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Bortle 5 - benefits (choice) of which NB filter to get


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I have a colour camera ASI294MC with an Esprit 100 F/5.5 scope and flattener. I'm also a newbie for imaging. Even so, I want to try using filters for narrow band targets - for me that is a new thing. Some choices in astro gear seem pretty clear cut, this one not so simple.

I have a 1.25" UV/IR Cut filter for putting inside the 21mm extension tube that comes with the ZWO. Now a second filter to replace it. There is an adapter ring that is 1.25" to T2 that takes no back focus and places the filter right in front of the sensor cover. I bought a 1.25" Optolong L-Pro broad band filter with the aim of reducing sky pollution and the effects of the moon but without an obvious hit to integration time.

I don't have any street lights and live about 5 miles from the nearest town and about 12 miles from the nearest large city. I'm not sure how to ascertain the true Bortle level but maps show it as Bortle 4 although I have my suspicions that it is higher e.g. closer to Bortle 5 in reality.

I'm looking at trying narrowband imaging and was looking at either the L-eNhance or L-eXtreme filters. At first I thought it was simply a case of picking the "new and improved" whiz-bang L-eXtreme, but after reading conflicting views I'm not so sure. On the one hand, it is claimed to be night and day better but equally the question of halos artefacts crops up. So the question is how much benefit will either filter provide and are the side effects of the L-eXtreme something to live with in exchange for perhaps increased contrast on moonlit nights? I was lucky enough to be given an L-eNhance filter, bought from FLO but I'm considering returning it to get the L-eXtreme - if that is a better choice and the additional cost is not so great with this size. There are other filters like IDAS but they are not available in my preferred size of 1.25".

My understanding is that a narrow band filter has good transmission characteristics in the pass band but still requires an integration time that is double not using a selective filter. Even so, the images I look at are considerably darker with the L-eXtreme so perhaps they need an even longer integration time? Some things I read about the L-eNhance are that it's not much better than L-Pro when helping out with Moon glow because it is already quite a wide pass band. Would either filter have an effect on the ability to achieve focus using a V curve (to help automate the process)? Are halos inevitable or are they a function of where the filter is placed and how far off-centre additional bright stars are? The choice to me is different to someone who was thinking of upgrading and taking a hit on the resale value (thanks FLO).

 

I am pretty confused as it's not obvious which of the two filters would be best. My original thought was to play it safe with the L-eNhance. False logic said that if I could use the L-Pro for normal targets then for NB the l-eNhance would be a good choice (if only having one filter that is available in my preferred 1.25" size). A few clouds later and I'm considering was that best. If I lived within a City the choice of cutting excessive light pollution vs getting artefacts would make the decision skewed towards the L-eXtreme. Is Bortle 4 or Bortle 5 a pivot point for which option to go for.

Simon

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The L-eXtreme is great, I'd go for that if I were you. (Or wait a little longer as new dual-band filters are on the way). Sure, you get some halos around very bright stars, but it's not that much of an issue really. You can minimise them during processing. For broadband targets, I doubt you really need any filter from Bortle 4/5.

For reference, I'm under Bortle 8 skies and use an L-eXtreme for narrowband targets, and no filter for broadband targets.

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In regards to L-eXtreme recommendation - check out this thread:

It appears that L-eXtreme filter does not play well with ASI294mc

There are flat issues, but also lights have strange gradients in them.

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4 minutes ago, vlaiv said:

In regards to L-eXtreme recommendation - check out this thread:

It appears that L-eXtreme filter does not play well with ASI294mc

There are flat issues, but also lights have strange gradients in them.

That's interesting, thanks @vlaiv Maybe worth waiting for the newer dual-band filters to see if they're any better then?

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7 minutes ago, Lee_P said:

That's interesting, thanks @vlaiv Maybe worth waiting for the newer dual-band filters to see if they're any better then?

Not sure if this is limited to 294mc camera.

That camera is sort of sensitive - people had issues with flats before (but that was down to how they were taken - flat assistant in APT did not work well).

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I have the ASI294MC Pro L-eXtreme combination and don't have any complaints - have a look at Astrobin for examples by putting "ASI294MC L-eXtreme" in the search window.

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Just now, GoldTop57 said:

I have the ASI294MC Pro L-eXtreme combination and don't have any complaints - have a look at Astrobin for examples by putting "ASI294MC L-eXtreme" in the search window.

What scope are you using those with?

