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June New Moon Period, NV Lessons Learned

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#1 Jeff Morgan

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Posted 21 June 2018 - 11:45 PM

The Dark Sky window for June is closed. Despite losing three nights to unseasonable weather right at New Moon, I did get in four nights and over 20 hours of time, primarily using my 16" f/7 Newtonian both prime focus and afocal. A few observations from this window.

 

1) 50mm SLR Camera Lens

 

Start with the fun stuff first: 50mm (1.8x magnification) is an awesome addition to the NV kit! The 22 degree field frames small and medium constellations nicely, and of course with the depth only NV provides. Using a 7nm h-alpha filter I was able to pick up the Zeta Ophiuchi nebulosity plainly. 50mm is a fun niche!

 

These lenses can be had at very fast speeds for moderate prices. The Canon FD lens I acquired is f/1.4 and it only cost $105 shipped from Japan. It’s a little bulkier than the native ENVIS lens, but still balances wonderfully in the hand. It’s a real treat at the end of a telescope session to flop down the in the Zero Gravity recliner and just wander for 20 minutes.

 

In the end I suspect I will add 85mm and 185-200mm telephotos to supplement my 50mm and 135mm lenses and cover the “sub-telescopic” framing options. Yes, low power NV viewing is that good.

 

2) Galaxy Summary

I used much of this window to finish out my Ursa Major, Leo, Leo Minor, and Virgo galaxy lists. Over 120 galaxies in this window. At last I have found the overall best strategy for delivering detail on galaxies! It is the same as with conventional eyepieces - use a bigger scope!

 

Magnification vs. Speed
Early on, I noted that star clusters are much less affected by slower f/ratios, whether it be from the scope, or an added barlow. The reason is that they are point sources. Since galaxies are made up largely of stars, I hoped they would also take magnification well. And by dumb luck, the first galaxy I tried this on (M82) had the correct morphology, surface brightness, and significant detail due to it’s proximity.

 

Unfortunately, that was more the exception than the rule. For the large majority of galaxies, just going with the fastest speed option gives the best result. Especially face-on spirals. The reason being that the intensifier craves the speed and truth be told, most galaxies just don’t have intricate detail to offer (at 16” aperture) so there is little upside to more image scale.

 

It’s a big sky and there are exceptions of course. If the galaxy type is Elliptical it will take magnification well, usually. But again, not much detail to offer (note - Centaurus A is already large, so the barlow added little).

 

Edge-on spirals and irregulars with high surface brightness also do well with barlowed intensifiers. In addition to a size that is more pleasing to the eye, sometime's these types offer gains in detail too. But sadly, by the numbers there are just not many M82’s out there. My galaxy technique is to start with either the 55 afocal or prime focus + 0.7x reducer first. If it has some promise, it's easy to remove the reducer and go native f/7 (103x in my scope) and evaluate the potential for further magnification.

 

Filters
I have been using a DGM GCE filter throughout my spring galaxy observing, with over 200 galaxies logged. For those not familiar, this is a filter intended for use in glass eyepieces that suppresses wavelengths centered around common light pollution wavelengths. Overall, it would be classified as a Broadband type. Results were best with face-on spirals, but even in these cases it was only a subtle enhancement. OTOH, I never felt the filter was harming the image either. All things considered, I don’t see this filter gaining widespread use with for NV galaxy work.

 

When I started with NV the Conventional Wisdom was you needed to have a long pass filter, so I bought the 640 based on my light pollution levels. Later I acquired the 685. The 685 makes the field quite dark. Initial results were not impressive, but more trials are needed.

 

However the trend is the more I observe, the less I use long pass filters. Best usage is for targets low in a light pollution dome. I almost never use them anymore at high target elevations, here “raw” is best. Inasmuch as there are some cases where long pass filters are useful and the cost is low, I’ll keep them around.

 

These days my advice to aspiring NV astronomers would be to get two h-alpha filters and save the long pass filter for your second round of accessory purchases. Part of a complete kit yes, essential, not so much ...

