mirrors are reasonably thick, and I find the image quality is usually fairly
crap unless steps are taken to get the mirror as close to ambient temperature as possible.
I find I have to get the scope out to its viewing position well before the daytime temperature
drops, and run the fan continuously until I want to view some hours later.
Without doing this, I find this scope fairly useless.
11-06-2011, 10:17 AM
With my skies I can rarely get above 200x on the planets. Only once in the year I've owned my 12" I was able to use 538x on Uranus, but that was ONCE since April 2010 when I picked the scope up! Most of the time 200-250x is the best you'll get with typical seeing conditions, according to my experiences anyway. On a bad night you might be limited to 150x, as a matter of fact I've seen nights when stars were bloating and fuzzing at just 60x! Don't worry, such horrible nights are as rare as the good ones!
When I was young I saved and saved to get a Nagler... I got a 4.8mm Nagler. Big mistake!!! I wish I had of kept saving that little bit extra and got something around 7mm or 9mm.
11-06-2011, 01:35 PM
Planetary 200 to 250x, wide field 40 to 100x.
Like Sab once or twice up to 500x BUT a very rare occurrence.
11-06-2011, 02:28 PM
I found my 7 mm nagler getting a little dusty after I got my 12"
went to a pentax 8.5 xf , while the jump dosn't look much on paper the
xf is usable often, the 7 wasn't.
An extra bonus with the xf is a high quality eyepiece for a very modest price.
11-06-2011, 04:35 PM
That looks like an impressive eyepiece within my budget. At 60* it'll be far easier to use than my 10mm Vixen (only 50* FOV). It'll give me 176x which I should be able to make good use of. Now the big decision is whether to bite the bullet, or shove this birthday cash in a drawer until I accumulate enough for the coveted XW10?:lol:
11-06-2011, 10:37 PM
The 10XW is a fine eyepiece! There have been some rumors about its availability after the earthquake/tsunami in Japan. I have no idea if these stories have any basis in reality. It might be worth a call to a dealer to check stock levels just in case...
14-06-2011, 07:05 AM
To test what mag you can get, leave your scope out &get up about 5, your eyes & scope should be ready to go (don't put lights on), Jupiter will be a good test
14-06-2011, 03:24 PM
Yep, a 9mm or 10mm most of the time. I have a 5mm that comes out occasionally.
Settle for 9mm or 10mm and have a 2x barlow in your box, just in case the seeing is good enough.
15-06-2011, 12:11 PM
I got a 10 inch 1200mm f/4.7 Dob as well. My first EP is LVW 13mm, next one will be a LVW 22mm as I realised in the inner-west suburb of Sydney, low-power EP will have much more chance than a high-power one.
But anyone can give me a suggestion between LVW8mm ($310) and XW10mm ($420)?
15-06-2011, 06:04 PM
This is a very location specific thing.
At my house on the Central Coast I regularly push both the 14" and 18" scopes to well over 300X on appropriate targets. Occasionally I get to 500X plus and 3 or 4 times a year I can get to 700X plus. On a couple of occasions over the past 5 years I have had the 18" Obsession to 1050X on the Moon and Saturn using a 5mm Pentax XW in a 2.5X TV Powermate.
I live right on the Southern edge (less than 50 metres to the water) of a very large lake. As some of you will know the temperature of a large body of water can take days to change by 1 degree and it is not affected by rising and falling day and night temperatures. Consequently, when there is no wind, the air above the lake can be very steady and provide excellent seeing.
Rod Berry (Rodstar), who lives less than 20km from me basically lives at the base of a large hill or ridge. This ridge causes an enormous amount of mid level air turbulence and Rod experiences "abysmal" seeing conditions at his house, where he can rarely get the 20" SDM to anything over 125X with his 20mm Nagler in the 20"/F5 scope. Under good seeing a month or two back at the Pony Club we had tack sharp images at 280X with a 9mm Nagler in the 20" scope. Rod has never been able to use that eyepiece at his house.
If you wish to observe the moon and planets at high power it sometimes pays to seek out a location which experiences very good seeing conditions. Places with good elevation are a good place to start as you automatically eliminate all the thermals emanating from the domestic environment. If you can get there on a night with no wind chances are you will get very good seeing and with a decent 12" scope you should be able to easily do 300X plus.
16-06-2011, 12:13 AM
Geez, a limit of 125x on a 20 incher!!!!! And I thought my seeing was a disaster :eek: I always get the feeling that often, it might be my immediate surroundings causing my bad seeing as opposed to atmospheric conditions. I observe from a tiny concrete courtyard bordered by my house to the west and south, about 5m from my scope. Plus some tall trees along the E and NE of the backyard, all this releasing heat and disturbing any airflow that may be happening. I live on top of a hill reaching about 15-20m above the surrounding terrain, don't know how this affects seeing.
16-06-2011, 06:18 PM
There is Pentax XW 10 mm for sale:
17-06-2011, 01:41 PM
Aargh! I ended up putting through an order for a 22mm Vixen LVW as I needed a good mid-low power eyepiece. Oh well, maybe next time with the XW. If the LVW is as good as the reports say I'll be pretty happy with it.
17-06-2011, 02:31 PM
Amen to that!!
It's interesting to see how varied observers' conditions can be. I find that if I'm getting good results with the 9mm - sharp stars and steadiness - then the 7mm and 5mm Naglers can be used. I've used the 5mm on about half the viewing nights since purchase. It's great for small galaxies and PN's and my brief experience with Pentax XW's is that at the short end they're even better. Obviously if anyone is struggling to use an 8-9mm on a regular basis, than anything shorter than that is a bit of a waste. Moving the scope to a better site is always an option if it's not too big.
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