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# How do I calculate the Rule of 500

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### #1 hdoraisamy

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Posted 17 October 2020 - 10:58 AM

How do I calculate the Rule of 500 when it comes to imaging with a DSLR attached to a telescope?

I have a Celestron Nexstar 8SE that will be connected to a modified Canon Rebel T6

How long can I take an image without noticing star trails? Will it decrease if i place a 2x Barlow? I read somewhere that the 8SE allows 15 seconds.

Thanks!

### #2 endlessky

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Posted 18 October 2020 - 02:26 AM

The 500 rule is a quick way of determining the longest exposure time, for a given focal length, before the stars trail too much in the final image.

It works like this: you take 500, divide it by the focal length of the lens/telescope you intend to use and that will be the longest advisable exposure. For example, for a 50mm focal length lens, 500/50 = 10s. For a 250mm focal length lens, 500/250 = 2s.

A Nextar 8SE has a focal length of 2000mm. So 500/2000 = 0.25s! With a 2x Barlow it's even worse, since the focal length will become 4000mm: 500/4000 = 0.125s.

It's not really a rule, but more of a guideline. From my experience, when I was using my 50mm lens for some untracked shots, I noticed I needed to stay around 7-8s, before stars trailed too much for my likings (so, the 500 would actually become 350-400, in my case = even lower than the "rule").

Rule or no rule, I doubt you'll ever reach 15s with a 8SE. You'll likely start seeing star trails well before 1s of exposure, if the rule applies.

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### #3 hdoraisamy

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Posted 18 October 2020 - 04:59 AM

Rule or no rule, I doubt you'll ever reach 15s with a 8SE. You'll likely start seeing star trails well before 1s of exposure, if the rule applies.

Thank you! I realized this last night. I also figured that the trails get worse if the object is much higher in the horizon (like the Veil Nebula).I started to see star bloat around 3 sec.

I do realize that an EQ mount is the only real solution, but I do not have the ability to invest in one at this time. Short of that, I would like to know if getting a 0.5x Focus Reducer would allow me to increase my exposure time by a factor of 2?

Last night, I tried to capture M27 with 3 second subs, but I don't see any hint of nebulosity in the single frames. I will try stacking, but it is a bit disappointing.

### #4 endlessky

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Posted 18 October 2020 - 05:07 AM

I do realize that an EQ mount is the only real solution, but I do not have the ability to invest in one at this time. Short of that, I would like to know if getting a 0.5x Focus Reducer would allow me to increase my exposure time by a factor of 2?

Last night, I tried to capture M27 with 3 second subs, but I don't see any hint of nebulosity in the single frames. I will try stacking, but it is a bit disappointing.

Reducing the focal length by 0.5x should allow you to double the exposure. The way I understand it, exposure limit and focal length are linearly related. So, if you can comfortably take 3s exposures at 2000mm, you should be able to reach 6s at 1000mm.

Most deep sky objects take a lot longer than 3s to be correctly exposed on a single sub. Then you need to stack many, many subs (think total integration time in multiple hours) to get satisfactory results.

To give you a comparison, I usually take anywhere from 90s to 240s single exposures, from my Bortle 5/6 sky. Then I aim to stack enough of them to reach a minimum of 2 hours. But the more the better, really. It's quite normal for people to go past 8-10 hours on a single subject, before switching to the next.

### #5 hdoraisamy

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Posted 18 October 2020 - 05:58 AM

Reducing the focal length by 0.5x should allow you to double the exposure. The way I understand it, exposure limit and focal length are linearly related. So, if you can comfortably take 3s exposures at 2000mm, you should be able to reach 6s at 1000mm.

Most deep sky objects take a lot longer than 3s to be correctly exposed on a single sub. Then you need to stack many, many subs (think total integration time in multiple hours) to get satisfactory results.

To give you a comparison, I usually take anywhere from 90s to 240s single exposures, from my Bortle 5/6 sky. Then I aim to stack enough of them to reach a minimum of 2 hours. But the more the better, really. It's quite normal for people to go past 8-10 hours on a single subject, before switching to the next.

Thank you very much

### #6 kathyastro

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Posted 18 October 2020 - 07:34 AM

The rule of 500 applies to a non-tracking mount like a tripod.  If your mount is tracking, which the SE mount should be doing, the rule of 500 does not apply.

The generally accepted rule for a tracking alt-az mount is 30 seconds before field rotation becomes evident.  That is an average: it will be longer in some parts of the sky and shorter in others.  Field rotation is independent of focal length.  If the field rotates 10 degrees, it will do so whether you are imaging at 400mm focal length or 2000mm.

