A few questions on basics

[davekelley]

Hi everyone

I have been observing for a few years,  using scopes from ST80 to 300mm Dob, from my suburban backyard. I am curious about a couple of things:

1. I don't seem to get much more detail in say the orion nebula, or much more resolving into stars of the hercules cluster, whichever size aperture i am using, which goes against everything I read.

2. I have recently obtained some nice 15x70 Helios Apollo bins, and it's been cloudy for weeks, but have managed to get a quick look at jupiter, and the disk of the planet seems as large to my eye as it does in my scope at more like 90x magnification!  Optical illusion?

I also wondered, my neighbours annoyingly tend to leave upstairs lights on all night long, anyone ever tried erecting some sort of curtain, like a beach windbreak? just to preserve a bit of night vision? are there such things you can buy?

Clear skies everyone, I have 1 hour clear sky forecast tonight, if I'm lucky!

Dave

[ronin]

1) Could be the set up of the scope. An achro will not be that sharp, the wavelength are focused at different positions and so the prime image is a little spread out. The consequence being not as sharp.

Reflectors are built to collect light, the secondary and the spiders again drop the contrast, they also have to be collimated pretty well.

Probably the best is through a good apo refractor, but they are costly.

Have read of one person who replaced the secondary and was amazed at the improvement, they reckoned that they just had a bad one, but 1 item can upset the system.

How good are the eyepieces ?

A 300mm reflector is likely to be fast (f/4.5?) so could need £250 eyepieces and a coma corrector.

The eye works as much or more on contrast then brightness so you lose things in a non-sharp low contrast image.

2) Suspect just optical. The eye+brain does things. The moon is actually small, but being bright is interpretted as big. Binoculars produce a small bright image, so every chance it gets converted to big.

Should be able to rig something up, but cannot help. Too many lights with me to make thinking about it worthwhile. I just take car and drive elsewhere.

[Northern Soul man]

Hi Dave, a very warm welcome to the SGL mate, i think the problem is that you say your observing from surburban skies - the 80 and the 300mm scopes probably have short focal lengths so < F5 or there abouts.  Ideally the planets need a much longer focal length for you to tease out detail - say around > F10 to even F15 - F20 (the longer focal lengths giving larger image size for any given aperture) larger apertures bring a much brighter image, allowing higher mags to a point where fainter detail becomes easier, but also you need aperture, so the more aperture you have together with a long focal length - the better you will do on the Planets - this is only a guide though.  Not sure how the 300 is mounted - you could try to stop the aperture down, say a cover over the front of the scope with an hole offset so that it misses the spider that holds the secondary mirror - this will give you a much longer focal length for your stopped down aperture ans importantly there will be minimal obstruction, guys with large dobs do this from time to time, as said though - a guide, best to observe when the planets at its highest in the sky, using a very stable atmosphere needs patience, from night to night you may not see any difference, then you get the odd night of near perfect seeing - these nights only come around once in a while.

Remember though larger apertures suffer more from poorer seeing - I know you just can't win!!

The bins will give you those nice widefield views, but the majority of the Messiers will be just smudges - EG M36, M37, M38 where as in a scope you will begin to resolve them much better.

Regarding the annoying neighbours light - see if you have a large blanket to place over your head whilst observing - this will increase contrast at the EP - looks stupid - but really works - use the dark blanket draped over your head and around the tube encircling the EP.

Hope that helps and keep us informed.  Paul.

[brantuk]

Have you tried an 8mm eyepiece in your dob? It'll give you 187x magnification which on a good night will show some very nice, sharp, and contrasty detail. The GRS for example would be no problem. The Orion Nebula will also benefit at this magnification - you'll get very clear whispy dust clouds with a very distinct hole containing the trapezium. A little more magnification and you may even split the "E" and "F" component doubles. I find the Hyperions are very nice for these objects - they're around £95 new but you can get them 2nd hand for around £60.

As for the neighbours I guess it depends how well you get on with them. There may be some tactful way of asking them to turn the light off. Or you could try getting them interested in a little astronomy by offering a view in your scope one night. Make sure it's directly under the offending light and see what they say..... Hth

[mitchelln]

A 300mm reflector is likely to be fast (f/4.5?) so could need £250 eyepieces and a coma corrector.

You do not need £250 eyepieces with a 300mm Dob. I started out with a couple of cheap 15 and 25mm Celestron plossls and they gave great views of Orion's Nebula and Jupiter. The most I've ever paid for a fixed EP was £69 for a William's Optics 6mm, which gives absolutely fantastic views of the planets.

All reflectors will have a degree of coma, but again, you also don't need a coma corrector unless you want absolute sharpness across the whole field. As you are tending to be concentrating on the object you have centred in the view, coma is not going to bother you. My 300mm Dobo is sharp across about 80% of the field without a coma corrector (which you can pick up pretty cheaply 2nd hand anyway).

Collimation is important with a big Dob otherwise you will lose detail and contrast, but once you master it it only takes a couple of minutes.

[davekelley]

Thanks everyone! I don't currently own a dob, I have bought and sold various scopes, I have used good eyepieces always, speers waler 10mm, and a few of the Hyperions, ultima barlow, and even televue powermate etc. I currently just have a 102mm f9 refractor but its an ED, semi apo. It gives nice contrast and sharpness, but the best I've seen is 2 bands on jupiter, and its a struggle to get the cassini division, all scopes I have owned, including 12 inch dob, have shown up DSO's as faint smudges, and nothing has really resolves into start even the edges of M15!  I suspect its my suburban location and my dark adaptation. I took my refractor to Brecon in summer but the sky was so full of stars and the milky way, I found it was difficult to even find my way around the constellations!  they just didn't stand out amongst so many stars!  I managed to locate M15....more or less the same bright fuzzy!!  Annoyed!