 

3) Open Clusters, Dark Nebula, and NV

One of the problems with NV on open clusters (and to a lesser extent dark nebula) is that these targets lie along the galactic plane. NV brings up so many foreground and/or background stars the target is overwhelmed.

 

In the past, I have used the Detachment rating from the Uranometria Field Guide to determine which clusters would respond best to NV with “well detached” being the best. For Not Well Detached, I had been using my glass eyepieces.

 

Previously I had been adjusting manual gain upwards until scintillation became noticeable, and pretty much leaving it there. Well guess what? Use manual gain control to slowly ramp that down and viola! The cluster (or dark nebula) emerges. Trumpler 30 is an amazing example of this. Sometimes it’s the simple things you forget first ...

 

Credit here to Alan Green (alanjgreen) who mentioned the technique as a method to “pop” the arms on galaxies where high gain settings emphasize galactic cores. I merely tried his idea on different targets. And it delivers!


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#2 The Ardent

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Posted 22 June 2018 - 12:26 AM

Your galaxy summary pretty much parallels visual observation. It's all about surface brightness. Elliptical and edge on's have it,  face-on's don't (exceptions like M51 - it's just bright) 

 

My galaxy experience last week was NGC 6665 in Lyra. In my 18" dob, not seen with a regular EP.  With NV faintly visible with averted vision. It's faint! 

 

I bought some Canon FD lenses from KEH.com. They have a good selection, and some under $50. 


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#3 Eddgie

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Posted 22 June 2018 - 09:24 AM

Very nice report Jeff. I very much enjoyed reading it.

 

My own comments...

 

Everyone knows I am a major fan of low power, but as you are seeing more and more, every magnification you use has far more effect than people would think.  As I said elsewhere, the old telescope model seems to make people think that the difference between 1x and 2x is kind of moot, but my point was (and is) that this is like changing from 50x eyepiece to a 100x eyepiece, and put that way, it starts to make more sense that the view is quite dramatically different.    

 

Filters..  I had made this same point maybe a year ago.   Filters remove energy and while NV is not as sensitive to green as red, it is not totally insensitive to these shorter wavelengths, and with a good lens (no chromatic aberration) or a mirror, this extra energy does contribute.  I find that there is a tiny limiting magnitude loss with filters and in my slower dob, I rarely use long pass fitlers anymore.

 

On the other hand, I do think that the loss is very small and hard to see, and it could be that filtering might increase the life span of the device.   Every photon that passes though it brings it closer to end of life, and a very bright sky is putting a lot of empty sky glow energy though it.  I don't know enough about how the difference in sky brightness might affect the tube life, but the sky brightness increases considerably when I go from 640 to no filter.  I do though run unfiltered a lot when using slower scopes because like you, I think the filter robs a tiny bit.  For my fast lenses though, I just have the feeling that a 650 is offering some protection at only a tiny cost in performance.   I could be wrong about this, but it makes me feel better to have the filter on when working at very fast speeds under very bright skies.

 

On the open clusters, this is exactly what I had reported from my Roswell session.  The field can be sooooooo rich under dark skies that fainter clusters totally merge into the background, and brighter clusters might have no defining border.

 

Me? No gain on the Micro, but I like to see it that way!  I feel as if it is more of a "True" view than we see in photographs or glass eyepieces.  These clusters are generally much closer to us, but they truly do sit on backgrounds of millions of stars!  Why not  accept seeing them that way?  It is the true reality of your galaxy!  I do though understand the desire to corral them because that is what we all used to do before NV. Still, I am learning to love seeing the sky the way it really is and in a way that I could never see using conventional gear.  Photo exposures and gain control help us cage them, but a tiger in the open Savannah is far more interesting to me than a tiger in a zoo landscape and I find myself now wanting to see clusters in their true appearance with respect to the Savannah of stars that they inhabit.  We get to see them both ways though but don't be eager to always put the tiger in a cage.  Spend some time enjoying it the way it really appears because that is what full gain is giving you!

A most enjoyable report though and I am as always thrilled to see you post and I always learn from you and everyone else!

 

NV Excites me so much more than regular astronomy does.  It is like being on a galactic safari in a spaceship. I never get enough. 


