Flat Fields Matter

For a couple of months, High Point Scientific had my new tripod on backorder. It is apparently quite popular, being solid yet inexpensive. I was pretty excited to get the notice that it was shipping.

As it is winter and Orion is quite prominent in the night sky, I thought it would be nice to capture the Orion nebula. Because of it’s location, basically formed around Orion’s dagger, it should be easy to find, at least compared to many, maybe most, deep sky objects.

For my first try, I had my CLS filter in place. We have a big sodium light that is basically in the same direction as Orion when I am set up in what is arguably a very handy place, in my driveway, just outside the garage. This filter, however, is pretty dark and it made it more difficult to find anything. At some point, I decided that maybe I was pointed the right direction and that I just couldn’t see the nebulosity in my test shots, so I set the thing loose taking 180 x 30 second subs. I spent some of the capture time in the house doing things that needed doing and some of it waiting in the car with the heater on, which was kinda novel.

I had started a little later than planned, plus all that attempting to find a nebula push my capture kind late. The exact place I had set up was inadvertently planned for my capture plan. This was where the camera was pointed at the end of 180 frames.

I captured a really pretty field of stars and I had only missed the nebula by this much:

As luck would have it, a couple nights later was clear and a Friday, so I set up again. This time I removed the CLS filter, hoping it would make it easier to find the nebula. I am not sure whether or not it made a real difference, but I did find it!

It was very exciting not only to see the nebula show up on the viewscreen, but also to be able to frame it so perfectly.

I captured another set of 180 x 30 second subs, about 30 darks, flats and bias. I set up a little bit out in the yard to keep from catching the house if capture went long. I also set up a heater and for the most part, sat with the equipment for most of the capture time.

I also ran a small test of two other bits of equipment, a dew heater for the astrograph and my Bluetti power station. While the power station was not purchased specifically for astrophotography (power loss during winter was the big thing), using it for possible dark site travel was a consideration.

This was the first time I had even powered up the dew heater. According to the Bluetti display, on high, it draws 6 watts. I could hardly even tell it was warm against the aluminum dew sheild of the Redcat. I suppose that all it has to do is keep it just warm enough to discourage condensation. Shrug. Whether or not it was succesful would come up soon enough.

We had plans for early Saturday afternoon, so I decided to stay up and do at least a preliminary stack of the capture. I scrolled fairly quickly through the subs and discovered a couple of where I presume I had bumped the tripod and excluded those subs. The final stack came out… ummm… odd.

This is after a bit of stretching in GIMP. There are two anomalies about this image. The most obvious to me is that the bottom 1/3 or so seems blurred, out of focus. I had run through checking the subs pretty quickly, but certainly none were way out of focus. It also strikes me as odd to only be out of focus on the bottom of the image. The top and middle seem to in sharp focus.

The other thing, and this was harder to notice because of the blurring, but there is a definite linear gradient from top to bottom.

It was too late and I was too tired to do much about it just then, so I hit it again the next morning. One of my more careful trips through the subs, I noticed that several towards the end of the capture seemed to have soft focus, so I excluded them and it was essentially unchanged. I reviewed my flats and noticed that they had a linear gradient to them and thought, oh, that makes sense, so I restacked again without flats: no change. It occurred to me later that I may have unchecked all the flat captures, but maybe not the flat master that the previous stack process created.

I posted a png of the stretched blurry image on the Nebula Photos Patreon community page, with some details about the capture. Nico took an interest and a few private emails later, I had much more carefully tried stacking without the flats. I cleaned all of the .info files out of the lights folder, moved the exclude lights to another folder, as well as moving the master flat to another folder. When I stacked this time, it was 131 lights, zero flats and the presumably good dark master and bias master. The image came out great and with a couple of stretches and a crop:

My favorite astro image thus far!

To further verify that the flats were the issue, I kept all the rest of the conditons the same and added back the flats and I got this different image. It may seem to be ok, but upon closer examination, it is still very wrong.

It is hard to tell at full size, but the bottom half of the picture, getting worse as it goes lower, the stars split into three divergent images of red, green and blue.

I will be the first to admit that I do not understand the inner workings of Deep Space Stacker and how it uses the calibration files, but it now seems obvious that if there is an issue with those files, it can damage your final image in probably unpredictable ways.

Some of the discussion with Nico was about my flat capture process and I am going to rework how I am doing that. For this session, flats were captured by holding a USB tracing pad up to the end of the dew shield and adjusting exposure until it was just a little underexposed, which turned out to be 1/2500″. This is probably way too fast and catching a pattern of sensor noise as well as PWM flicker and shutter artifacts from the brightness control of the panel itself.

There are several ways to address this and I will report on what works well for me.

The Gate Keeper

I have mentioned that I have a camera mounted by our front gate, powered by a dedicated solar panel, charger and battery.