[davekelley]

Soory peeps, when I say M15, I mean M13, of course!

Thanks everyone! I don't currently own a dob, I have bought and sold various scopes, I have used good eyepieces always, speers waler 10mm, and a few of the Hyperions, ultima barlow, and even televue powermate etc. I currently just have a 102mm f9 refractor but its an ED, semi apo. It gives nice contrast and sharpness, but the best I've seen is 2 bands on jupiter, and its a struggle to get the cassini division, all scopes I have owned, including 12 inch dob, have shown up DSO's as faint smudges, and nothing has really resolves into start even the edges of M15!  I suspect its my suburban location and my dark adaptation. I took my refractor to Brecon in summer but the sky was so full of stars and the milky way, I found it was difficult to even find my way around the constellations!  they just didn't stand out amongst so many stars!  I managed to locate M15....more or less the same bright fuzzy!!  Annoyed!

[Moonshane]

even with poor collimation and the basic 'free' eyepieces, I'd expect you to see substantial differences in e.g. M42 and to resolve stars to the core in the brighter globs in a 12" dob vs a smaller frac. also, I'd expect you to see good detail on Jupiter in the dob. without seeing the scope etc it's hard to say.

as for lights I did this

http://stargazerslounge.com/topic/113007-observing-screen/

[Northern Soul man]

Nice 1 , I had a 12" revelation Dob - one of the early ones I think - must say to be honest - I never really got on very well with it - at the time i had an 8" Meade LX 90, I know there's an aperture difference and not that you can compare like for like, but I much preferred the views in the SCT - yes the image was brighter in the Dob  - but most say that you loose fine detail in the SCT, I found quite the opposite compared to the Dob and with the jump up to the 11" I found that I just seem to get on with the SCT - I know if you probably placed them side by side and if you could compare mag for mag taking the differences into account most would probably choose the 12" Dob, I'm only going from memory here, but the views through the Meade gave the edge for me when considering an upgrade to a larger SCT or a larger Dob, hence the larger SCT.

I must be honest though, the views of Jupiter near opposition around March time have probably been the best I've seen for a very long time and I've been at this for 25 years or so - probably on parr to the Shoemaker Levy impact, I was absolutely astounded when the first few fragments hit Jupiter - everyone was saying "No chance -  the Amateur in his back garden aint going to see anything of these impacts with Amateur instruments" I had an 8" F8 hinds Optics homemade Newt at the time, Jupiter resembled an owl looking back at us.  Going back to the views around opposition this year, i had the CPC set up with the Binoviewers in and had the best views for such a long time - even in the temperate latitudes towards the poles of Jupiter were showing detail that I have not seen before - the GRS was the reddest I've ever seen it with the ovals the most distinct I have ever seen them, the actual "trough" where the GRS sits was just as "contrasty" as the ovals following - all from an instrument thats not known for its contrasty views - never owning a Refractor or a large Mak - I bet the views were even more breathtaking - but the SCT, for me, has been a firm favorite, from the 8" Meade to my 11" CPC, I've found them to be a very capable instrument - I know - not a light bucket  -but its a keeper from me !!.

Paul.

[Charic]

davekelley.........Hi, I often use garden furniture to hide from the offending light sources, and with the vegetation almost back on the trees, street lighting is somewhat more controlled. But I often hide my head/eyes under a dark bath towel. If I cant see the light, it cant see me! and mye eyes are better protected. Just remember not to cover the end of the OTA?

Northern Soul Man........ "Remember though larger apertures suffer more from poorer seeing".......  I said that once! Just waiting for the fallout? apparently that's not the case for  light pollution, as pointed out by several here at SGL. I was quite surprised to learn why.

[cotterless45]

I think that your views of nebulae and clusters are affected by light pollution. I've seen M13 in pristine dark skies and it looks completely stunning. The widest stars look like diamonds thrown onto black silk. The core and surround resolve to give a propellor and myriad swirls of stars. Showing newbies this from town is quite under whelming.

Similar views with nebulae, M42 shows fine expansive filaments , M1 and M27 resolve into a bright filamentous targets standing out clearly from the background, not just bits of fuzz. NGC 7789 will blow you away and live up to it's description of "Caroline's Rose".

I use 10 washing line poles in holes in the ground and dark throws between them , held on by big plastic clips. The whole thing can be packed away and stops the light from neighbour's nocturnal illuminations and the ever annoying street lights,

Nick.

[chiltonstar]

Re local light pollution: I've tried various solutions, but for me a 1 to 1.5m square of black cloth works best. Set the scope up, pointing at the target, then drape the cloth to cover your head and scope, but leaving the front end uncovered (and the RACI in my case). The night vision will improve quite rapidly, and at this time of year, keeps the head warm as well!

Chris

[jason.p]

Hi Dave.

I've used various light screens over the last couple of years. I started with some old curtains hung over a washing line to this purpose made screen made of some left over roofing felt. Simple, cheap but effective!

[m37]

Thick black pillowcase over your head as a sort of viewing isolation booth when at the scope, blocks out the lights. And when you're moving around/going indoors, stick a kid's pirate* eye-patch over your viewing eye to preserve dark adaptation. Also use it to cover my other eye when observing so I don't get a squinting headache!

cheers

Chris

* pirate design optional but looks awesome.