Edited by Eddgie, 22 June 2018 - 09:28 AM.

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#4 NiteGuy

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Posted 22 June 2018 - 11:39 AM

Yikes! I had no idea that NV devices had a lifespan short enough to be of serious consideration. How many hours are these things typically rated to yield before they end up in the trash can? With the high expense involved, if they're somewhat short-lived, one of these will probably never find its way into my shopping cart.

 

Please just tell me something good/positive regarding the life span of NV devices to help my Friday along until Happy Hour gets here.



#5 Jeff Morgan

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Posted 22 June 2018 - 11:59 AM

Generally, 12,000 hours of use under whatever conditions the US Military specifies. I doubt they use filters like we do (but I did not read the contract requirements).

 

Eddgies comments are interesting, I would view them more in the context of extending that 12,000 hours. But let's look at it the other way:

 

During this observing window, I racked up around 20 hours - on the clock, not the tube. Since I turn my device off when I pull my eye away, there was significantly less than 20 hours put on the tube. (Indeed, approaching two years in I'm on my 4th CR123 battery, each gives about 40 hours of running time).

 

If I am in any danger, it is wearing out the on/off switch in the housing!

 

But supposing I did rack up 20 hours of actual tube time. There are 12 dark sky observing periods per year.

 

20 * 12 = 240 hours.

 

12,000 hour life / 240 hours per year = 50 years of tube life.

 

I'm actually very hopeful that I am buying a new NV eyepiece on my 107th birthday. 


Edited by Jeff Morgan, 22 June 2018 - 12:00 PM.

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#6 Starman81

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Posted 22 June 2018 - 01:55 PM

The Dark Sky window for June is closed. Despite losing three nights to unseasonable weather right at New Moon, I did get in four nights and over 20 hours of time, primarily using my 16" f/7 Newtonian both prime focus and afocal. A few observations from this window.

 

1) 50mm SLR Camera Lens

 

Start with the fun stuff first: 50mm (1.8x magnification) is an awesome addition to the NV kit! The 22 degree field frames small and medium constellations nicely, and of course with the depth only NV provides. Using a 7nm h-alpha filter I was able to pick up the Zeta Ophiuchi nebulosity plainly. 50mm is a fun niche!

 

These lenses can be had at very fast speeds for moderate prices. The Canon FD lens I acquired is f/1.4 and it only cost $105 shipped from Japan. It’s a little bulkier than the native ENVIS lens, but still balances wonderfully in the hand. It’s a real treat at the end of a telescope session to flop down the in the Zero Gravity recliner and just wander for 20 minutes.

 

In the end I suspect I will add 85mm and 185-200mm telephotos to supplement my 50mm and 135mm lenses and cover the “sub-telescopic” framing options. Yes, low power NV viewing is that good.

 

2) Galaxy Summary

I used much of this window to finish out my Ursa Major, Leo, Leo Minor, and Virgo galaxy lists. Over 120 galaxies in this window. At last I have found the overall best strategy for delivering detail on galaxies! It is the same as with conventional eyepieces - use a bigger scope!

 

Magnification vs. Speed
Early on, I noted that star clusters are much less affected by slower f/ratios, whether it be from the scope, or an added barlow. The reason is that they are point sources. Since galaxies are made up largely of stars, I hoped they would also take magnification well. And by dumb luck, the first galaxy I tried this on (M82) had the correct morphology, surface brightness, and significant detail due to it’s proximity.

 

Unfortunately, that was more the exception than the rule. For the large majority of galaxies, just going with the fastest speed option gives the best result. Especially face-on spirals. The reason being that the intensifier craves the speed and truth be told, most galaxies just don’t have intricate detail to offer (at 16” aperture) so there is little upside to more image scale.

 

It’s a big sky and there are exceptions of course. If the galaxy type is Elliptical it will take magnification well, usually. But again, not much detail to offer (note - Centaurus A is already large, so the barlow added little).

