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Bottletopburly

sub lengths

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Bottletopburly

So last time i was out i was doing 120 sec  guided subs ,i tried 300 and stars were eggy ,160 was ok ,phd says PA was 1.2" i assume one arc  min 2 sec is that right ? so to get 300 sec subs do i need to get the PA under 1 arc  min, or what sub length do you recommend ,camera is a canon 1000D,  guiding with EQ6 belt modded mount ,200PDS, 130mm fl guidescope 

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Bino-viewer

I'm obviously far from being an expert, but could it be a Newtonian thing ?

Is your collimation nailed ?

Could it have shifted a tiny fraction during the long exposure ?

Could it thermal / seeing issues ?

Could the wind have picked up just for a second or two and 'flexed' the tube

just enough to elongate the stars a bit ?

 

Visually my Newtonian never gives me the really tight starlight points of light

my old refractor did. Its not at all bad, just not quite as good as the frac.

Probably a Newtonian thing. Certainly more likely at F4.

Just a thought.

 

 

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Graham

It could be an imbalance of focal lengths between the guide scope and the newt.

Fast guide scopes on slow imaging scopes is not ideal. 

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Bottletopburly
7 hours ago, Bino-viewer said:

I'm obviously far from being an expert, but could it be a Newtonian thing ?

Is your collimation nailed ?

Could it have shifted a tiny fraction during the long exposure ?

Could it thermal / seeing issues ?

Could the wind have picked up just for a second or two and 'flexed' the tube

just enough to elongate the stars a bit ?

 

Visually my Newtonian never gives me the really tight starlight points of light

my old refractor did. Its not at all bad, just not quite as good as the frac.

Probably a Newtonian thing. Certainly more likely at F4.

Just a thought.

 

 

 

6 hours ago, Graham said:

It could be an imbalance of focal lengths between the guide scope and the newt.

Fast guide scopes on slow imaging scopes is not ideal. 

Food for thought , I will double check collimating I do think this may be out a tad , will check guiding formula  found calculator here http://www.wilmslowastro.com/tips/autoguiding.htm

may be turn the fan down  or even off  the scope is F5 and the guidescope F4.3 I will play with sub length again and see next time .

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Ron Clarke

I had this problem earlier this year with my MN190. Graham suggested I get an OAG instead of using a finder/guider so the speed was equal. Whilst I have bought an OAG, I haven't tried it yet but on paper I should solve the issue. I always use 180 sec subs with my 1000d, seems to suit the camera!

 

Cheers

Ron

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Bino-viewer

I have also enquired about guiding 'off axis' 

but have been told they can be a double edged sword of sorts.

 

The chap i spoke to, a well respected vendor & imager, said although they allow 

using the same field of view as the main scope and also allow for movement in the main scope,

they have a far reduced field of view and can be a nightmare to set up.

 

I was a bit puzzled by this.

Surely the main scope dictates how big the field of view is rather than the OAG ?

If you are using a fast scope with plenty of aperture you are going to have a big field of view ?

And on paper, from what i've read they seem fairly easy to set up ?

More, much more reading up to do, i think 🙂

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Graham

OK.

Off axis guiding uses a small prism to pick off light from the edge of the focused light cone just out side of the cone used by the imaging camera. 

This is what causes the reduced fov.

Never had much of an issue setting it up and personally I would never go back to using a guide scope. 

 

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Bottletopburly

Apparently my guide ratio is 

My guiding setup ratio

wouldn’t know if that’s good or bad the box is green hope that means it’s fine for guiding 

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Graham

Nor would I.

Never been one for charts and graphs.

You can read all the specs and information till the cows come home.

The absolute proof of what works is in your images. 

Edited by Graham

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Bottletopburly
36 minutes ago, Graham said:

Nor would I.

Never been one for charts and graphs.

You can read all the specs and information till the cows come home.

The absolute proof of what works is in your images. 

I’m sure it’s fine , I will see how the next couple of sessions go if I can get three min subs then I’d be happy with that .

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Ibbo

I think what the vendor meant was that the guider when fitted to an OAG gives a much narrower FOV than when used with what you are using now.

Sometimes it can throw up a lack of guide stars

 

If you go the OAG set it up in the daytime or on a bright object like the moon to get the focus points of both the imaging cam and the guider cam.

 

a pencil mark on the focuser tube can help you work out how much difference the focus is and if you need any spacers.

 

An OAG is not that difficult to set up if you do it in a methodical way.

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Sunny Phil

I'm no expert on sub lengths for guided or driven shots but one thing NOBODY mentions is that when you get close to the north or south celestial pole, you can use much longer subs without star trailing. I would experiment to find out what sub length works with objects near the celestial equator. Then work out the declination of the nearest point in your field of view to the celestial equator (i.e. the lowest declination in your field of view). You then divide the sub length that works near the equator by the cosine of the declination at the edge of your field of view.

