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Trying to understand focal length for '500' rule

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#1 Shawn_slt130

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Posted 14 August 2019 - 03:01 PM

As stated in another thread, the '500' rule is simply 500 / focal length = maximum exposure time without star trails.

Sounds simple... but when you put an eye piece into your telescope does that affect the focal length?

 

For example if I use a 32mm EP, is my effective focal length now 500/32 = 15.6 seconds?

So if i were to take a picture with my iPhone, I could take a 15 second exposure?

 

But if I used a DSLR camera with no lens, and the scopes focal length is 650, the longest exposure i could take is .75 seconds?

 

However, I do have a motorized mount, all be it an Altazimuth, so it won't track proportionally.... but does it increase the exposure time at all?

 

Thanks!

 

 

 

 

 

 



#2 sg6

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Posted 14 August 2019 - 03:45 PM

Eyepieces are for eyes and they do not integrate the exposure time into one simple exposure so trailing is not applicable. Slightly strange but with an eyepiece in the focal length become infinity, or maybe zero - never thought about it. Optically it should be collimated light in and collimated light out. Also the eye delivers an image, so by rights no image with an eyepiece without an eye when "focussed" correctly.

 

When it comes to astronomy rules the first one I suggest is "Forget all the rules that follow".

Most fail almost immediatly.

 

Az mounts and AP can be a problem. Better then no motors and it may depend on where the oject is in the sky.

Due South objects move horizontally, Due East they move more or less vertically up, and Due  West they move more or less vertically down. Basically messy.

 

Can the mount be set to RA only and placed in an approximate Eq set up?



#3 photoracer18

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Posted 14 August 2019 - 03:47 PM

Putting an eyepiece between the scope and the camera is called eyepiece projection and most don't do that kind of imaging anymore. Putting more glass between the objective and the camera tends to degrade the image.

As for your question, no the equation is not that easy. When you add an eyepiece you are effectively increasing the focal length of the telescope. When talking about cameras the power is relative to the camera sensor size. If you use a full frame camera, then 1x is equal to every 50mm of focal length so if you had say a 200mm lens on that camera that is about 4x. A telescope with a 1000mm fl in the formula means the max exposure without trails is about 0.5 seconds (500/1000). That is just the telescope used as a lens. Adding an eyepiece changes the equation. For a full frame sensor the formula for a 1000mm telescope and a 25mm eyepiece would be 500/(1000/25)x50= 500/2000= .25s or 1/4s in photo terms.

 

The more magnified the image the shorter to exposure. Most people start out with an 80mm refractor for imaging, which generally have focal lengths around 500mm which give you a max of 1s. Smaller lenses will increase that to multiple seconds. And that is for a totally unguided or non-tracking setup. Even though your SLT 130 is an alt-az go-to scope and has field rotation issues with imaging you will still get exposure lengths greater than 1 second with it before the rotation of the field starts having an effect. That is assuming the go-to alignment is spot on and keeps the object right in the center of the field.


Edited by photoracer18, 14 August 2019 - 03:49 PM.


#4 Shawn_slt130

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Posted 14 August 2019 - 04:00 PM

So I guess there are really 2 questions here.

If I use an iPhone connected to the telescope via a phone adapter how long of an exposure would you estimate?

I assume less exposure time with the moon, and perhaps longer if I am locked onto a bright DSO?

 

If I attached a DSLR camera, in this case a Nikon D3200, with no lens attached, how long of an exposure would you estimate?

based on the formula above, if I have a 650 focal length and used the scope as a lens, I might get about .75 seconds.  500/650

If I have trouble getting the Camera to focus, it has been mentioned I could use a Barlow somewhere in the mix and this may resolve the focus issue....



#5 S.Boerner

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Posted 14 August 2019 - 04:37 PM

The problem you get with long exposures on an AltAz mount is called field rotation.  It differs in different parts of the sky.  You can probably get by with a 2 minute exposure in the east or west, one minute in the north or south, and as little as 15 seconds due north.  At one time there was an excellent pdf on the internet that did all the math.  I no longer remember the author's name nor can I find it.  I know it had "field rotation" and Boston in the title.  I'm going to start another thread to see if anyone remembers it, has a copy, or can find it.



#6 kathyastro

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Posted 14 August 2019 - 04:40 PM

As stated in another thread, the '500' rule is simply 500 / focal length = maximum exposure time without star trails.

Sounds simple... but when you put an eye piece into your telescope does that affect the focal length?

 

For example if I use a 32mm EP, is my effective focal length now 500/32 = 15.6 seconds?

So if i were to take a picture with my iPhone, I could take a 15 second exposure?