 

Edge-on spirals and irregulars with high surface brightness also do well with barlowed intensifiers. In addition to a size that is more pleasing to the eye, sometime's these types offer gains in detail too. But sadly, by the numbers there are just not many M82’s out there. My galaxy technique is to start with either the 55 afocal or prime focus + 0.7x reducer first. If it has some promise, it's easy to remove the reducer and go native f/7 (103x in my scope) and evaluate the potential for further magnification.

 

Filters
I have been using a DGM GCE filter throughout my spring galaxy observing, with over 200 galaxies logged. For those not familiar, this is a filter intended for use in glass eyepieces that suppresses wavelengths centered around common light pollution wavelengths. Overall, it would be classified as a Broadband type. Results were best with face-on spirals, but even in these cases it was only a subtle enhancement. OTOH, I never felt the filter was harming the image either. All things considered, I don’t see this filter gaining widespread use with for NV galaxy work.

 

When I started with NV the Conventional Wisdom was you needed to have a long pass filter, so I bought the 640 based on my light pollution levels. Later I acquired the 685. The 685 makes the field quite dark. Initial results were not impressive, but more trials are needed.

 

However the trend is the more I observe, the less I use long pass filters. Best usage is for targets low in a light pollution dome. I almost never use them anymore at high target elevations, here “raw” is best. Inasmuch as there are some cases where long pass filters are useful and the cost is low, I’ll keep them around.

 

These days my advice to aspiring NV astronomers would be to get two h-alpha filters and save the long pass filter for your second round of accessory purchases. Part of a complete kit yes, essential, not so much ...

 

3) Open Clusters, Dark Nebula, and NV

One of the problems with NV on open clusters (and to a lesser extent dark nebula) is that these targets lie along the galactic plane. NV brings up so many foreground and/or background stars the target is overwhelmed.

 

In the past, I have used the Detachment rating from the Uranometria Field Guide to determine which clusters would respond best to NV with “well detached” being the best. For Not Well Detached, I had been using my glass eyepieces.

 

Previously I had been adjusting manual gain upwards until scintillation became noticeable, and pretty much leaving it there. Well guess what? Use manual gain control to slowly ramp that down and viola! The cluster (or dark nebula) emerges. Trumpler 30 is an amazing example of this. Sometimes it’s the simple things you forget first ...

 

Credit here to Alan Green (alanjgreen) who mentioned the technique as a method to “pop” the arms on galaxies where high gain settings emphasize galactic cores. I merely tried his idea on different targets. And it delivers!

 

Jeff, great reporting, thanks. Some queries:

 

- With regard to dropping the longpass filter... I do believe you live under 'dark' skies (yellow zone?) though you did not mention it in your report. I think that factor is very important, as many of us NV users are in Bortle 7/8/9 zones where the Longpass is indispensable. 

 

- On the OCs, did you experiment with varying the mag/speed to see how they respond? 

 

- About the galaxies and fast being better (in general)--this is what I had surmised. The best way to get more image scale is more aperture!

 

- Can you post more details of the SLR lenses you are using and what adapters were needed to use them?  Maybe even some pictures smile.gif. I am getting cautiously excited but I am thinking they cannot easily accept filters, but that must be a misconception since you mentioned running a 7nm h-alpha...



#7 Eddgie

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Posted 22 June 2018 - 04:38 PM

Yikes! I had no idea that NV devices had a lifespan short enough to be of serious consideration. How many hours are these things typically rated to yield before they end up in the trash can? With the high expense involved, if they're somewhat short-lived, one of these will probably never find its way into my shopping cart.

 

Please just tell me something good/positive regarding the life span of NV devices to help my Friday along until Happy Hour gets here.

Tubes don't just stop working at 10,000 hours or 12,000 hours.

 

This is my general understanding.  The tube life is based on the amount of time the tube will be able to deliver 50% brightness.  Every time you turn on the tube, and use it, the unit dims an imperceptible amount.  

 

As I understand it though, the brighter the image at the photocatode, the more quickly the tube might reach that point of 50% and the dimmer the average exposure, the more brightness it might preserve at that 10,000 hour mark.

