27 replies to this topic

[PollAirUs]

Hi folks,

 

I'll be starting astrophotography in about 2 weeks. But I'll have to begin without a light pollution filter.

I think it's nice to start without a LP filter, I think it will really make me see the impacts of the LP in our pictures, and know better the sky of my backyard. Of course I'll be buying a filter as soon as I can, but I'll have to stick without it for a few months.

I'll be using a Nikon D5300, a Rokinon 135mm and the Star Adventurer. So I think it's a good setup to start.

 

Anyway, I live in a yellow zone, bortle 5 - about 19.85 mag/arc². And I wonder which exposure time it's the best for me to shoot at 135mm F/2.8 and ISO 400 (best dynamic range for Nikon D5300).

I don't want to shoot too short exposures and lose details that are possible to get even without a filter. But I know if I shoot 2 minute exposures my sub exposures will be blown over.

 

I know that's something I'll probably have to try by myself in the field. Probably looking at the histogram of my sub exposures. Trying to make the histogram stay always at the left.

 

I just want an orientation. Know what you have to say about it.

I can't wait to start astrophotography!

 

 

 

[Madratter]

A rule of thumb with consumer cameras is expose so the peak of the histogram is 1/3 of the way from the left. A better rule is to make sure that all 3 colors are separated from the left edge.

[nimitz69]

Also, total integration time is more important than getting exposure time ‘perfect, more is better ’. My skies are 19.5 and I can do exposures of 3mins with LRGB without issue. I would think you can easily do 2 min subs @sqm 19.8. Focus on getting several hrs of imagers at the minimum

Whether your subs are 1min, 2 mins or 3 mins won’t matter as much as whether you collect 1 hr, 4 h4s or 8 hr of data - and Cal frames are A must as well ...

Edited by nimitz69, 30 November 2020 - 08:59 AM.

[jonnybravo0311]

As you guessed in the original post, you'll have to get outside and take test exposures to see what works best for your situation. As long as all three channels are well away from the left edge of the histogram and your highlights aren't clipping, you'll be fine.

 

Light pollution filters aren't really recommended for broadband targets like M31. I think you'll get better images without one. Also, to reinforce what nimitz69 wrote, total integration time is the key. Examples...

 

30 minutes of data:

 

 

10 hours of data:

 

 

Both of those were taken under Bortle 6/7 skies with no filters.

[endless-sky]

Agree with all of the above.

Ballpark: histogram peak 1/4 to 1/3 from the left, as seen from the camera display.
Long answer: determine the read noise of the camera and make sure you expose enough such that it doesn't matter. I have your same camera, but I astromodified it. I normally expose anywhere from 800 to 1000+ mean background ADUs (read on an unstretched, uncalibrated, RAW sub).

When in ballpark, total integration time is the most important thing. With my only good as a paperweight kit 70-300mm zoom lens I couldn't bare make myself waste more than one or two hours on a single subject. Now that I have optics that I actually like, I have the opposite problem: I don't know when to stop. I took almost 9 hours on my first posted image (the Heart Nebula), 10 hours on the California Nebula (not processed, yet), about 6 on M33 (going for at least 10), and 11 on IC 405/410 and planning at least 15.

I am shooting from a Bortle 5/6 zone, using an Optolong L-Pro filter, and my subs are usually between 3 to 5 minutes.

Edited by endlessky, 30 November 2020 - 10:51 AM.

[sn2006gy]

The only time its "shooting too short" is if you're not correctly sampling your sensor. Integration time is additive with diminishing returns.  Target a min above your bias and a good mean with no low clipped pixels and few high clipped pixels.  On a dslr/color this is faster than most people expect even with an LP filter.

 

obviously more subs is a trade off in compute integration time but i find there is more compute time than clear skies

 

Integration time matters most and with color cameras, shorter exposures usually survive LP gradients better and they have higher dynamic range.

 

For example, I shot Orion Nebula - 355x45 seconds.  Had I gone any longer i would not have been able to see the trapezium and had I actually gone a bit shorter, the resolution of the trapezium may have been a tad bit better (hfr of the stars is high because of their extreme brightness)

Edited by sn2006gy, 30 November 2020 - 10:53 AM.

[PollAirUs]

Agree with all of the above.

Ballpark: histogram peak 1/4 to 1/3 from the left, as seen from the camera display.
Long answer: determine the read noise of the camera and make sure you expose enough such that it doesn't matter. I have your same camera, but I astromodified it. I normally expose anywhere from 800 to 1000+ mean background ADUs (read on an unstretched, uncalibrated, RAW sub).