 

But if I used a DSLR camera with no lens, and the scopes focal length is 650, the longest exposure i could take is .75 seconds?

 

However, I do have a motorized mount, all be it an Altazimuth, so it won't track proportionally.... but does it increase the exposure time at all?

 

Thanks!

The 500 rule (or 400 rule for cropped-frame cameras) is for prime focus only.  If you are going to do eyepiece projection (eyepiece but no camera lens) or afocal (eyepiece + camera lens), determining an effective focal length is difficult.  Your best bet in those cases is to do trial and error.

 

If you use a 650mm scope and a DSLR at prime focus, your maximum exposure time is 500 / 650 = .77 seconds for a full-frame camera, or 400 / 650 = .62 seconds for a cropped-frame camera.

 

Using a barlow with a camera at the (barlowed) prime focus will change the effective focal length of the scope.  A 2x barlow with that 650mm scope will become a 1300mm scope.  So the maximum exposure times will be 500 / 1300 = .38 seconds or 400 / 1300 = .31 seconds, depending on the sensor.

 

A tracking alt-az mount, if properly aligned and tracking well, can give you exposure times up to 30 seconds.  Beyond that, field rotation will become a problem.


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#7 17.5Dob

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Posted 14 August 2019 - 08:07 PM

I wouldn't even worry about the "Rule of 500".. You have a mount than will let you track many times longer than a fixed telescope.The "Rule of 500 was based on old film cameras and the first generation of small 3-6Mp digital cameras and can't be used with today's 24-36Mp++ dSLRS .

If you want a more accurate way of determining a fixed tripod exposure, USE THIS

 

 

Back to eyepiece projection/afocal shots. Your eyepiece will act as a SUPER barlow, multiplying your fl many,many times. You can't determine it easily as changing the distance between your camera and lens changes your focal length as much as changing eyepieces in the case of afocal shots. Lets just say it will be a tiny fraction of what the exposure calculatore shows for prime focus.....



#8 Alex McConahay

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Posted 14 August 2019 - 08:10 PM

There is a lot of stuff in here......some of it I think needs my opinion also.

 

The "Rule of 500" is about how long one can image without noticing trailing in an unTRACKED mount. That is, one pointing to a specific point in ALT-AZ, and not following the movement of the stars by rotating on the right ascension axis. It is not about field rotation or polar alignment, or some of the other things mentioned. 

 

It has nothing to do with the size of the sensor and it does not change according to pixel size. Now, that statement needs qualification.  The image one gets on the middle of any sensor is the same as that of any other sensor as far as the amount of trailing, as far as the eccentricity of the star in the image. What may change is how many pixels that trailing covers. But the number of arc-seconds stays the same. When DISPLAYED at the SAME magnification, the center of the image will look essentially the same regardless of the size of the sensor. However, the outer edges of the sky will target area will show in the larger sensor and not in the smaller sensor. They have been cropped in the "crop" sensor. Now, if you pixel peep, and blow the pixels up, you will see that the sensor with the smaller pixels will show more trailing when counting the number of pixels in the star image......but the star will also show more pixels in its cross section. The eccentricity will be essentially the same.

 

If you add an eyepiece to the imaging train, you will increase the magnification of the image. This would have the apparent effect of lengthening the focal length. So that is fairly correct. This is properly called eyepiece projection, and is usually what is used in cell phone photography. Since the image has been magnified, the "rule of 500" no longer applies. You are right, you have to correct it.  

 

Now, if you have a motorized mount, the equation has to change again. With an Alt-Az motorized mouont (not on a wedge), you can actually image longer than if you have a static mount. SO, you can safely use a "Rule of 1000," or maybe more (I really don't know how much more). And then, if your mount can be made to follow the celestial sphere, by rotating the RA axis or sitting on a wedge, then you can bump it to the "Rule of gazillions or something." (not really).

 

Where does field rotation come in? Well, if you would like to figure that out, may I suggest you go mess around with

 

 http://celestialwond...xErrorCalc.html

 

You will see that Field Rotation is not the killer you might have thought. You can actually go for some time, even when somewhat misaligned. It depends on a number of things, including focal length, how far your guide star is from the center of the frame, and where your target is in the heavens.  

 

Of course, all this "rule of XXX" depends on your tolerance for trailing. If you plan to display your pics on a cell phone, you can easily bump that rule to 1500, or 2000 probably. The stars on a cell phone are so small they just don't show much trailing. If you want to display that photo at 11 x 17, you had best keep it to the "Rule of 100" or something. (I made up those numbers. My point is, what you find the number to put into the formula "Rule of XXX" depends heavily on what you find tolerable. )

 

 

As one wise sage put it:

 

"Forget all the rules that follow".
Most fail almost immediately."