 

But the point is the tube does not just stop working at this time.  It just slowly degrades over time.   For someone using the device under dark skies, I would guess that tubes will be bright for many thousands of hours of use and perhaps well past the 10,000 hour mark.   My concern though is that pointing an image tube to a bright, light polluted sky accelerates that dimming.   

 

I try to never point my tubes at any kind of intense light source like the moon or porch lights or car headlights.   The damage is almost infinite, but over the years, it could start to show up if this is a common exposure. 

 

Cnoct told me very specifically that the hours on a tube don't really matter that much.  He said what was important was the illumination levels the tube was subjected to over time. 

 

(Also this.  Tubes that have a lot of hours on them are easy to see.  Every now and then you see a PVS-7 tube for sale on Ebay that has a very uneven and dim looking display.  My guess these are military surpless with many thousands of hours that got removed from service because they did not meet spec anymore.  I have seen a number of them over the years.  The uneveness in the view I have come to believe is just due to the wear on the tube's phosphor screen and coating that is thinner is where it burned though first.) 

 

Plasma and LCD TVs often have the same issue. In time, they dim. A 40,000 hour life of a plasma TV was not rated on how long it would work without failure, but rather on how many hours it would go before it had dimmed to (as I recall) half of he the original brightness. Now 40,000 hours is a lot but TVs are on a lot.  And my guess is that this lifetime is based on standard programming demands. If the TV were left with a white screen showing, I bet the lifetime would be very greatly reduced because all of the pixels would be firing at full strength. 

And I think that what Cnoct was basically saying to me is that the same is true with NV.  The brighter the average image over the test illumination level, the sooner the tube will fall to 50% brightness. 


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#8 Jeff Morgan

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Posted 22 June 2018 - 07:25 PM

Jeff, great reporting, thanks. Some queries:

 

- With regard to dropping the longpass filter... I do believe you live under 'dark' skies (yellow zone?) though you did not mention it in your report. I think that factor is very important, as many of us NV users are in Bortle 7/8/9 zones where the Longpass is indispensable. 

 

- On the OCs, did you experiment with varying the mag/speed to see how they respond? 

 

- About the galaxies and fast being better (in general)--this is what I had surmised. The best way to get more image scale is more aperture!

 

- Can you post more details of the SLR lenses you are using and what adapters were needed to use them?  Maybe even some pictures smile.gif. I am getting cautiously excited but I am thinking they cannot easily accept filters, but that must be a misconception since you mentioned running a 7nm h-alpha...

 

I'm about 7 miles north of downtown Prescott. Sadly, my skies are steadily getting worse - yellow zone is becoming an optimistic estimate.

 

There are three active sports fields within two miles of me (city and university). It was devastating when the university installed lighting to make the students "safer". Never mind that the hush-hush assaults all occurred indoors at parties involving alcohol and people known to each other - why let facts get in the way when we can just go with primal fear of the dark to "solve" the PR problem?

 

Unfortunately, Prescott is somewhat of a boomtown being overrun with Kali refugees. A 3500 home development was approved less than two miles north of me. Great for property values, lousy for astronomy futures. The light pollution ordinances are on the books appear to be totally ignored in Prescott Arizona.

 

For now I am still good unfiltered. But my long pass filters may be getting more of a work-out in the near future. Like I said, no plans to sell them.

 

***

 

On the OC question, "yes". Generally I treat these like I was using standard eyepieces. That is, I keep adding power until either framing or seeing stops me. Stars dim somewhat at slower speeds, but not like nebula do so I can get away with a lot. Unfortunately, those background/foreground stars are mostly along for the ride. Adjusting the gain drops them out though, isolating the cluster much in the same way a H-alpha filter isolates the nebula. Come to think of it, Manual Gain is a filter.

 

***

 

lol.gif  Yes, expectations were my worst enemy with respect to galaxy viewing. Before the days of NV I fell into the same Expectations Game upgrading from a 12.5" to a 16". 

 

***

 

On to the happy topic! Just go to Amazon or eBay and search "Canon FD to C-mount" (or whatever lens brand you prefer). These run about $20 shipped. Stick with the same brand telephoto and you only need one adapter.