When in ballpark, total integration time is the most important thing. With my only good as a paperweight kit 70-300mm zoom lens I couldn't bare make myself waste more than one or two hours on a single subject. Now that I have optics that I actually like, I have the opposite problem: I don't know when to stop. I took almost 9 hours on my first posted image (the Heart Nebula), 10 hours on the California Nebula (not processed, yet), about 6 on M33 (going for at least 10), and 11 on IC 405/410 and planning at least 15.

I am shooting from a Bortle 5/6 zone, using an Optolong L-Pro filter, and my subs are usually between 3 to 5 minutes.

Thanks! I'll probably also shoot until I get a high integration time like you do. A few questions: How did you modified your camera? And could you explain that part of your text that I highlighted in red?

 

The only time its "shooting too short" is if you're not correctly sampling your sensor. Integration time is additive with diminishing returns.  Target a min above your bias and a good mean with no low clipped pixels and few high clipped pixels.  On a dslr/color this is faster than most people expect even with an LP filter.

 

obviously more subs is a trade off in compute integration time but i find there is more compute time than clear skies

 

Integration time matters most and with color cameras, shorter exposures usually survive LP gradients better and they have higher dynamic range.

 

For example, I shot Orion Nebula - 355x45 seconds.  Had I gone any longer i would not have been able to see the trapezium and had I actually gone a bit shorter, the resolution of the trapezium may have been a tad bit better (hfr of the stars is high because of their extreme brightness)

I really liked what you said here! So with M42, for exemple, I can shoot some longer exposures to catch the fainter details and then take some shorter exposures to catch the bright core, and then combine both into an HDR image, right?
 

[jonnybravo0311]

I really liked what you said here! So with M42, for exemple, I can shoot some longer exposures to catch the fainter details and then take some shorter exposures to catch the bright core, and then combine both into an HDR image, right?

 

That's exactly correct and how a lot of people create their M42 images.

[endless-sky]

Thanks! I'll probably also shoot until I get a high integration time like you do. A few questions: How did you modified your camera? And could you explain that part of your text that I highlighted in red?

I followed some (written) tutorials (for example, LifePixel's) and mainly a video tutorial by Digital Shaman dismantling a D5300 to replace the shutter ("How to replace shutter on the Nikon D5300. Lesson.") - it's not in English, but you really don't need the audio to understand what he's doing. I only watched it until he reached the sensor and its stock filter, which was what I was interested in.

 

Here are some tips that I came up while I astromodified mine:

 

- mark with a fine point permanent marker the three screws that hold the sensor down, before removing them and count how many turns it takes you to unscrew each one. You'll need to get them pretty much exact when you screw them back in, if you want to keep the sensor flat (otherwise you'll have tilting in your images)

 

- get yourself some kind of UV/IR cut filter. You do not want to leave the camera full-spectrum, otherwise you'll have bloated stars and possibly halos around them (like if you were taking photos with an achromatic refractor). I couldn't find any pre-cut, rectangular filter that would have the same dimensions of the stock Nikon filter that I replaced. I resorted in buying a 2" UV/IR cut filter and cutting it to size with a glass cutter. With my calculations, 4 full rectangular filter should fit easily in the 2" circle - so if you are careful, at least one attempt should succeed

 

- clean the replacement filter the best you can, on both surfaces, before mounting it. Or get ready to become the best you can at taking flats. Even if the filter looks clean, it won't be. You'll most likely have residual dust spots somewhere, so flats will become a necessity

 

- be careful with the ribbon cables and connectors, they are flimsy and delicate. Remove the battery quite sometime before opening the camera, so the capacitors can have time to fully discharge. You do not want to short anything (I also made a bracelet with some wire and attached the free end of the wire to my mount tripod - without the mount head - to ground myself)

 

- mark all the screws and their positions. They are all different sizes and lengths. What I did was I made a rough drawing of each layer of the camera I removed, I marked the holes on each drawing, and while I was taking the screws off, I taped them with clear tape next to the corresponding hole in the drawing. This way, it was very easy to remount everything, just following the drawings in the reverse layer order

 

- take pictures before and after, as you remove and disconnect cables and ribbons, so you can go back to the pictures to see how they were placed, if they were twisted in a certain way, ect.

 

It's pretty straight forward, but many things can go wrong. So be careful, clean the room well before you do it, keep order on the table, and be ready to dedicate it at least a few hours.