 

 

Alex



#9 17.5Dob

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Posted 14 August 2019 - 10:43 PM

There is a lot of stuff in here......some of it I think needs my opinion also.

 

The "Rule of 500" is about how long one can image without noticing trailing in an unTRACKED mount.

 

It................. does not change according to pixel size.



Where does field rotation come in? Well, if you would like to figure that out, may I suggest you go mess around with

http://celestialwond...xErrorCalc.html

?????

The Canon 1-d had  11.7um pixels, the type of cameras "The Rule of 500" was written around.

 

A 183 series astro cam has 2.4um pixels. For any given static tripod exposure, using the same optics, the 183 will show 5X  the "pixel" smear. Of course the amount of actual travel in arc-sec is exactly the same, but the smaller pixels will show 5X the movement.

A pixel smear of 4-5 pixels is almost invisible unless you pixel peep at 200% , but 5 pixels of smear on a 1-D equates to 25 pixels on a 183., when viewed at 100%. You might as well shoot star trails.


Your link regarding "field rotation" due to a bad PA has nothing to do with the field rotation caused by using the OP's Alt/Az mount. That's comparing Apples, to a German Shepard. 

Alt Az mounts don't "track" they simply point in a jagged stair step pattern, there is no PA alignment that can even be done, as they are Alt/Az, not an EQ, there is no Polar axis to align....................

"If" you really want to learn about field rotation in an Alt/Az system, take a couple of aspirin, and read this treatise, because your head will explode................

(If you don't want to read it, using a 560mm fl OTA, (a hyperstarred CPC 1100 on it's stock mount) you are safe shooting 10 sec subs using an IMX071 sensor, anywhere without rotation. By 30 sec exposures you are going to have significant areas of the sky that will show visible trailing)



 


Edited by 17.5Dob, 14 August 2019 - 11:18 PM.


#10 Alex McConahay

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Posted 14 August 2019 - 10:46 PM

What is "pixel smear?"

 

Alex



#11 17.5Dob

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Posted 15 August 2019 - 12:38 AM

Pixel smear aka star trailing across multiple pixels.

There are so many "new" exposure calculator "Aps" out there, and as most people can't do simple division in their head anymore, (without pulling out their "Smart Phone"), it's time to let the "Rule of 500" die. Even most of my FaceBook astrophotography sites users have dumped it in favor of an real calculator taking into account pixel size, pixel smear tolerance, target declination, etc.they have stored on their phones

BTW, this is 12 X 30", UN-TRACKED, static light weight photo tripod, 560mm fl refractor, APS-c camera. Most of the "trailing" is due to not having a FF at the time.

That's why simple antiquated guidelines don't work......

48493035401_765e39b5e3_o.jpg


Edited by 17.5Dob, 15 August 2019 - 12:42 AM.


#12 Alex McConahay

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Posted 15 August 2019 - 09:26 AM

I would argue that "pixel smear" does not matter in the Rule of 500. The Rule of 500 is about tolerance of eccentricity (elongation) in the final image. In the final image one cannot see individual pixels. One can only see the shape of the stars. If the stars look elongated, then one has exposed too long, regardless of how many pixels are represented in the shape of a star. 

 

What matters is whether a star is longer than it is wide, the ratio of width to length. And this is a measure of distance, regardless of the units one uses to measure the distance.  

 

It is true that if one sensor has pixels half the size of another camera, then a star will "smear" over twice as many pixels. But that same star will also have a cross section that is twice the number of pixels across. Therefore the ratio of width to length (the definition of eccentricity) is the same, and the star will look just as elongated in the final picture. This is true whether measured in arc seconds, millimeters, or number of pixels.

 

Yes, changing the units of measurement, as one does when measuring in different size pixels means that there will be more or less units of measurement. But it does not change the eccentricity any more than measuring with the metric system or the "standard" inches and feet changes the shape of an object.

 

That is why changing the size of the pixels does not change the value of the "Rule of 500." 

 

But I agree with you that there are other measures of how long one can expose. 

 

Alex



#13 17.5Dob

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Posted 15 August 2019 - 10:15 PM

Real Life example showing how bad the Rule of 500 is...

From Dr. Roger Clark's excellent site

( My D5300 has pixels 1/2 the size of those, multiply by 2)


 


Edited by 17.5Dob, 15 August 2019 - 10:21 PM.


#14 S.Boerner

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Posted Yesterday, 07:05 PM

I'd mentioned a good article on field rotation earlier in the thread.  Member ButterFly found and sent me the link:

 

https://visns.neocit...Rotation V3.pdf




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