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Ciaran Meier

Advice on increasing contrast.

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Hello folks

Been observing from home lately given current curtailments.  LP eliminates all but the brightest stars and make DSO hunting a tad tricky.  Has anyone any experience on boosting contrast on these objects.  I know there's no miracle fix for this but it would be nice to tease out a bit more detail if possible. 

Current setup:

8 inch F6 newt.

6mm EP.  1mm exit pupil. x200

12mm EP. 2mm exit pupil.  X100

25mm EP.  4.2mm exit pupil.  X48

Any advice much appreciated. 

Ciaran. 

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That's a tough one. Light pollution will always interfere in your observations. A light pollution filter may help a very little bit, but not so much. For emission nebulae a good UHC or OIII filter will be of benefit.

Try to switch eyepieces on the deep-sky objects you observe. Sometimes, a bit less or more magnification will improve contrast just enough to make an object visible or to see more detail.

And although not everyone will agree: a larger telescope will show you more, even under heavy light pollution.

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Cheers.  

Yep, I had a feeling it was a long shot.  Aperture really is the only answer. 

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You can drive a lot of times to a dark spot with a small telescope for the price of a large telescope 🙂. But if you're restricted due to corona (don't know your country-specific regulations), then I guess aperture is the key. Or patience.

Maybe EAA can be of some help, but my experience is virtually nil in that area.

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I recently took my scope [130 f/7 ED Triplet] out to a dark site [dark blue on the light pollution map] for a couple nights. I have some relatives that live out in the countryside away from all the city lights. One thing I noticed was that a UHC filter is much more effective with dark skies than it is in mild/moderate light polluted skies. Case in point, the Rosette Nebula [NGC 2244]. From my light polluted back yard, regardless if I'm using a 10" SCT, 10" Dob, or 5" Apo [*with UHC filter], I can usually only see slight differences in brightness & darkness around the Rosette. I basically have to stare at it for several minutes before I start to notice any nebulosity surrounding the central cluster of stars, and it's still hardly distinguishable. From a dark site, it just pops right out clear as day. Even my relatives, who have no experience whatsoever with observing, could see it without any trouble. "It looks like a Christmas wreath." I used to think that UHC filters were better for blocking out light pollution [to an extent] so the nebulae could shine through with less interference. However, they perform much better from a dark location by increasing the contrast features of certain nebulae, just as their name implies. 

 

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As I suspect you are constrained in a similar way to many of us currently to observing at home, I would suggest concentrating for now on classes of deep sky objects that are least affected by light pollution such as globular and open clusters and the brighter planetary nebulae. Some of the brighter galaxies might also be available but many, and especially the face on ones, will be very hard to make anything of. Multiple stars and asterisms count as DSO's as well I think.

When travel becomes possible then you can seek out darker skies and expand your DSO repertoire.

 

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It might sound odd, but try increasing your magnification..higher magnification increases contrast and often makes details jump out more.

A good example of this is M13 in Hercules, a fine globular which looks fabulous in my 5" Apo at x175-x250. I start to resolve a lot of 11th and 12th magnitude faint stars with higher power. In your 8" you should be able to go at least this high and once your eyes are dark adapted you will see more. M57 in Lyra (the Ring planetary nebula) is another target that can take high power and stand out more and will be well places in the rising wast over the coming months.

Also, get a dark hood, or towel to put over your head, and consider winged eyeguards for your eyepieces if possible. And finally, if you have any artificial light sources overlooking your garden try to position yourself with your body between the source and your observing object (I appreciate this may not always be practicable).

Double stars are often a rewarding target if you suffer from light pollution..again, higher power = darker background and your 8" reflector if well collimated should split hundreds of close and colourful doubles quite easily, even with LP to contend with.

Good luck!😊👍

Dave

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Thanks for the helpful hints chaps.  I did suspect that there was little to be done about the LP.  F15rules mentioned double stars and as it happens I'm just back in after a few hours with my 15×70's tracking a few down.  

Look after yourselves 

Ciaran. 

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On 30/03/2020 at 04:55, Ciaran Meier said:

Hello folks

Been observing from home lately given current curtailments.  LP eliminates all but the brightest stars and make DSO hunting a tad tricky.  Has anyone any experience on boosting contrast on these objects.  I know there's no miracle fix for this but it would be nice to tease out a bit more detail if possible. 

Current setup:

8 inch F6 newt.

6mm EP.  1mm exit pupil. x200

12mm EP. 2mm exit pupil.  X100

25mm EP.  4.2mm exit pupil.  X48

Any advice much appreciated. 

Ciaran. 

The primary thing to do in an urban LP environment is use a bit higher powers than normally used at a dark site.  Yes, this will dim the object as well as the background, but the larger size and darker background will make most extended objects a bit easier to see.  For any star or star cluster (open, globular), higher power yields improved contrast and allows you to see fainter stars.