 

These are the dimensions of the stock filter:

 

34.5mm x 21.5mm

 

If you cut a UV/IR cut filter to size, I would suggest you to cut it a little smaller (for example 34mm x 21mm): if you cut it too big, it won't fit inside the rubber frame, so it's better to err on the smaller side, rather than the big one. Too small and it will fall in on top of the sensor and won't be held in place by the filter bracket, so be careful.

 

As far as ADUs go, this thread - warning: lots of math! - (Help Me Optimizing My D5300 Capturing and Calibrating Workflow) explains the process on how I came up with those numbers. You open an uncalibrated RAW light frame in PixInsight and run the Statistics process on it - this will tell you the mean ADU (and other useful things). To determine the read noise of your camera (which should be pretty similar to mine), instead, you need two bias frames, two flat frames and two dark frames (these taken at two different exposure times - I used 60s and 600s).

 

If you go through the thread, you will find a lot of different opinions on the "best" numbers to use. At the end, I settled for 800 ADUs or a little higher (sometime I go as high as 1200). If you go too low, you'll end up with many frames to stack. Too high, and you might saturate stars / lose dynamic range / have other problems (tracking, guiding, wind, vibrations, etc. - the longer the exposure, the more you have to lose if one of these things go wrong).

Edited by endlessky, 30 November 2020 - 03:01 PM.

[Stelios]

Since you haven't actually *done* any astrophotography, I'll make what will seem like a patronizing suggestion, but really comes from experience: Don't do ANYTHING yet--and for a couple of months at *least* after you've started. You sound like you've read a lot about the hobby--reading is essential, but you need experience to put things in context.

 

The best guide is the simplest--the already mentioned histogram. As long as the histogram "mountain" stays in the 1/4 to 1/3 from the left region for each sub(exposure), you will be more than fine, no matter what any calculations may say. Total integration time (total sum of sub times) is what matters in the end. 

 

The Orion nebula is a relative weirdo in that optimal imaging requires HDR techniques. It's the exception rather than the rule. Overall you will be best served by keeping things simple--keep your exposures and ISO consistent. At 135mm it will be pretty hard to get detail in the Trapezium anyway. 

 

As far as a filter is concerned--from a Bortle 5 area I think the only filter you will need is something like the L-Extreme for capturing the fainter emission nebulae. Definitely no filters for galaxies, globs and reflection nebulae like M45. They offer little benefit and tend to introduce annoying color casts. As for modding the camera, I wouldn't--today there are cooled OSC (one-shot-color) dedicated astro-cameras at very competitive prices. 

 

But start simple, and after a while your own pictures will tell you what you need to improve. You may find you have different concerns, that your problem is getting good focus, or that most of the objects are way too small.  You may find you need to focus on processing. The detailed workings of an internal combustion engine don't much benefit someone who hasn't yet sat behind the wheel. 

[TareqPhoto]

I laugh a lot when people say light pollution and then mention Bortle 4 or 5 or 6, so what i should call out sky which is under Bortle 8 or 9?

[jonnybravo0311]

I laugh a lot when people say light pollution and then mention Bortle 4 or 5 or 6, so what i should call out sky which is under Bortle 8 or 9?

Daylight

[TareqPhoto]

Daylight

Ok, and what i should call the daylight here when the sun is out centered the sky?  

[jonnybravo0311]

Ok, and what i should call the daylight here when the sun is out centered the sky?  

LOL... you can tell the difference?

[TareqPhoto]

LOL... you can tell the difference?

Yes, Bortle 8/9 without a sun and daylight has the sun out and blue sky   

[PollAirUs]

I followed some (written) tutorials (for example, LifePixel's) and mainly a video tutorial by Digital Shaman dismantling a D5300 to replace the shutter ("How to replace shutter on the Nikon D5300. Lesson.") - it's not in English, but you really don't need the audio to understand what he's doing. I only watched it until he reached the sensor and its stock filter, which was what I was interested in.