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1 minute ago, GoldTop57 said:

I have the ASI294MC Pro L-eXtreme combination and don't have any complaints - have a look at Astrobin for examples by putting "ASI294MC L-eXtreme" in the search window.

Agreed plenty of folk are ok. I wish I was or could work out what I'm doing wrong.....

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3 minutes ago, GoldTop57 said:

I have the ASI294MC Pro L-eXtreme combination and don't have any complaints - have a look at Astrobin for examples by putting "ASI294MC L-eXtreme" in the search window.

Never mind scope - what size of filter do you have?

I just had a look at Astrobin and images are produced with 2" version of filter. This means that filter is used in front of any corrective optics - like field flattener.

1.25" version is usually used in front of sensor and between sensor and field flattener - that might be the issue.

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Aside from the fact that some users of the 294mc report issues with the l-extreme, in theory it would be better due to it's narrower passband, blocking more unwanted light and increasing contrast. 

There should be no issues with autofocus routines, but you'll probably have to expose for longer, so it'll take a little longer. I have the mono version of the 294, and use 10s exposures for narrowband autofocusing. Takes maybe 90 seconds to complete the routine. 

1 hour ago, SimM said:

My understanding is that a narrow band filter has good transmission characteristics in the pass band but still requires an integration time that is double not using a selective filter.

This is not quite right. The optimal minimal exposure time is determined by your equipment and sky conditions. Ideally, you want to expose until your sky background level swamps your camera read noise. Obviously, using narrower bandpass filters will reduce the amount of sky background which reaches your sensor, so longer exposures are needed to do that. 

Depending on the gain setting you use, for you this would be somewhere in the region of about 3 minutes (high gain) to about 5 minutes (unity gain) with the l-extreme. In broadband, you'd probably find you only actually need subs of around 20 - 30 seconds. 

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Just now, GoldTop57 said:

I have a filter holder so its quite close to the sensor now, I didn't really like the images when threaded on to the flattener to be honest.

That somewhat deepens the mystery - I was convinced it was down to filter placement, but think I can't explain this - except that there is difference between 2" and 1.25" version of filter?

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I have Scope - field flattener - extension tube - OAG - ZWO 21mm extension tube (with 1.25" adapter inside) - ZWO 294 camera. If I had a filter drawer (more for mono use IMHO) then I would replace the 21mm extension tube. So for me a filter drawer would be behind the flattener but not quite as close as the 1.25" adapter - this puts the filter very close to the camera sensor e.g. you couldn't opt to put the filter in from the other side of the holder it really is that close. The adapter has two small holes and so getting it out is really easy e.g. spin it round with an Allen key and you have a carrier to mount the filter. I like the arrangement because everything is inside and the setup is closed. Swapping filters isn't something you do every five minutes so either a filter drawer or a filter wheel are not something I want to go for considering it's a colour camera and mono isn't something I want to try anytime soon. I would like to use the L-eNhance or L-eXtreme for a pseudo SHO Hubble "look" although I probably won't get a SII filter for a while. Having the filter code to the camera sensor should minimise halos if they are a reflection of a reflection? As the filter is moved forward it exaggerates any issues is how I (misguidedly) look at it?

I would like to use autofocus and did wonder if any NB filter affects this and if the L-eXtreme would make it even harder. Getting auto focus to work is harder than it looks so for me there are probably several issues to work through. Interesting that auto focus through the guider works and with an OAG the two focus is directly linked with the same image scale hence the guider sees the brighter image before the filter via the little prism. Any NB filter will cut down the light so would be beneficial in getting a longer focusing sub that averages the seeing and variability and so be beneficial? I have a trial of SGP Pro which I plan to get to try it, first I have to establish the focus zone and step size and other stuff for my focuser (and walk before I run).

 

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4 minutes ago, SimM said:

I have Scope - field flattener - extension tube - OAG - ZWO 21mm extension tube (with 1.25" adapter inside) - ZWO 294 camera. If I had a filter drawer (more for mono use IMHO) then I would replace the 21mm extension tube. So for me a filter drawer would be behind the flattener but not quite as close as the 1.25" adapter - this puts the filter very close to the camera sensor e.g. you couldn't opt to put the filter in from the other side of the holder it really is that close. The adapter has two small holes and so getting it out is really easy e.g. spin it round with an Allen key and you have a carrier to mount the filter. I like the arrangement because everything is inside and the setup is closed. Swapping filters isn't something you do every five minutes so either a filter drawer or a filter wheel are not something I want to go for considering it's a colour camera and mono isn't something I want to try anytime soon. I would like to use the L-eNhance or L-eXtreme for a pseudo SHO Hubble "look" although I probably won't get a SII filter for a while. Having the filter code to the camera sensor should minimise halos if they are a reflection of a reflection? As the filter is moved forward it exaggerates any issues is how I (misguidedly) look at it?

