Permanently Solving Differential Flexure

I’m sure there are a few people out there who shared my experience getting into longer focal length imaging, and by that I mean anything longer than about 1000mm. When I first got my Celestron C8, I had a 50mm Orion mini guide scope to work with, and the guiding I got from it was completely unworkable. My guiding seemed fine, but the difference in focal length caused massive drift issues that I wasn’t going to be able to get around. Lucky for me, right around this time, a new plugin for NINA got released, which promised to correct this issue. Before I get to it, though, I think it’s important to understand why this is needed in the first place.

Differential Flexure, Explained

Differential flexure is something that isn’t encountered too often by astrophotographers and it’s a bit of a lesser known issue because most of the time, if you’re buying a big enough scope for it to matter, you generally understand that you need to upgrade your guider with it. Normally, people will simply buy an off axis guider, which means the guiding is actually using the light and by extension the stars coming through your main telescope. This obviously replaces the need for a separate telescope that sits on top of your main one to do the guiding, so things are generally simpler and balancing is usually easier. Differential flexure comes into play when the difference in focal length or angle on the sky between your main telescope and guiding scope are so different that guiding cannot be done properly. Your guide scope simply doesn’t have the resolution to guide to an accurate enough degree that your main scope will produce sharp, round stars. The other scenario that produces this issue is if your guide scope is pointing at a different area of the sky than your main scope. Both situations will mean your images will come out with star trails even though your guiding may look great. The solution in the past has been to just get a bigger scope for guiding or switch to off axis guiding.

The problem with off axis guiding is that it is more expensive than using a guide scope and the amount of stars available to guide on is tiny compared to an SSAG, sometimes there are even no stars visible. When I got my off axis guider, I encountered this issue almost immediately, so I went to Cloudy Nights and asked what I was doing wrong. Evidently, I needed a bigger OAG and a more sensitive camera. Some said I could get away with just a more sensitive camera. I didn’t have the money for an ASI174mm at the time, so I got a 178 instead, which many people said worked for them. For reference, I was using the standard ZWO OAG (not the OAG-L). After putting the rig back together, I still had one, sometimes no stars, and my exposure time for guiding was pushing six seconds. In the end, I couldn’t make it work with my light pollution and didn’t want to spend any more money on off axis guiding, especially the hundreds more it would take to get what everyone said would be bullet proof, a Lodestar X2 and Celestron OAG. So instead, I bought an 80mm Meade ST80 clone and set out to figure out the issue. Just days after the new scope arrived, this plugin released, and my issues disappeared.

You could of course just get a bigger SSAG, but a big scope to guide with adds weight, balancing issues, and a lot of cost. Even a cheap 100mm guide scope is going to run you a few hundred dollars, and it is going to be big, heavy, and difficult to use. You really don’t want your guide scope to be physically larger than your imaging scope, that is just asking for a nightmare when it comes to balancing the mount.

The New Plugin

If you have been using NINA already for your image acquisition and sequencing, this new plugin is free and easy to install. If you click on the plugins button on the left side of NINA and click on “available”, you’ll be greeted by all the available plugins for NINA. Scroll down until you see “Flexure Correction” and at the top right click install. You’ll need to restart NINA for it to take effect, but once it does you’re all set. I’ve found that no adjustments to the settings are needed. The plugin works by taking an image before and after your sequence images your target and calculates the amount of drift between the two images. It then sends a command to PHD2 that shifts the center point of the star by the amount of drift recorded, effectively speeding up or slowing down your guide speed and adjusting your corrections according to the drift. None of the correction images are saved so they won’t take up any space on your imagine PC, and the plugin is dead simple to use even if you don’t entirely understand the problem. My images since using it have been sharp and it’s allowed me to take longer exposures and have more confidence in my guiding throughout the night. It isn’t perfect, I still do have a tiny amount of drift, so I slew and center after each filter change and that keeps everything perfectly on target. It is more than good enough for the simple 2-5 minute images I’m taking though. My Celestron C8 sits at about 1350mm with the reducer on and my guide scope is just an ST80 clone sitting at about 400mm. To contrast to off axis guiding, I now have more than a dozen stars available at all times and even have multi star guiding available for atmospheric corrections, unlike with my OAG. My exposure times also shrunk, and I’ve found that my sweet spot is at 1.5 seconds.

Some Things To Consider

If you’re going to use this plugin, here’s some things I’ve learned since starting to use it. First, the exposure time for it is based off your plate solving exposure time, so don’t use something too long like 15 seconds or you’ll lose a lot of imaging time. I keep mine low at about 5 seconds for LRGB imaging. You want it to be the minimum possible without having a failed solve. Second, the first image you take after initiating the plugin is going to be sacrificial. Since there is no previous data, the plugin doesn’t know how much drift to correct for until there’s at least one image taken. What this means is that the first image you take will show some drift, but I’ve found that even the second image becomes usable and subsequent images are basically flawless. Point is, don’t worry if your first image doesn’t look good. Third, don’t push it with the hardware you try to use with the plugin. I actually tested the 50mm guide scope with the plugin, and while images looked better, even the plugin has limits and it couldn’t completely correct the drift. It was close, but for an 8 inch SCT I’d say the 80mm guide scope is a good spot to be and I wouldn’t go smaller than that. Fourth, use ASTAP for all your plate solving needs. I think ASTAP is what most people are using, but if you aren’t, use it for both your main plate solve and your blind solve. It’s faster than anything else I’ve tried, works locally on your machine, and is very accurate. Lastly, if you do multiple filters in a single night, my best results came from resetting the plugin after each filter change. All I do for this is add a flexure correction trigger in each filter’s sequence box in NINA. All this means is each filter will have its own drift calculation done starting from when the filter is first switched to. It may sound unnecessary, but I’ve just found it cleans up flexure correction’s calculations after hours of imaging and makes later images a little sharper.

This might be the single most impactful piece of software I have gotten for astrophotography, simply because of its novelty to the hobby. NINA is fantastic software, but I was already imaging on other programs before it, so in terms of novelty this is truly the first solution to this problem I’ve ever seen. Even if you don’t think you have drift, I promise you have some, so give this plugin a shot and see if it makes your images any sharper, my guess is it will.


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