 

Here are some tips that I came up while I astromodified mine:

 

- mark with a fine point permanent marker the three screws that hold the sensor down, before removing them and count how many turns it takes you to unscrew each one. You'll need to get them pretty much exact when you screw them back in, if you want to keep the sensor flat (otherwise you'll have tilting in your images)

 

- get yourself some kind of UV/IR cut filter. You do not want to leave the camera full-spectrum, otherwise you'll have bloated stars and possibly halos around them (like if you were taking photos with an achromatic refractor). I couldn't find any pre-cut, rectangular filter that would have the same dimensions of the stock Nikon filter that I replaced. I resorted in buying a 2" UV/IR cut filter and cutting it to size with a glass cutter. With my calculations, 4 full rectangular filter should fit easily in the 2" circle - so if you are careful, at least one attempt should succeed

 

- clean the replacement filter the best you can, on both surfaces, before mounting it. Or get ready to become the best you can at taking flats. Even if the filter looks clean, it won't be. You'll most likely have residual dust spots somewhere, so flats will become a necessity

 

- be careful with the ribbon cables and connectors, they are flimsy and delicate. Remove the battery quite sometime before opening the camera, so the capacitors can have time to fully discharge. You do not want to short anything (I also made a bracelet with some wire and attached the free end of the wire to my mount tripod - without the mount head - to ground myself)

 

- mark all the screws and their positions. They are all different sizes and lengths. What I did was I made a rough drawing of each layer of the camera I removed, I marked the holes on each drawing, and while I was taking the screws off, I taped them with clear tape next to the corresponding hole in the drawing. This way, it was very easy to remount everything, just following the drawings in the reverse layer order

 

- take pictures before and after, as you remove and disconnect cables and ribbons, so you can go back to the pictures to see how they were placed, if they were twisted in a certain way, ect.

 

It's pretty straight forward, but many things can go wrong. So be careful, clean the room well before you do it, keep order on the table, and be ready to dedicate it at least a few hours.

 

These are the dimensions of the stock filter:

 

34.5mm x 21.5mm

 

If you cut a UV/IR cut filter to size, I would suggest you to cut it a little smaller (for example 34mm x 21mm): if you cut it too big, it won't fit inside the rubber frame, so it's better to err on the smaller side, rather than the big one. Too small and it will fall in on top of the sensor and won't be held in place by the filter bracket, so be careful.

 

As far as ADUs go, this thread - warning: lots of math! - (Help Me Optimizing My D5300 Capturing and Calibrating Workflow) explains the process on how I came up with those numbers. You open an uncalibrated RAW light frame in PixInsight and run the Statistics process on it - this will tell you the mean ADU (and other useful things). To determine the read noise of your camera (which should be pretty similar to mine), instead, you need two bias frames, two flat frames and two dark frames (these taken at two different exposure times - I used 60s and 600s).

 

If you go through the thread, you will find a lot of different opinions on the "best" numbers to use. At the end, I settled for 800 ADUs or a little higher (sometime I go as high as 1200). If you go too low, you'll end up with many frames to stack. Too high, and you might saturate stars / lose dynamic range / have other problems (tracking, guiding, wind, vibrations, etc. - the longer the exposure, the more you have to lose if one of these things go wrong).

I really appreciate your time to have written the instructions on how to modify the DSLR, I'll definitely keep them in case I want to try to make the modification myself in the future. Just like the thread you mentioned. I haven't finished reading it yet, but it is certainly helping me a lot to understand the concepts.

 

 

Since you haven't actually *done* any astrophotography, I'll make what will seem like a patronizing suggestion, but really comes from experience: Don't do ANYTHING yet--and for a couple of months at *least* after you've started. You sound like you've read a lot about the hobby--reading is essential, but you need experience to put things in context.

 

The best guide is the simplest--the already mentioned histogram. As long as the histogram "mountain" stays in the 1/4 to 1/3 from the left region for each sub(exposure), you will be more than fine, no matter what any calculations may say. Total integration time (total sum of sub times) is what matters in the end. 

 

The Orion nebula is a relative weirdo in that optimal imaging requires HDR techniques. It's the exception rather than the rule. Overall you will be best served by keeping things simple--keep your exposures and ISO consistent. At 135mm it will be pretty hard to get detail in the Trapezium anyway. 

 

As far as a filter is concerned--from a Bortle 5 area I think the only filter you will need is something like the L-Extreme for capturing the fainter emission nebulae. Definitely no filters for galaxies, globs and reflection nebulae like M45. They offer little benefit and tend to introduce annoying color casts. As for modding the camera, I wouldn't--today there are cooled OSC (one-shot-color) dedicated astro-cameras at very competitive prices. 

 

But start simple, and after a while your own pictures will tell you what you need to improve. You may find you have different concerns, that your problem is getting good focus, or that most of the objects are way too small.  You may find you need to focus on processing. The detailed workings of an internal combustion engine don't much benefit someone who hasn't yet sat behind the wheel. 