 

For filters, you can either glue a 1-1/4" filter thread on the inside of the adapter (workable, but clumsy for filter changes), or bite the bullet and use 2" filters with the appropriate step down. This is what I eventually did. Much nicer. The 50mm lens uses a 55 to 48 step down. Also available cheaply on the aforementioned sites.

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#9 SMark

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Posted 23 June 2018 - 03:18 AM

Tubes don't just stop working at 10,000 hours or 12,000 hours.

 

This is my general understanding.  The tube life is based on the amount of time the tube will be able to deliver 50% brightness.  Every time you turn on the tube, and use it, the unit dims an imperceptible amount.  

 

As I understand it though, the brighter the image at the photocatode, the more quickly the tube might reach that point of 50% and the dimmer the average exposure, the more brightness it might preserve at that 10,000 hour mark.

 

But the point is the tube does not just stop working at this time.  It just slowly degrades over time.   For someone using the device under dark skies, I would guess that tubes will be bright for many thousands of hours of use and perhaps well past the 10,000 hour mark.   My concern though is that pointing an image tube to a bright, light polluted sky accelerates that dimming.   

 

I try to never point my tubes at any kind of intense light source like the moon or porch lights or car headlights.   The damage is almost infinite, but over the years, it could start to show up if this is a common exposure. 

 

Cnoct told me very specifically that the hours on a tube don't really matter that much.  He said what was important was the illumination levels the tube was subjected to over time. 

 

(Also this.  Tubes that have a lot of hours on them are easy to see.  Every now and then you see a PVS-7 tube for sale on Ebay that has a very uneven and dim looking display.  My guess these are military surpless with many thousands of hours that got removed from service because they did not meet spec anymore.  I have seen a number of them over the years.  The uneveness in the view I have come to believe is just due to the wear on the tube's phosphor screen and coating that is thinner is where it burned though first.) 

 

Plasma and LCD TVs often have the same issue. In time, they dim. A 40,000 hour life of a plasma TV was not rated on how long it would work without failure, but rather on how many hours it would go before it had dimmed to (as I recall) half of he the original brightness. Now 40,000 hours is a lot but TVs are on a lot.  And my guess is that this lifetime is based on standard programming demands. If the TV were left with a white screen showing, I bet the lifetime would be very greatly reduced because all of the pixels would be firing at full strength. 

And I think that what Cnoct was basically saying to me is that the same is true with NV.  The brighter the average image over the test illumination level, the sooner the tube will fall to 50% brightness. 

My ANVIS binocular has a pair of older, identical tubes with the same dates. Yet one tube is noticeably weaker than the other. I have always assumed that the weaker tube was simply nearing the end of it's useful life and should continue to weaken. But, to be honest, maybe it's just as likely that one tube is higher or lower gain than the other? There are no gain controls with my unit, so they are what they are. I wouldn't know how to tell the difference between the two scenarios. What do you think is more likely... That someone originally loaded a pair of tubes in this unit that had vastly different gain, or that they were originally matched and one tube seems to have aged faster than the other? And is there a way of knowing the difference? hmm.gif



#10 alanjgreen

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Posted 23 June 2018 - 07:08 AM

2) Galaxy Summary

I used much of this window to finish out my Ursa Major, Leo, Leo Minor, and Virgo galaxy lists. Over 120 galaxies in this window. At last I have found the overall best strategy for delivering detail on galaxies! It is the same as with conventional eyepieces - use a bigger scope!

 

Magnification vs. Speed
Early on, I noted that star clusters are much less affected by slower f/ratios, whether it be from the scope, or an added barlow. The reason is that they are point sources. Since galaxies are made up largely of stars, I hoped they would also take magnification well. And by dumb luck, the first galaxy I tried this on (M82) had the correct morphology, surface brightness, and significant detail due to it’s proximity.

 

Unfortunately, that was more the exception than the rule. For the large majority of galaxies, just going with the fastest speed option gives the best result. Especially face-on spirals. The reason being that the intensifier craves the speed and truth be told, most galaxies just don’t have intricate detail to offer (at 16” aperture) so there is little upside to more image scale.