I would like to use autofocus and did wonder if any NB filter affects this and if the L-eXtreme would make it even harder. Getting auto focus to work is harder than it looks so for me there are probably several issues to work through. Interesting that auto focus through the guider works and with an OAG the two focus is directly linked with the same image scale hence the guider sees the brighter image before the filter via the little prism. Any NB filter will cut down the light so would be beneficial in getting a longer focusing sub that averages the seeing and variability and so be beneficial? I have a trial of SGP Pro which I plan to get to try it, first I have to establish the focus zone and step size and other stuff for my focuser (and walk before I run).

 

In general - reflections are less pronounced if there is larger distance between reflecting elements. This is because reflection is unfocused light - usually from bright star and if you spread it over larger area - it's intensity goes down. Large enough reflections fall below noise floor and can't be seen.

It is a bit like dust shadows - close to sensor they are very "black" in flats - and those further away are greyish and sometimes not that noticeable.

As far as impact on your particular setup - I won't hazard a guess of how it might behave.

Maybe best thing to do is get filter from retailer with good return policy and then just try how it behaves on your setup?

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5 hours ago, vlaiv said:

Maybe best thing to do is get filter from retailer with good return policy and then just try how it behaves on your setup?

Indeed. I was keen to have a go with my L-eNhance filter, especially as tonight the Moon is pretty prominent (Waxing Gibbous) and only three days off a full moon.

Normally I wouldn't consider this as being practical but I thought why not try NB! Target is the Rosette Nebula and NGC 2244.

I followed all the suggestions to read on CN about ASI294MC issues and the L-eXtreme filter e.g. flats and colour casts, so perhaps I shouldn't push my luck and stick with what my instincts say might give trouble for little additional benefit e.g. very little upside and great potential for disappointment.

The filter is 1.25" and very close to the ZWO camera in the ZWO extension tube in a small adapter. I will try to target the Rosette Nebula and NGC 2244 and answer the question of any issues with the OSC and filter position and of course my "somewhat dubious" newbie imaging skills.

Simon

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On 14/01/2022 at 14:02, SimM said:

Indeed. I was keen to have a go with my L-eNhance filter, especially as tonight the Moon is pretty prominent (Waxing Gibbous) and only three days off a full moon.

It was a clear night up to about 1:30 am and then ended abruptly with thick cloud seemingly to appear in an instant (so the forcast was right). So in the end I didn't manage to quite solve all my issues, so a "target" was a bit too much (this time).

I had the L-eNhance fitted within the 21mm ZWO spacer that attaches to the ASI294MC.  It was my first time with only a scope with a (noticeable) filter, but the experience of only having NB was (for me) a bit weird. I would have preferred the ability to switch filters, so I'll definitely be getting an EFW - even for OSC use, so that I have a "closed" environment to call up NB, BB or similar without having to open stuff up . Some tasks that are normally straightforward become much harder and difficult when that choice is removed. I've often thought about having a filter drawer or a manual filter wheel for OSC use, but just a few hours with only access to one captive (NB) filter convinced me that an EFW isn't really a luxury, especially as filters are pricey and need looking after.

I did find that plate-solving worked very well even with the filter in-place. Also getting focused - I haven't so far, setup auto focus, but I could visibly check and adjust the focus OK and see HFR numbers as well. Considering how bright the moon is, the filter really helps with the background. So I'm both pleased with how it went but also a bit frustrated with some other issues not related to using the filter. I would have liked the ease of switching out the filter but I was reluctant to mess with it.

With the placement of the filter inside my ZWO 21mm extension, there really is only one way round it can fit e.g. with the front facing the sky and the threads back towards the camera - otherwise it would clash with the sensor. Optolong had an article that about identifying the AR side of a filter and placing it towards the camera. For my two Optolong filters this would mean putting them with the threads away from the camera - which seems wrong? With an EFW it would mean having to put the motor side facing towards the sky. So not as the instructions suggest (or finding some filters that are the other way) or trying to reverse the filters in their holders. Everything points to filters facing towards the sky - so perhaps the Optolong article wasn't right!

Simon

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