Thank you! I agree with a simple start and then observe in my results what and where I need to improve. I'm so excited to finally start shotting! Because yes, I already read a lot about the hobby. Since February I have been researching what would be the best equipment to start and everything that is needed.

 

I laugh a lot when people say light pollution and then mention Bortle 4 or 5 or 6, so what i should call out sky which is under Bortle 8 or 9?

Well, I'm sorry for people who have to deal with bortle 8 or 9 class skies, but that's what filters are made for. However, you cannot deny that bortle 5 have light pollution, even at a lower level.
 

[TareqPhoto]

I really appreciate your time to have written the instructions on how to modify the DSLR, I'll definitely keep them in case I want to try to make the modification myself in the future. Just like the thread you mentioned. I haven't finished reading it yet, but it is certainly helping me a lot to understand the concepts.

 

 

Thank you! I agree with a simple start and then observe in my results what and where I need to improve. I'm so excited to finally start shotting! Because yes, I already read a lot about the hobby. Since February I have been researching what would be the best equipment to start and everything that is needed.

 

Well, I'm sorry for people who have to deal with bortle 8 or 9 class skies, but that's what filters are made for. However, you cannot deny that bortle 5 have light pollution, even at a lower level.
 

It has light pollution, but that LP compared to Bortle 8 or 9 or even 7 is like nothing, it is still much more manageable than 7/8/9, i mean how possible to shoot Andromeda or M45 or even some dark nebulae from B5 than 8 or 9.

[endless-sky]

I really appreciate your time to have written the instructions on how to modify the DSLR, I'll definitely keep them in case I want to try to make the modification myself in the future. Just like the thread you mentioned. I haven't finished reading it yet, but it is certainly helping me a lot to understand the concepts.

Glad I could help!

 

But, yes, Stelio's advice is good: take your time, start practicing and as you go, you'll also get a better understanding on what you actually need or don't need at all. I have re-evaluated my priorities/my path many times, over the course of a year, since I started getting into this hobby.

 

For example, it may well be the case that you skip a modified DSLR camera and go straight to a dedicated astrocamera.

[sbharrat]

I laugh a lot when people say light pollution and then mention Bortle 4 or 5 or 6, so what i should call out sky which is under Bortle 8 or 9?

 

 

...

 

Well, I'm sorry for people who have to deal with bortle 8 or 9 class skies, but that's what filters are made for. However, you cannot deny that bortle 5 have light pollution, even at a lower level.
 

So I have seen different responses to this question about imaging under Bortle 8 skies (me unfortunately  ). Some caution to not use LP filter because of the color cast and to instead post-process. Some use LP filter to allow longer exposure time. Are these approaches really equivalent? Or is it invariably something more complicated like "it depends on the target"?

 

My concern with overly short exposures is that it would *seem* that the probability of getting a photon at the pixel in every sub depends on the exposure time. It would follow (??) that at below some threshold, averaging or median in the stacking will result in nothing. So if we are talking about let's say the top 10 beginner objects, what is the minimum exposure that stacking with sufficient integration will get good results? Is 30s good enough? 

 

PS> For me at least, I can't even get a 2m sub (histogram pegged at right edge). Even 30s at 1600 iso is well into the right half. Will move down to 800 but my understanding for digital sensors is that that is just math since sensitivity is fixed.

[TareqPhoto]

So I have seen different responses to this question about imaging under Bortle 8 skies (me unfortunately  ). Some caution to not use LP filter because of the color cast and to instead post-process. Some use LP filter to allow longer exposure time. Are these approaches really equivalent? Or is it invariably something more complicated like "it depends on the target"?

 

My concern with overly short exposures is that it would *seem* that the probability of getting a photon at the pixel in every sub depends on the exposure time. It would follow (??) that at below some threshold, averaging or median in the stacking will result in nothing. So if we are talking about let's say the top 10 beginner objects, what is the minimum exposure that stacking with sufficient integration will get good results? Is 30s good enough? 

 

PS> For me at least, I can't even get a 2m sub (histogram pegged at right edge). Even 30s at 1600 iso is well into the right half. Will move down to 800 but my understanding for digital sensors is that that is just math since sensitivity is fixed.

Talking about a narrowbanding based targets or broadband based targets? 

[sn2006gy]

I don't agree with the refusal to use LP filters.

 

They are not meant to be the best - you are losing some light. no one disputes that.