 

It’s a big sky and there are exceptions of course. If the galaxy type is Elliptical it will take magnification well, usually. But again, not much detail to offer (note - Centaurus A is already large, so the barlow added little).

 

Edge-on spirals and irregulars with high surface brightness also do well with barlowed intensifiers. In addition to a size that is more pleasing to the eye, sometime's these types offer gains in detail too. But sadly, by the numbers there are just not many M82’s out there. My galaxy technique is to start with either the 55 afocal or prime focus + 0.7x reducer first. If it has some promise, it's easy to remove the reducer and go native f/7 (103x in my scope) and evaluate the potential for further magnification.

 

Filters
I have been using a DGM GCE filter throughout my spring galaxy observing, with over 200 galaxies logged. For those not familiar, this is a filter intended for use in glass eyepieces that suppresses wavelengths centered around common light pollution wavelengths. Overall, it would be classified as a Broadband type. Results were best with face-on spirals, but even in these cases it was only a subtle enhancement. OTOH, I never felt the filter was harming the image either. All things considered, I don’t see this filter gaining widespread use with for NV galaxy work.

 

When I started with NV the Conventional Wisdom was you needed to have a long pass filter, so I bought the 640 based on my light pollution levels. Later I acquired the 685. The 685 makes the field quite dark. Initial results were not impressive, but more trials are needed.

 

However the trend is the more I observe, the less I use long pass filters. Best usage is for targets low in a light pollution dome. I almost never use them anymore at high target elevations, here “raw” is best. Inasmuch as there are some cases where long pass filters are useful and the cost is low, I’ll keep them around.

 

These days my advice to aspiring NV astronomers would be to get two h-alpha filters and save the long pass filter for your second round of accessory purchases. Part of a complete kit yes, essential, not so much ...


 

Credit here to Alan Green (alanjgreen) who mentioned the technique as a method to “pop” the arms on galaxies where high gain settings emphasize galactic cores. I merely tried his idea on different targets. And it delivers!

Jeff,

 

Firstly, thanks for the mention above smile.gif
Background: I was told that reducing manual gain increases the SNR of the device and that's why I started backing off gain as described by Jeff.

 

As far as broadband filters go then my results back up your comments. I live with SQM 21.6 skies and now use an Astronomik UHC broadband filter for non galaxy viewing on low to the horizon targets or when the moon is out. Filters provide little benefit at higher altitudes.

 

I tested and compared Astronomik CLS, Astronomik UHC and Baader 610nm on non-nebula targets. The only filter that gave me any extra on face-on Galaxies was the Astronomik UHC (and not really a noticeable gain over unfiltered).

- I sold the CLS and I am just saving the 610nm for more testing as it was relatively cheap to buy.

- The Astronomik UHC was the winner in my 3-way comparison but we are talking small gains though.

- The Astronomik UHC did give a nice dark background to work with whilst observing which means you can turn the gain down and increase the SNR. I had some success low down in Sagittarius on Messier targets with the UHC.

 

From my location face-on Galaxies are best unfiltered and as you say, the faster you can go, the greater the chance of seeing the spiral arms.

- The 55mm Plossl gives good results and I also then try the 35mm Panoptic for more scale too...

- I have seen the thick spiral arms of M101 at x1 from my location! Yes, it was TINY but they were visible.

 

I have now purchased a 12nm Astronomik Ha (thanks to winning a £100 voucher in a competition) to go alongside my 6nm Astronomik Ha filter and would be happy to receive any tips from others as to when the 12nm is better than the 6nm??
- I am interested to see how the 12nm goes with (a) smaller aperture refractor (b) overly bright nebula like M42
- any other tips/experiences from other folks most welcome...

 

Thanks,

Alan


Edited by alanjgreen, 23 June 2018 - 07:12 AM.

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#11 pwang99

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Posted 23 June 2018 - 07:41 AM

I find the 12nm to be great in darker skies (Bortle 3), and useless in my typical downtown Bortle 5 skies.

