jetstream

2 hours ago, scarp15 said:

when we find ourselves next time at a very dark place on a cold winters night just getting on with observing, the rest will be washed away, forgotten. 

Yes, its all about the experience, at least for me.

22 hours ago, GTom said:

Watching a dozen mirror making videos and reading reports, I am tempted to grind a 20-24" piece.

I waited over a year to get my 24" f4.1 from a reputable maker. It is 2" thick which was agreed on because a thick mirror is so much easier to work on and test. There are few professional makers that can actually produce VG large mirrors around f4.

Not trying to sound discouraging but if these guys are challenged by making them a first time amateur maker might just have some challenges too. A truly thin mirror like Lockwood produces might be almost impossible for a beginning amateur to make with any sort of accuracy.

then there is the structure...

16 minutes ago, estwing said:

Jumping up from a 20"....that's a mighty leap to see a real benefit. 

Yes, its nice to see a magnitude gain on stars between aperture increases IMHO- it will be hard to gain real benefit in an aperture jump from a 20" unless going to 28"-32" . In my case a 36" would be a nice increase.

Just my thoughts.

3 hours ago, Ships and Stars said:

my wife and newborn daughter

Congrats to you and your wife for the birth of your daughter!

You made an excellent choice for the big dob btw IMHO... and hope you can use it soon as things smooth out for you guys.

37 minutes ago, Ships and Stars said:

we have a new family member and two of us (not me) were in hospital for a little bit,

I hope all is well and they are recovered and also that you can get some time to observe.

3 hours ago, mapstar said:

Eternal cloud and barmy high temperatures for January? Weather is a bit odd at present?

Here too, my weather has been cloudy/rainy/snow etc for over a year... I used to observe for weeks at a time in the fall and winter. Clouds again today and the forecast is the same for 10 days. That'l teach me to buy a big dob lol!

Unusually there will be an odd unforecast very clear night here and there and I then go running to wheel the scope out. Astronomy has been forced to the backburner for me, but nothing will take my passion to "look up" away.

52 minutes ago, mapstar said:

NGC891 has to be up there as one of the best to view. One of my personal favourites and I think the star hop from almach is burnt into my brain. 

Had some excellent views over the years, transparency is a must as it really gives up loads of fine detail when the conditions are right. Our skies over here are rarely that good though. 

The 22" picks up all the faint outer regions, I imagine the 24" and better skies is mind blowing. 

Nice read Gerry thanks.

 

Thanks Damian, these larger dobs do show a lot more in a lot of objects for sure and I eagerly wait for your reports!

5 hours ago, Piero said:

Great report Gerry!

I was out with my dob on the 30th Dec. Really pleased with the views of M42 and other targets.

Also glad to hear that the pushfix adapter is working well with you.

Thanks Piero!

The Pushfix works VG and so does the Docter at f4.1 now, excellent actually. Also, the TV intravel adapter is perfect for the 17.3mm Delos. All in all a 6-7 week wait to get them lol!

Congrats for a fine session Stephan, these dobs with the handles attached really are fast to use. I do the same thing.

from Vogel, not sure if you saw this Vlaiv, very interesting to me the stages of dark adaptation. When fully dark adapted my 21.8 sky is grey... and I can see much outside unless in a thick forest etc.

" All cats are gray in the dark "

http://www.reinervogel.net/index_e.html

8 hours ago, vlaiv said:

two mirrors at 94%

I was told by a reputable coater that many reflectivity specs are "optimistic" or "nominal". My top notch mirror coatings are a true 93%, the sec 96%. They also will vary in reflectivity across the surface a little. Mass produced mirrors rarely reach this level and many corrector plates are not near the level you quote at 99.5%... IMHO.

If you take a realistic mass produced mirror and corrector % it might be .90x.90x .95= .77 or 77% transmission. Roland Christian has an essay somewhere noting this stuff. It get worse with poor eyepieces too.... My 24" dob runs .93x.96=89% reflectivity.

@John do you still have Rolands essay link?

4 hours ago, vlaiv said:

So you are saying that you think first image of 18.5 mag is "over estimated" - in sense it shows more of galaxy then you would expect and in second case it is under estimated - it is less bright view than you would expect?

Yes, in both cases. and

this is overly optimistic at 20.4 in a 16" dob. One issue with some observers and sketchers is reporting things better than reality dictates IMHO and I hope I didn't offend anyone.

In the first case drop any hint of spiral arms and in the second M51 will have a glow about it more so than distinct separated spirals at 21 SQM IMHO. Under 21.5+ it all gets larger, brighter and the 2 spirals arms emerge from the glow nicely.

I use an 8" f3.8 31% CO and a 17mm EP and also a 10" ff4.8 25% CO with a 20mm EP. Increasing the mag will reduce the glow and pull out the spirals better with good eyepieces ie 12.5mm Doc, 10 Delos and the super 10mm BCO. Any high transmission eyepiece works well.

M51 is a bright galaxy pair.

8 hours ago, vlaiv said:

I'm sure that quality varies between nights but on a single night with two or more repeated measurements in same direction - there probably is no much difference in obtained value?

Yes, if transparency is stable-aerosols,dust and whatever else- any change here can change the reading.

Of note and not mentioned much is the effect of the Milky Way-under excellent skies the MW can show up to almost .5 magnitudes brighter, with my average here just over .25 mag. This is huge...

If the SQM picks up the edge of the MW one time and not the next the readings will differ.

The SQM is accurate and points out that sky darkness is not a static quality IMHO. Oh yeah, they need a few readings to heat the sensor up or they read high. I usually take 4 or 5 readings and then use the ones after that- when the readings stabilize.

