Contrast

Don't be shy! If you're just starting out, here's the place to ask that first question

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The Bat
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Contrast

Post by The Bat »

Hi, my name is Rachel and this is my first post on the forum. I have long had a passing interest in all things astronomical but have only been actively observing since April. Last December I just about knew about 4 constellations by sight, but after avidly devouring some stargazing guides that I was given as a present, I went on to research and buy my first telescope, a Celestron C8-S and now I have ticked off something like 39 Messier objects, 15 or so NGC’s and all the superior planets. All without a Go-to, so I am feeling rather pleased with my efforts so far.

Although I am star-hopping my way around OK, I am having problems actually seeing detail in galaxies. Most appear as fuzzy smudges and all I can make out is which way up they are, whereas all the magazines talk about being able to make out dust lanes and occasionally spiral arms. I have read plenty about observing tips such as averted vision and proper dark adaptation, what I want to know is, is it a problem with my observing skills, light pollution, summer sky, eyepiece, diagonal or all of the above? How can I achieve better contrast? Should the background sky in the eyepiece be black (because it isn’t)? At the moment I am using the diagonal that came with the telescope, but I was advised from the outset that I should consider upgrading that at some point. I have Televue 32mm and 15mm Plossl eyepieces and a 2x Barlow. I live in a small market town with some light pollution, but it’s not actually too bad and they do turn the streetlamps off at 1am which disappointingly didn’t make a huge difference. Should I up the power to see detail or not?

I have an ever growing wish list which includes a new diagonal, dew heaters, funky eyepieces and imaging equipment (a whole new minefield), but as I don’t have a bottomless bank account, I want to prioritise. Presumably, there is no point in splashing out loads on imaging if my diagonal is mediocre, likewise with expensive eyepieces. What I would like to know is how much difference will a decent diagonal make? I have in mind a 2” William Optics that was recommended to me (as I will one day want to go for 2” eyepieces), but I don’t want to splurge the best part of £200 and find it makes no difference to my viewing. Any comments would be appreciated.
Celestron C8-S XLT
CG5 mount, dual axis motor driven
Imaging Source DFK21AF04.AS camera
North Essex Astronomical Society
A
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Post by A »

Hi and welcome.

I've used several diagonals over the years. At the moment I have Meade, Skywatcher, Celestron and William Optics. But also used the more expensive ones on the market. To be honest I cannot see any real noticeable difference in the eyepiece between them.

The stock ones that come with the telescope are not to bad, the problem is that many of them are out of alignment. I recently had to take my Meade one apart for this very reason.

I always thought that the contrast issue that you are experiencing is more down to the design of the telescope rather than say a problem with the diagonal.
I'm sure others will be along with their views.
brian livesey
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Post by brian livesey »

Bear in mind, Rachel, that galaxies are at vast distances from us, so the amount of detail we see with the eye through small to moderate-sized 'scopes is necessarily limited: they don't look anything like the intricately detailed galaxy pictures we see in astro-mags, more like smokey-grey smudges.
Despite the visual limitations, it's satisfying to be able to see the ghostly impressions of objects lying millions of light years away.
There's definitely one galaxy's dust-lanes you can see in an 8-incher and that's the Black Eye galaxy in Coma Berenices. There are others too that other forum members might like to recommend. The Great Andromeda Galaxy (M31) also shows dust lanes in an 8-inch 'scope.
brian
The Bat
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Post by The Bat »

Thanks for the replies. I am under no illusions about seeing Hubble-esque images down the scope, but the impression I am getting is that what classes as a 'large scope' in Sky at Night magazine (>150mm SCT) is not quite large enough for seeing the kind of detail they tend to describe. If it's a limitation of my scope, I can live with that. I didn't want to spend loads of money on accessories to find nothing makes much difference. I just need to set my expectations a little lower.

I will certainly revisit the Blackeye Galaxy when it comes round again. This was one of my first DSO's, but that was when my observation wasn't up to much and my scope was somewhat out of collimation which I didn't realise at the time. Hopefully my view next time round will be better. If there are any others that people can recommend - finding DSO's is one thing, but knowing which are the best ones with the most distinguishing features is something else.

It's still a thrill finding a new object. I get quite excited about the fact that photons that have travelled millions of lightyears are coming to my eye. And just finding stuff is satisfying.
Celestron C8-S XLT
CG5 mount, dual axis motor driven
Imaging Source DFK21AF04.AS camera
North Essex Astronomical Society
mike a feist
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Post by mike a feist »

if my diagonal is mediocre

What an interesting point! I have many times come across information and questions about the quality of the main mirrors, eyepieces ( cheap and fancy), and even sky diagonals (mirror or prism) but never about the quality of the diagonal. The question is, of course, by how much do diagonals vary? How optically-flat is optically-flat and how much difference does it make? I would imagine that the reflectively of the aluminium coating on all mirrors is generally similar when new, after all aluminium is aluminium is.... (unless they have an extra coating !) and would also think that if anything it would be the reflectivity that would be the key element when deep-skying. maf
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Post by A »

I think you find that there are two types of people when it comes to diagonals, those that spend lots of dosh and say they can see a big difference, and those that spend a lot of dosh and see extremely little difference when comparing against the cheaper units on the market.
I am one of the latter.

The only benefit I can see from the dielectric 1/10th wave units is that the coating withstands cleaning and will last longer than the standard mirror coatings.

My dielectric unit only produces a very,very slight difference on DSO, but the same detail is still visible in the cheaper unit. On the planets,
I see no difference, and I also see no difference in the sky background betweeen the two.
I have also seen reports that state that the 2"inch versions can have un-even coatings because of the dielectric coating process.

The only problem with the stock units is that most are out of collimation. Once adjusted, they work extremely well.

One interesting thing nowadays is that the manufacturers of the lower end APO scopes now seem to be messing around with the wavelengths of the lenses. As a result, you may find that a faster APO scope will produce better results when used with a prism diagonal.
Eclipse
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Post by Eclipse »

My stellarvue will take 2" eyepieces, so I splashed out about 140 beer vouchers on a 2" star diagonal. It certainly seems to be more robustly made than its 1.25" brethren, and is actually collimated nicely.
Coronado PST with SME-40 double stack H@ filter
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