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markclaire50

Limiting Magnitude - stories in real world!

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Hi. 

This thread is all about members real life experience with limiting MAGNITUDE (LM). 

1. What is best reference source for calculating a scopes theoretical LM? 

2. Do you ever reach this figure in a dark sky, by which I mean a 6.5 naked eye limited sky? 

3. If you can only see magnitude 5 stars, due to LP, do you also see 1.5 magnitudes knocked off your scopes theoretical LM in real world situations. 

4. Diffuse objects like comets- how does its magnitude relate to ease of viewing in a given scope- example I've seen members with real problems seeing Iwamoto at mag 10-11, with apertures that would find this magnitude easy if it was a star. Anyone know how to link comet magnitude with point source magnitude? Example would a mag 11 comet be equivalent to looking for a mag 13 star? 

ANY OTHER STORIES ARE MOST WELCOME! Stories of how you observed stars in your scope to its theoretical magnitude limit. Stories about the dimmest DSOs you have detected, with or without LP. Anything you think relates aperture and magnitude. 

Thanks 

Mark

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35 minutes ago, markclaire50 said:

Interestingly, difference between novice and expert is huge! 

I have a paper somewhere (which I can post if you are interested) that gives the algorithms...

Chris

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3 minutes ago, chiltonstar said:

I have a paper somewhere (which I can post if you are interested) that gives the algorithms...

Chris

Hi

Thanks Chris. I would be interested in that. 

Mark

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I find Mel Bartels site very useful for calculating these things, including diffuse object visibility.

https://www.bbastrodesigns.com/VisualDetectionCalculator.htm

I use an SQM-L meter to measure sky background brightness which is an interesting and useful way of gauging how good a sky is. I know that my skies have improved from around mag 18.5 to around 19.1 using this measure since my streetlights were upgraded to LED.

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26 minutes ago, Stu said:

I find Mel Bartels site very useful for calculating these things, including diffuse object visibility.

https://www.bbastrodesigns.com/VisualDetectionCalculator.htm

I use an SQM-L meter to measure sky background brightness which is an interesting and useful way of gauging how good a sky is. I know that my skies have improved from around mag 18.5 to around 19.1 using this measure since my streetlights were upgraded to LED.

Very interesting Stu. 

Thank you. 

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30 minutes ago, Stu said:

I find Mel Bartels site very useful for calculating these things, including diffuse object visibility.

https://www.bbastrodesigns.com/VisualDetectionCalculator.htm

I use an SQM-L meter to measure sky background brightness which is an interesting and useful way of gauging how good a sky is. I know that my skies have improved from around mag 18.5 to around 19.1 using this measure since my streetlights were upgraded to LED.

Hi Stu

There is a bug in the calculator. 80mm aperture gives 16 mag LM, but 3 inches gives about 10.looks like inches work, mm doesn't. 

Mark

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Another interesting aspect relates to whether there is a 'Comfort-Gap' i. e. a gap between what theoretically can be seen and what is seen. Example-- if my scope is theoretical mag 11, under dark skies, would I find it hard to see a mag 11 star under dark skies? Would I need some extra leeway, say LM of 11.5 to see it comfortably? Take the E and F stars, mag 10.3. I simply can't see these in either 80mm frac or 127 mm mak. Angular seperation from trap stars is well within the scopes capability. I've tried several times with both scopes, different nights throughout Jan/Feb, but simply cannot pick out e or f. If seperation isn't the issue, is this a magnitude "too close to my LM" issue or some other factor. I probably need to look at sky safari and pick some mag 4-6 stars then determine actual naked eye limit and see if I can determine LM for my scopes, when moonless. 

Edited by markclaire50

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41 minutes ago, markclaire50 said:

Hi Stu

There is a bug in the calculator. 80mm aperture gives 16 mag LM, but 3 inches gives about 10.looks like inches work, mm doesn't. 

Mark

That is correct Mark, should have mentioned it. Inches work, mm don't.

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27 minutes ago, markclaire50 said:

Another interesting aspect relates to whether there is a 'Comfort-Gap' i. e. a gap between what theoretically can be seen and what is seen. Example-- if my scope is theoretical mag 11, under dark skies, would I find it hard to see a mag 11 star under dark skies? Would I need some extra leeway, say LM of 11.5 to see it comfortably? Take the E and F stars, mag 10.3. I simply can't see these in either 80mm frac or 127 mm mak. Angular seperation from trap stars is well within the scopes capability. I've tried several times with both scopes, different nights throughout Jan/Feb, but simply cannot pick out e or f. If seperation isn't the issue, is this a magnitude "too close to my LM" issue or some other factor. I probably need to look at sky safari and pick some mag 4-6 stars then determine actual naked eye limit and see if I can determine LM for my scopes, when moonless. 

This is a good point Mark, I struggle with E and F at times and I do wonder if it is more related to limiting magnitude than separation. Last night in my 8" I was getting down to about mag 11.5, possibly a little deeper, and the calculator is saying between 11.4 and 12.4 for my scope and skies. I guess a mag 10.3 star close to the mag 5.1 C component just gets swamped by the star and LP.