9 hours ago, vlaiv said:

What is not to like about it?

I agree! some have reservations about accuracy, but they are deadly accurate. Thats the problem really- they show up very small differences in darkness, which is affected by transparency.

9 hours ago, vlaiv said:

I don't know the answer to a simple question - if SQM reading is mag20 and nebula brightness after extinction is mag23 - will contrast be enough to be detectable by human eye (I can tell you in percentage how brighter nebula will be compared to background but that tells me nothing if it will be visible by human eye)?

Have you heard of Mr Clark and Mr Blackwell Vlaiv?

" A low-contrast object is more easily detected if it is larger. For an extended object such as a galaxy viewed in a telescope, magnification does not change the contrast with the background, because both the sky's and the object's surface brightnesses are affected equally "

Understanding extended objects is key to seeing them well.

https://clarkvision.com/articles/visastro/omva1/index.html

59 minutes ago, vlaiv said:

What should we adapt as being correct scale?

I really dislike the Bortle scale... all this, all that, can you see this can you see that. On top of this transparency changes the Bortle rating IMHO.

If the MW is bright and jagged, sharply defined with the Opi split showing down to the horizon things are about as good as they get. 21.7-21.8 skies will reflect the MW on a nice shiny car hood...or a calm lake.

The MW will go soft in less than transparent skies and when the readings go below 21.3 or so IME.

Vlaive, I wish you could come up with a simple sky rating for people.

1 hour ago, bomberbaz said:

Another option for me a little nearer is the ribblehead viaduct, loads of parking with a beautiful clear southern horizon and the sqm is given as 21.71, that is now top of my list to try out as its less than an hour away too.

Excellent Steve.

A friend from UK has done extensive SQMing (not near you though) and finds that posted SQM readings can be optimistic... this place sounds like a VG starting point for sure. If however that sky listed at 21.7mag doesn't "startle" or "dazzle" you it most likely is not at that level.

A true 21.7mag is extremely good. I'm excited for you in your pursuit of dark skies.

6 hours ago, PeterW said:

I was at a mag21.5 site and the Nebula visibility was dramatically different on different nights.. same size reading.
Once you get to around 21.6 or so I’d look at the bortle descriptions in preference as the numeric differences are slight, but the visual differences keep coming (allegedly as 21.5 is the best I’ve come across),

Peter

I think that when you get a truly high SQM reading-over 21.6 IME- the skies are very transparent.  This low level of transparency also minimizes light domes and their extent. Obviously clouds can give false high SQM readings.

Great report Stephan and a wish you all the best in 2020.

1 hour ago, alan potts said:

Few people seem to have seen G and H even though they are within the compass of my 18 inch

Glad you are feeling better Alan and I eagerly wait reports.

I see G and H at times but honestly I don't focus my efforts on this kind of thing. I get so mesmerized by the core of M42 that much is overlooked. Another great object is the Running Man figure, out in full stride the other night. The 24" does really well on PN so this has now become a favorite set to obs and bright galaxies.

33 minutes ago, GavStar said:

Yes in the country by the sea as PeterW said. When I took the phone shots the sqm was 20.8, it later improved to 21.2 but by then Orion was behind my house.

Happy New Year Gerry! Hope it’s not too cold where you are.

Happy New Year to you Gavin!

Not cold but cloudy and we got about 18" of snow yesterday with drifts up around 3 ft. The NV set is working well with the dob and I'm eagerly waiting reports!

17 minutes ago, vlaiv said:

For some reason I related use of the word "snaps" as meaning photography .

I've only observed from Bortle 4 / mag 21.1 skies and was blown away by views (was rather transparent sky that particular evening), so I can only imagine what it looks like in mag 21.7 skies.

 

Vlaiv, it is a sight to behold.

1 minute ago, bomberbaz said:

Thanks gentlemen. I think that a recce with perhaps some simple snaps of some brighter dso's (M45, 42, 33, 15 for example) and a use of pair of bins and making notes of the sites all on the same night might be the best option to see whether it's significant enough to warrant going for the slightly darker skies. 

For me I judge sky conditions by the appearance of the Milky Way- the sharper and more jagged it appears the better and is a very easy method to asses with. That Copernicus site is interesting...

2 minutes ago, vlaiv said:

In perfect transparency we would have absolutely no issues with light pollution.

Yes, this coincides with my variable skies- after a good storm and with no snow on the ground I can get repeatable 21.9 mag readings. Under less transparency my readings go down as low as 21.4 with snow on the ground.

Thanks for the excellent links Vlaiv- I love that Copernicus site which I have now saved.

11 minutes ago, vlaiv said:

For "normal" angles it is just 1/cos(angle)

You can use short cut cos values for 30, 45 and 60 degrees

and that will give you air mass at each angle as:

0 degrees = 1

30 degrees = 1/(sqrt(3)/2) = 2 / sqrt(3) = ~1.1547

45 degrees = 2/sqrt(2) = ~1.4142

60 degrees = 2

But as you start approaching horizon - above formula will give wrong results. It assumes flat earth scenario, while earth is of course curved.

Different models give different results for horizon but most are about 38 air mass value.

See here for different models:

https://en.wikipedia.org/wiki/Air_mass_(astronomy)

 

Thanks Vlaiv- when I can see numerous stars just above the trees from my high vantage point observing will be good. I had heard that the horizon was 2 airmasses but figured it had to be more.

Do you have any idea what the effect on a star, say 10 mag at the different angles? I'm wondering about extinction. When the airmass effect is calculated I wonder what transparency they are assuming?

I've thought about this for a while now.