 

For example, if you are imaging near the pole and the lowest declination of your field of view is 80 degrees, you can use an exposure time of just over 90 times what you can do near the equator! However, if the corresponding declination is 60 degrees, the exposure time doubles what you can do near the equator.

 

This is one reason, I like imaging the pole and its surroundings. The only snag is that you sometimes need a lower ISO because of noise.

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Tweedledee
7 hours ago, Sunny Phil said:

I'm no expert on sub lengths for guided or driven shots but one thing NOBODY mentions is that when you get close to the north or south celestial pole, you can use much longer subs without star trailing. I would experiment to find out what sub length works with objects near the celestial equator. Then work out the declination of the nearest point in your field of view to the celestial equator (i.e. the lowest declination in your field of view). You then divide the sub length that works near the equator by the cosine of the declination at the edge of your field of view.

 

For example, if you are imaging near the pole and the lowest declination of your field of view is 80 degrees, you can use an exposure time of just over 90 times what you can do near the equator! However, if the corresponding declination is 60 degrees, the exposure time doubles what you can do near the equator.

 

This is one reason, I like imaging the pole and its surroundings. The only snag is that you sometimes need a lower ISO because of noise.

Very good point Phil. 👍

 

Check your calculator Phil. I think 80 degrees corresponds to nearly 6 times the sub length. 

 

If your edge of field was just 1 degree off the pole (89 degrees) then the sub length could theoretically be about 57x that used at tbe celestial equator.

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Sunny Phil

According to Pugh's Rule of 6th form maths, if two students get the same answer, then they are probably right.

 

If they get different answers they are probably both wrong.

 

I looked up the cosine of 89 degrees and it is about 0.017. If you divide this into 1, you get a result of just under 60. This proves we were both wrong.

 

Fortunately, I was right about a declination of 60 degrees, as I remembered that the cosine of 60 degrees is 0.5. However, the cosine of 90 degrees is zero, which suggests that there would be no star trailing at all because the exposure time becomes infinite. However, as you know I've posted a few images near the pole and found that the practical limit to exposure time is more affected by the lightness of the sky background than Earth's rotation.

 

So, for the less mathematically minded, if you image at a declination of less than about 30 degrees, you should stick to the same exposure times that you use at the celestial equator. Once you get to 45 degrees the exposure time increases by a factor of SQRT(2), which is just over 1.4. The exposure time starts to lengthen quite quickly as you get nearer to 90 degrees but you are probably better off looking up the cosine of your declination online, rather than relying on any of us here!

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Tweedledee
1 hour ago, Sunny Phil said:

I looked up the cosine of 89 degrees and it is about 0.017. If you divide this into 1, you get a result of just under 60. This proves we were both wrong.

Isn't that what I said? It is actually a radian (180/pi).

 

1 hour ago, Sunny Phil said:

This proves we were both wrong.

I think it proves that it's not your calculator that was wrong.

If we were mathematicians several hundred years ago I might have challenged you to pistols at dawn over that, or at least sliced off your ear with my sword. 😉

But, what do I know about sub lengths. I haven't taken any yet, unless you count the crescent moon I did with my phone whilst walking the dogs one evening. 🙂

According to Pete's rule on pre-GCSE maths - "Read the question at least (1/cos(60)) times, where the angle is in degrees". 😊

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Sunny Phil
1 minute ago, Tweedledee said:

Isn't that what I said? It is actually a radian (180/pi).

 

I think it proves that it's not your calculator that was wrong.

If we were mathematicians several hundred years ago I might have challenged you to pistols at dawn over that, or at least sliced off your ear with my sword. 😉

But, what do I know about sub lengths. I haven't taken any yet, unless you count the crescent moon I did with my phone whilst walking the dogs one evening. 🙂

According to Pete's rule on pre-GCSE maths - "Read the question at least (1/cos(60)) times, where the angle is in degrees". 😊

Further to other threads, you are going to find out about sub lengths first hand, soon!  

 

For undriven exposures, the maximum exposure time is:

 

500/(focal length * cosine of the lowest declination in your field of view)

 

Most people forget about the declination.

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Bottletopburly

Math was never my strong subject ,thank god for  guiding and phd2 , i will give winging it a go first 😀

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Tweedledee
2 hours ago, Bottletopburly said:

Math was never my strong subject ,thank god for  guiding and phd2 , i will give winging it a go first 😀

I'm sure I'll be whingeing once I get started, with or without the maths. 😂

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