It is not an exact science though. Visibility depends on dark adaptation, exit pupil, transparency, observer experience and more I'm sure.

Well worth checking out your NELM and also LM through the scope at a variety of mags so you get a flavour for it. You should see fainter stars at higher mag because the sky background gets dimmer but the star remains the same surface brightness because it is a point source.

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OK, so tonight I nipped outside about 10pm before Satan had risen much in the west. I could detect mag 4.2 stars just on edge of direct vision. 

I also remember seeing stars in same fov last night as iwamoto, and just checked. One I definitely remember, as it was in a triangle formation, is mag 10.77. 

This means that last night I could see a dimmer star than the E and F stars, but not either of the latter. So, neither angular separation or magnitude could be the source of problem. I wonder if the bright cloudy nebulosity haze in front of these stars destroys the contrast just enough to prevent my scopes cutting through. Maybe the angular separation is tight enough that together with the haze and mag difference between the adjacent stars have a cumulative effect making them out of reach for me. ?

Edited by markclaire50
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8 hours ago, markclaire50 said:

OK, so tonight I nipped outside about 10pm before Satan had risen much in the west. I could detect mag 4.2 stars just on edge of direct vision. 

I also remember seeing stars in same fov last night as iwamoto, and just checked. One I definitely remember, as it was in a triangle formation, is mag 10.77. 

This means that last night I could see a dimmer star than the E and F stars, but not either of the latter. So, neither angular separation or magnitude could be the source of problem. I wonder if the bright cloudy nebulosity haze in front of these stars destroys the contrast just enough to prevent my scopes cutting through. Maybe the angular separation is tight enough that together with the haze and mag difference between the adjacent stars have a cumulative effect making them out of reach for me. ?

I'm sure it is a combination of factors Mark. The nebulosity will presumably increase the sky background a little, making it harder to pick out these stars, but the main factor will be glare from the nearby stars masking them. Don't forget the effects of seeing, cooling and collimation on this too. Seeing can be local eg thermals from houses and central heating etc, it can be due to the jetsream or weather systems, but is also very dependent on the altitude of the object. We have a much harder time with these objects than the chaps in the Southern Hemisphere.

Scopes which have excellent contrast and produce tight star images are most likely to succeed in this challenge. The best view I've ever had of these bar none, was with Gavstar's 160mm TEC triplet. With top quality glass, no diffraction spikes or central obstruction, and not being affected by tube currents in the same way as compound scopes, plus observing over fields gave a beautiful image with the main four stars controlled very tightly and E and F plainly and constantly visible as lovely tiny pin points with direct vision. Amazing!

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1 hour ago, Stu said:

I'm sure it is a combination of factors Mark. The nebulosity will presumably increase the sky background a little, making it harder to pick out these stars, but the main factor will be glare from the nearby stars masking them. Don't forget the effects of seeing, cooling and collimation on this too. Seeing can be local eg thermals from houses and central heating etc, it can be due to the jetsream or weather systems, but is also very dependent on the altitude of the object. We have a much harder time with these objects than the chaps in the Southern Hemisphere.

Scopes which have excellent contrast and produce tight star images are most likely to succeed in this challenge. The best view I've ever had of these bar none, was with Gavstar's 160mm TEC triplet. With top quality glass, no diffraction spikes or central obstruction, and not being affected by tube currents in the same way as compound scopes, plus observing over fields gave a beautiful image with the main four stars controlled very tightly and E and F plainly and constantly visible as lovely tiny pin points with direct vision. Amazing!

Hi Stu. 

Yes, I've been reading a plethora of random success and failure stories about this. It is clear that seeing (another topic in its own right!) is the devil incarnate if it's going against you. I also learned that refractors seem to get relatively more success ( inch for inch) than other scopes (well, I knew that anyway) . I do have plans to get a bigger weapon to deal with these stars (that's not the only reason ?). I'm hopeful that a C9.25 will do the trick, as currently it has moved above a 180mak in my list of possible choices. Interestingly I do see many reports of 150mm newts picking them out with ease but other people with 200mm and 250mm newts struggling! What's that all about? Seeing and transparency, I must conclude! I also see discussion about goldilocks magnifications, not too low or high, but then others who say aperture AND high mag is the key to crushing these tricky upstarts!  I can believe all are true depending on location of observer and/or seeing/transparency. 

For me, in my particular situation, I am going down the bigger aperture plus higher mag option to crack this one. Tempted by a sw120, but new C9. 25 scopes can probably be bought in same price range and I'm sure can outmatch a 5" frac. 

If anyone has used a C9.25 to successfully see E and F, I'd like to hear your comments. ?

Mark

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I think a decent quality longer focal ratio 150mm newt like an f8 or even an f11 would be a good option. That aperture will still cut through poor seeing quite well, whilst having enough resolution and brightness to see them relatively easily.

My seeing is quite poor and I don't recall much success with my C925 but that may have been down to cooling and collimation too.

What are your observing conditions like? It is worth understanding because throwing aperture at the problem may not be the best option if you have poor seeing or observe over houses for instance.