 

I'd rather image more in (high bortle) B8 and see galaxies/clusters/globulars and have a 95% awesome image with an L-PRO than to avoid it all costs because the internet says its not worthy.

 

Many LRGB filter sets are notched with similar notches to block the same LP that an LP filter is notched for. You can of course purchase non notched filter sets to have full pass through and shared band passes between filters.

 

You can also gradient nuke to remove gradients - but sometimes the IQ is better running at a lower gain + slightly longer sub with an LP filter in extremely high bortle skies than nuking massive gradients and some of the data they share in a non filtered broadband image.

 

Experiment around. The l-pro was kind of surprising how well it worked - especially for dark nebulas.

[endless-sky]

I, also, have nothing but praise for my L-Pro filter. Of course, so far, I have only used it on H-alpha rich targets (Heart, California, Tadpole and Flaming Star Nebulae), so I have no clue on what it will do on broadband targets.

 

I have read left and right that light pollution filters are bad for broadband, because they not only cut the light from light pollution, but also light coming from the target. Therefore doing nothing or very little (or even be harmful) to the SNR of the image.

 

Am I going to do 10 hours on the Pleiades (or M31 or M33) with, and then 10 hours without, just to test it out? That surely would be the best approach, but depending on which session came out ahead, the other 10 hours would be wasted (since they could have been 20 with the best scenario).

[sn2006gy]

I would be curious if someone took a notched L filter and used it in comparison to an L-PRO to see what the differences are. Heck, i guess i could go compare the marketing material and compare the band passes.

 

Sometimes though the notch isn't even universal... for example astronomiks have their l1, 2 and 3 filters that are used to correct for apo/achromatic issues and quality of imaging train while others are notched more for common LP filter ranges such as sodium.

 

Also, I always see anecdotal claims of 'hey, it reduces your SNR, use a gradient removal"...

 

well, hate to break it to ya people, but removing gradients is removing signal and with an LP filter you may actually get more DESIRED signal.

 

Think of is as narrowband - you're taking MUCH longer exposures to target specific desired signal and it works great.

 

Nothing stops people from doing L-PRO to build up a synthetic lume and wait for a new moon to shoot naked OSC or RGB to add color 

 

my experience shows me i do not lose resolution with the L-PRO if i dither/bayer drizzle and if i want to improve the color i can target that with similar techniques to LRGB imaging.

[sbharrat]

Talking about a narrowbanding based targets or broadband based targets? 

I am using a regular DSLR (Canon 1000D) at this point. Really just starting this so it is the "starter set" of targets, M31, M42, ... TBD

Edited by sbharrat, 04 December 2020 - 01:56 PM.

[TrustyChords]

So I have seen different responses to this question about imaging under Bortle 8 skies (me unfortunately  ). Some caution to not use LP filter because of the color cast and to instead post-process. Some use LP filter to allow longer exposure time. Are these approaches really equivalent? Or is it invariably something more complicated like "it depends on the target"?

 

My concern with overly short exposures is that it would *seem* that the probability of getting a photon at the pixel in every sub depends on the exposure time. It would follow (??) that at below some threshold, averaging or median in the stacking will result in nothing. So if we are talking about let's say the top 10 beginner objects, what is the minimum exposure that stacking with sufficient integration will get good results? Is 30s good enough? 

 

PS> For me at least, I can't even get a 2m sub (histogram pegged at right edge). Even 30s at 1600 iso is well into the right half. Will move down to 800 but my understanding for digital sensors is that that is just math since sensitivity is fixed.

I am in the same boat as you so to speak. Bortle 8 here in my backyard. Your results match mine, more or less.

 

With short exposure times, you will get enough photons to work with (it's overall integration time that matters)-- however it's the read noise which could be an issue. I don't think it's as big of a deal with current CMOS cameras as it used to be, FWIW.  Certainly not more of an issue in terms of noise than the LP you'll be dealing with. YMMV.

 

At f/4.9 (with reducer), on m31 I'm doing 20-30 second subs using an un-modded DSLR @800 ISO. This lands my histogram at around 1/3-1/2 peaked. (Edit: I'm not using any LP filter.)

 

I'm getting pretty decent results with this, considering.

 

FWIW, even 10 seconds on M42 with these settings blows out the core.

 

On a positive note, at 20 seconds, no guiding is necessary and my stars are perfectly round.

Edited by TrustyChords, 04 December 2020 - 02:29 PM.