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### #7 hdoraisamy

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Posted 18 October 2020 - 09:17 AM

The rule of 500 applies to a non-tracking mount like a tripod.  If your mount is tracking, which the SE mount should be doing, the rule of 500 does not apply.

The generally accepted rule for a tracking alt-az mount is 30 seconds before field rotation becomes evident.  That is an average: it will be longer in some parts of the sky and shorter in others.  Field rotation is independent of focal length.  If the field rotates 10 degrees, it will do so whether you are imaging at 400mm focal length or 2000mm.

Thanks! I wonder - Is it possible that I started seeing trailing > 3 seconds because I attempted to use a solar system align to track DSOs?

### #8 kathyastro

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Posted 18 October 2020 - 10:51 AM

Thanks! I wonder - Is it possible that I started seeing trailing > 3 seconds because I attempted to use a solar system align to track DSOs?

Aligned is aligned.  Solar system alignment is less precise than a star alignment, but should still work for DSOs.  Possibly the alignment was less precise than you expected.

Also possibly, your target was near the zenith, where field rotation is quite rapid.

### #9 endlessky

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Posted 18 October 2020 - 12:03 PM

The rule of 500 applies to a non-tracking mount like a tripod.  If your mount is tracking, which the SE mount should be doing, the rule of 500 does not apply.

The generally accepted rule for a tracking alt-az mount is 30 seconds before field rotation becomes evident.  That is an average: it will be longer in some parts of the sky and shorter in others.  Field rotation is independent of focal length.  If the field rotates 10 degrees, it will do so whether you are imaging at 400mm focal length or 2000mm.

I assumed the original poster wasn't tracking at all, my mistake. The rule I stated is, indeed, valid only if you are not tracking at all.

### #10 hdoraisamy

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Posted 18 October 2020 - 02:02 PM

Aligned is aligned.  Solar system alignment is less precise than a star alignment, but should still work for DSOs.  Possibly the alignment was less precise than you expected.

Also possibly, your target was near the zenith, where field rotation is quite rapid.

That's what i thought too. I tried 2 different targets - The Helix and the Veil Nebula. One is low in the horizon, while the other is at the zenith. I could not get more than a few seconds on either target which makes me question do I have something else going on?

As a side note - I tried to plate solve the stacked outputs of these 2 second subs and it was off.

### #11 MarMax

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Posted 18 October 2020 - 07:04 PM

Last night, I tried to capture M27 with 3 second subs, but I don't see any hint of nebulosity in the single frames. I will try stacking, but it is a bit disappointing.

Since you are running an Alt-Az go to mount you should be able to get up to 30 second exposures when the mount is tracking at its best. I've found that my mount has "great" tracking about 10% of the time and have no real explanation for this as I set it up the same each time. It's never terrible but usually I can only count on about 15 seconds of exposure.

For your M27 shot you'll need more like 15 seconds of exposure and the mount needs to be tracking well. Here is a picture of M27 with a 12 exposure at ISO 3200 and you can still see trails.

### #12 hdoraisamy

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Posted 19 October 2020 - 06:12 AM

Since you are running an Alt-Az go to mount you should be able to get up to 30 second exposures when the mount is tracking at its best. I've found that my mount has "great" tracking about 10% of the time and have no real explanation for this as I set it up the same each time. It's never terrible but usually I can only count on about 15 seconds of exposure.

Thanks! I am not sure if this is a contributor to my problem, but both M27 and M57 are very high in the sky this time of the year where I live (Pennsylvania). My rig involves a DSLR attached to the scope pointed near vertical, I wonder if the weight of the DSLR is causing the alignment to shift somehow?

I was able to image all the way to Neptune with no issues. I struggle to find a good target in the southern sky which is not so high in the horizon.

### #13 MarMax

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Posted 19 October 2020 - 10:11 AM

My rig involves a DSLR attached to the scope pointed near vertical, I wonder if the weight of the DSLR is causing the alignment to shift somehow?

Yes, weight and imbalance affect the tracking performance. I have a counterbalance weight which I should have mentioned. I have rails on the top and bottom but I only put the weight on the bottom rail. There is a lot of info out there on the best way to balance but I just shoot for neutral balance at about a 45 degree angle with my heaviest EP.

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### #14 hdoraisamy

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Posted 19 October 2020 - 10:17 AM

Yes, weight and imbalance affect the tracking performance. I have a counterbalance weight which I should have mentioned. I have rails on the top and bottom but I only put the weight on the bottom rail. There is a lot of info out there on the best way to balance but I just shoot for neutral balance at about a 45 degree angle with my heaviest EP.

Thanks! I will check this out

### #15 photoracer18

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Posted 22 October 2020 - 05:47 PM

While not the best option a wedge is cheaper than a GEM.

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