I chose this Renogy Wanderer 10A solar charge controller specifically because it has USB power outlets built in, elegantly feeding the USB power cord to the camera.

The battery box lid, designed for indoor use apparently, had air vents on the top and allowed rain inside. The first controller died a wet death. I covered those air vents. It’s not as if the rest of the lid fits with a hermetic seal, so the box is still well ventilated, but now it is at least rain tight.

Wintertime is tough on solar stuff. So many overcast days result in less efficient charging. The camera draws 100-200mA;. The granularity of the charge controller’s output meter is 0.1A and it’s usually says 0.1A, but sometimes it’s 0.2A. Maybe when it is darker and the IR illuminator is on?

A couple of overcast days is enough for the lawn tractor battery to fall behind and drop power, especially after dark. The tractor battery is not intended for deep cycle use, although the gate controller is only on it’s 2nd battery in about 10 years. I think the constant drain of the camera, even at only 200mA, is probably the issue.

I have considered changing the battery to an actual deep cycle battery. WalMart carries an inexpensive ($80-ish) marine battery in the 24 size class. Real specifications for that specific battery were elusive, but I found several 24 size marine batteries from other sources. If they mentioned it at all, the amp-hour rating was 75-80 AH, so that seems likely for the EverStart at WalMart.

A more useful number for power service is the watt hour, literally how many watts for how long. It’s really handiest when comparing power systems that might not be running the same voltage. Conversion of AH to WH is easy enough. Voltage times amperage is wattage. For example, an off-grid solar system running 12V with 100AH of battery can be compared directly to a 48V system with 25AH of battery because they would both provide 1200 watt hours of power.

12V x 100AH == 48V x 25AH

In the case of the EverStart, 80AH x 12V = 960WH. A 12 volt battery is rarely actually 12 volts. Lead-acid batteries like to charge at 2.25 or so volts per cell, so a 12V battery’s ideal charging voltage is 13.8V. The open circuit voltage for a fully charged battery is about 2.1 volts per cell, so 12.6 is typical. Plug that number into the formula and a 12.6 volt 80AH battery should be good for 1008 watt hours.

Once you have a watt hour rating, figuring out how long a charged battery should last is pretty straight forward. In the above example, under ideal conditions, you should be able to draw one watt for 1008 hours, 2 watts for 504 hours, 50 watts for 20+ hours, and so on.

In practice, lead acid batteries can be permanently damaged if discharged too deeply. Deep cycle batteries are optimized to limit this damage, but you can’t eliminate it. It’s just how the chemsitry in there works. It is good practice to limit discharging a lead acid battery to no more than 50% charge, so derating that watt hour rating by half would be wise. One watt for 504 hours, 2 watts for 252 hours, etc.

Assuming 200mA for the camera and the specified 10mA for the charge controller itself, an 80AH battery should be able to run the camera for 192 hours to the 50% charge level, or about 8 days. That shoudl be plenty.

Then again, lead acid batteries need maintenance, especially during the summer months. All the cool kids are using lithium iron phosphate batteries in their solar systems, mostly because they enjoy certain efficiencies, but also because they are essentially zero maintenance, so I started looking into that.

LiFePO4 batteries can be charged and discharged faster than lead acid, are much lighter and because of smart onboard electronics, don’t really need a super smart charger. They are not particular cheap, however.

I found that 20AH LiFePO4 batteries run similar in price to 80AH lead acid batteries. 20AH works out to 96 hours and these batteries protect themselves at about 75% discharge, so 72 hours is likely.

So, roughly the same price as the big lead acid batter and only 3 days without a charge, but zero maintenance and an arguably longer service life. I decided it was worth trying one out at the very least.

Via Amazon, I chose at Vestwoods 20AH battery. The Renogy charge controller has a lithium mode. Part of setting that mode involves setting the charge voltage. The labelling on the battery indicates that it is a 12.8 volt battery, so I took that as the best guess.

It was delivered at 7:24 on December 30, ironically about a foot from where it would be installed. Here’s Amazon’s delivery photo. The battery box nearest to it is where it goes and the camera above it is what it powers.

It’s about the size of a large motorcycle battery.

I connected it almost immediately, by flashlight.

According to the recordings from that camera, it ran from 9:05PM on December 30 until 12:37 AM New Years Day.

The immediate presumption was that changing the battery type caused an issue. I didn’t have a lot of free time to troubleshoot, so it took me a few days to determine that what had actually happened was that the power output from the charge controller had failed. All indications are that it is on, but there is no power on either the USB jacks or the screw terminals.

Before I found the Renogy charge controller, I had purchased a 12V to USB adapter, assuming I would be using something like that connected directly to a battery. Using that, I was able to at least get the camera operational again, pending the arrival of a replacement charge controller.