Many if not most beginners use too low a magnification for most objects and that's OK in dark skies, even though they'd see more at higher power.

With most DSOs (not the largest objects) observed in an urban setting, you probably shouldn't go lower in power to view them than 0.5D in millimeters, i.e. a 10" scope at 127x, or a 6" scope at 76x.

The lower powers can be quite nice, but not particularly when the skies are light-polluted.

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22 minutes ago, Don Pensack said:

With most DSOs (not the largest objects) observed in an urban setting, you probably shouldn't go lower in power to view them than 0.5D in millimeters, i.e. a 10" scope at 127x, or a 6" scope at 76x.

Cheers.

My 12mm gives me 100x, half the 200mm diameter.   

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One of the best things you can do to improve your views of DSO's from a light polluted area, is to block out the surrounding light. There's little you can do about general sky glow, but the light that strikes your eye at the eye/eyepiece interface is destructive to dark adaption. Only when you are properly dark adapted will you see your scope perform at its optimum. Cheap solution - buy a dark lightweight blanket such as a dog bed blanket and hang it over your head and eyepiece. Make sure you are comfortable because you'll be under it for at least 15 to 20 minutes before your eye becomes super sensitive to the fuzzy stuff. If you study a DSO for a reasonable amount of time, you'll soon begin to detect subtle detail you might assume is beyond your scopes capability. If you sketch, you should use a dim red light torch. Shielding your eye from surrounding light sources, even dim ones, will greatly improve even a small scopes ability to deliver contrasty deep sky views. 

A useful addition to a deep sky observers kit would be a UHC filter. Better in many ways than an OIII as it allows the starlight to pass through more easily.

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1 hour ago, mikeDnight said:

Cheap solution - buy a dark lightweight blanket such as a dog bed blanket

The other half does a bit of sewing (quite a bit actually) and I'm pretty certain she's got some heavy duty canvas fabric tucked away somewhere.  If I"m well behaved  I could convince her to put together a decent sized tailored light proof shroud.  Perhaps incorporate a bit of ventilation as well.   I'll drop a wee hint tomorrow 😀

Cheers and keep safe

Ciaran. 

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16 hours ago, Ciaran Meier said:

The other half does a bit of sewing (quite a bit actually) and I'm pretty certain she's got some heavy duty canvas fabric tucked away somewhere.  If I"m well behaved  I could convince her to put together a decent sized tailored light proof shroud.  Perhaps incorporate a bit of ventilation as well.   I'll drop a wee hint tomorrow 😀

Cheers and keep safe

Ciaran. 

Ciaran, it might be useful to explore the terms "contrast" and also the concept of Suiters "wobbly stack" (preservation of the arriving image, through the scope).

Contrast is the fixed difference between the sky brightness and the objects brightness. This points out that sky conditions (darkness) are the only the thing in our control re contrast. This includes local light intrusion etc.

Then there is the scope and features of its use ie scatter control, flocking (stray light) and magnification used etc.

Lots of info out there on all this and stray light control for your dob is a worthwhile endeavour IMHO.

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59 minutes ago, jetstream said:

Ciaran, it might be useful to explore the terms "contrast" and also the concept of Suiters "wobbly stack" 

Cheers. Quick google search on "wobbly stack" returned some very interesting threads here on SGL. Have to say it's a new concept to me. 

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Its a very useful concept:

 

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Unfortunately light pollution is really a problem when it come to faint fuzzys. It depends if it is what I call "skyglow" pollution, that is due to the large amount of light in the area due to it being a town , city or such. Or localise pollution, next door neighbours lights entering your viewings space, as such a hood or blanket over your head does then help. But with "Skyglow" pollution then there is very little you can do. You can go for the brighter DSO and depending on the level of pollution then this is achievable. Or just accept for the time being that you will have to limit your observation to planets, Luna and double stars. No matter how bad your light pollution is there are always certain objects available to view. Concentrate on what you can see from your light pollution Sky's for the time being.

But when you can eventually get to a "True dark site" then this wait will be even sweeter, as you will really appreciate how faint fuzzys just pop to the eyes under ink black Sky's , a treat for the eyes

 

 

 

 

 

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Posted (edited)
38 minutes ago, Timebandit said:

Concentrate on what you can see from your light pollution Sky's for the time being.

Yep,  I've been spending a lot more time on the doubles lately, and enjoying it.  Was out a few nights ago with a recently  "de-mothballed" refractor.  Bresser Messier 102mm supplied with an EQ5 mount which was bought in a local Lidl store about 15 years ago.  Loads of CA on Venus etc but a cracker on doubles. Quick setup too.  Took me a while to come to terms with the different viewing arrangement but really enjoying it now. 

Cheers all and let's hope we're over the worst soon. 

Ciaran. 

 

20200405_100755.jpg

Edited by Ciaran Meier
Typo.
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