But under those darker skies, it nicely gives the "enhanced H-alpha contrast" that we like for big diffuse nebula, with much less scintillation, and more background stars showing through.


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#12 Eddgie

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Posted 23 June 2018 - 11:21 AM

 

I have now purchased a 12nm Astronomik Ha (thanks to winning a £100 voucher in a competition) to go alongside my 6nm Astronomik Ha filter and would be happy to receive any tips from others as to when the 12nm is better than the 6nm??

 

Thanks,

Alan

You will have them both so you will get your answer soon enough, but here are my own comments.

 

The biggest issue with narrower pass filters is band shift of the off center light rays, and this gets worse as the scope gets faster. 

 

I have a 12nm, 10nm, 7nm and 5nm and in all faster scopes, the band shift in the 5nm is the must severe and the 12nm is the least severe and even at very fast focal ratios, it is hard to see.

 

To me, this means that nebula targets that are big in scopes that are fast can loose a considerable amount of brightness.

Example..  Barnard's Loop under dark sky.   If 5nm and 7nm with 1x, you only the nebula that is close to the center of the field will be bright and away from center, the nebula can almost disappear.   In the 12nm, the loop and even Angelfish will all be visible in the field.

 

Under bright skies though, this can flip.  Barnard's loop can be so dim using the 12nm that it is really hard to see, but the better suppression of the fast fitlers means that what part you can see are brighter than in the 12nm.

 

In slow scopes, the noise can get bad with the 5nm and I hear people say they prefer the 12nm under those conditions, but I find the narrower pass filters almost always bring out the most detail, though this is best seen when it is near the center of the field. 

 

In general though I think if you made me pick, one or the other, I would pic a 5-7nm over a 10-12nm.   In the city for sure, I rarely use the 12s.      

 

In the binocular, I use the 7s a lot because this is a good compromise between the size of the brighter part of the field and the contrast, but in the slower scopes, I almost always reach for the 5nm.

 

 

Under bright skies, I think the 12nm is not very useful. Under very dark skies it will give a result closer to the 5-7nm, but most of the time, if it fits into the center 2/3rds of the field, the narrower filters I think do better. 

 

My opinion then is the 12nm is most useful when viewing large targets under darker skies using faster focal ratios and you want to see the target with an even level of illumination (or a realistic level of relative brightness). 

 

I don't limit myself though, and that is why I am a fan of scopes that can use a filter wheel.   I have several filters mounted and I can flip back and forth to select the one I think does best for the area in the field of view.


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#13 moshen

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Posted 23 June 2018 - 02:34 PM

 

- I have seen the thick spiral arms of M101 at x1 from my location! Yes, it was TINY but they were visible.

 

That's really incredible. I'm going to have to try this next time I'm somewhere dark.



#14 Jeff Morgan

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Posted 23 June 2018 - 08:44 PM

I have now purchased a 12nm Astronomik Ha (thanks to winning a £100 voucher in a competition) to go alongside my 6nm Astronomik Ha filter and would be happy to receive any tips from others as to when the 12nm is better than the 6nm??

 

Not sure if I can answer that directly because it involves aperture, speed, environment, and target. Oh, and then there is Manual Gain. Head spinning yet?

 

But observational technique is easy enough:

 

Case 1

You start with the 12nm and the image looks good (and it almost always will vs. long pass or unfiltered). The questions then are two:

 

  1. Can it take more?; and
  2. How much more?

 

Two questions, and quite a dilemma if you happen to gave three H-alpha filters to choose from!

 

Case 2

You start with your most restrictive filter. Now the only question is:

 

  1. Is the tube showing signs of photon starvation?

 

One question, with a simple solution: drop down to the looser filter. And if you had three filters as above, you're done if the Mama Bear fixes the photon starvation.

 

While this is not a direct answer to your "Rules of Thumb" question, the strategy does tend to minimize filter swaps. Unless of course you are curious about the "looser" filter. Perhaps likely since this hobby generally draws perfectionists and OCD types lol.gif And then the issue is moot because you will cycle through all of the filters anyway!


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