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11 minutes ago, Stu said:

I think a decent quality longer focal ratio 150mm newt like an f8 or even an f11 would be a good option. That aperture will still cut through poor seeing quite well, whilst having enough resolution and brightness to see them relatively easily.

My seeing is quite poor and I don't recall much success with my C925 but that may have been down to cooling and collimation too.

What are your observing conditions like? It is worth understanding because throwing aperture at the problem may not be the best option if you have poor seeing or observe over houses for instance.

Hi Stu

I'm quite a new observer, so not sure I'd know good seeing vs bad seeing yet. I'd need more observation hours under the belt with the same scope. However I can say there are houses under my line of observation. But I'm not in a town or city.I'd be surprised if we'll insulated roofs (fairly modern houses) were causing the issue. I am rather concerned to hear your failure with a C9.25?

Don't say I have to start my research over again ????

Perhaps I should get both. I had been toying with getting an OO vx6L which has an F8 ratio. Asking for 1/10wave mirror option for good measure. These scopes are not too expensive and would certainly be portable. Might even be OK for DSO? Final question Stu. Please tell me that you have had superb views of the planets in your C9.25? The pup? Any positives to persuade me.... it is worth getting, rather than a 180mak?

Thanks 

Mark

 

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On ‎22‎/‎02‎/‎2019 at 15:10, markclaire50 said:

ANY OTHER STORIES ARE MOST WELCOME! Stories of how you observed stars in your scope to its theoretical magnitude limit. Stories about the dimmest DSOs you have detected, with or without LP. Anything you think relates aperture and magnitude. 

Thanks 

Mark

Dark adaption, permitting enough time for your pupils to dilate and gain their full potential night vision, particularly sensitizing the rods (light sensitive cells) at the periphery of the retina. Often challenging from home unless a hood or something is used to assist, far easier at a remote and dark sky location. Observing at home, built up environments, emit thermals from peoples central heating systems, any air turbulence or overhead plane contrails will create a fluctuation in seeing conditions. Not being sure of your location Mark, perhaps if you could venture somewhere a little darker away from any buildings, if just occasionally. It isn't necessarily about aperture, magnitude or which type of scope. Often it is much more about getting out and investing spending time learning and gaining as much as you can with what you currently have and in doing so taking the good with the bad and the inconclusive. Through this process, you will forge a much stronger conviction in terms of a future instrument. If you were interested in getting a scope with premium mirrors, just marking up on one you'd mentioned, an OOUK VX8L F6 on a dobsonian base with the mirror spec, do occasionally come up pre owned and are a great and portable all rounder, just my personal bias though.  

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8 minutes ago, scarp15 said:

Dark adaption, permitting enough time for your pupils to dilate and gain their full potential night vision, particularly sensitizing the rods (light sensitive cells) at the periphery of the retina. Often challenging from home unless a hood or something is used to assist, far easier at a remote and dark sky location. Observing at home, built up environments, emit thermals from peoples central heating systems, any air turbulence or overhead plane contrails will create a fluctuation in seeing conditions. Not being sure of your location Mark, perhaps if you could venture somewhere a little darker away from any buildings, if just occasionally. It isn't necessarily about aperture, magnitude or which type of scope. Often it is much more about getting out and investing spending time learning and gaining as much as you can with what you currently have and in doing so taking the good with the bad and the inconclusive. Through this process, you will forge a much stronger conviction in terms of a future instrument. If you were interested in getting a scope with premium mirrors, just marking up on one you'd mentioned, an OOUK VX8L F6 on a dobsonian base with the mirror spec, do occasionally come up pre owned and are a great and portable all rounder, just my personal bias though.  

Thanks Scarp. I'm beginning to realise that I need to find a dark site. Preferably one without the general public around. They'd probably wonder what I was up to! ?

The set up I have now is highly portable with a 10Ahr tracer lithium battery ( very good!) to keep me powered for hours. I have a sw az alt WiFi mount which creates its own WiFi network and links to my phone and to sky safari via syn scan. I've learnt that goto and tracking is for me, based on past experience. But I've also learnt it is essential to have a raci finder too, as sometimes goto is off target and correcting it via main scope is a nightmare! So, there is nothing stopping me going to a dark site if I can find one in Nottinghamshire. Then I can really see what my scopes can do! 

Cheers

Mark

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I had a session recently where I noted that naked eye I could see to approx Mag 3.5 (this is what I noted but experience tells me this might be slightly underestimating), in a 50mm finder (9x magnification) I could see to Mag 7.5, and in a 120mm refractor at 60x magnification I could see to approx Mag 10.4

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1 hour ago, Paz said:

I had a session recently where I noted that naked eye I could see to approx Mag 3.5 (this is what I noted but experience tells me this might be slightly underestimating), in a 50mm finder (9x magnification) I could see to Mag 7.5, and in a 120mm refractor at 60x magnification I could see to approx Mag 10.4

Hi Paz. This fits with my belief that you can deduct naked eye mag (3.5) from max mag naked eye under perfect dark sky (6) in this case giving 2.5.deduct this from theoretical limiting mag of a 5 inch(13.4) and you get 11. Not far off. I know this is a bit rough, but maybe more evidence needed of whether it's semi-valid. Would need other reports. 

Mark

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