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Old 16-06-2013, 07:24 PM
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Joshua Bunn (Joshua)
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refraction at the horizon.

Hi,

Ive been getting my camera chip orthogonal to the optical axis while pointing at the zenith - so far so good (not without many hrs of work under the skies though). When i point to about 30 deg above horizon to see if the focuser has any appreciable flex that shows in images, i get star elongation.

Now,Im using a Hedrick focuser on a CDK with only an stl11000 attached and the focuser is a pretty solid kit. My question is, could this elongation be a result of starlight refraction through the low altitude atmosphere?

The elongation was over the whole chip, all in the same direction, maybe upto 50 - 75% elongation over the round star image. The elongation was always in a direction on my chip which was parallel with a line going from the horizon up to the zenith - no mater what orientation the scope and camera were in.

Thanks
Josh
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Old 17-06-2013, 10:12 AM
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Hi Joshua,

I have a 10inch F4 reflector and my kit isn't anywhere nearly as rigid as yours, but I too have noticed something similar. Its visible even when focusing with 300ms exposures.
in my case it was always along the E-W direction or in RA and when pointing low.

I thought it could've been mirror slop or secondary flex or collimation as mine's an F4, but interesting to hear your issue.

Do they appear in short exposures as well, like 300 or 500ms when focusing?
do you have any examples?

Cheers
Alistair
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Old 17-06-2013, 02:54 PM
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Thanks Alistair,

I will get some examples and post them, yes the CDK is F8.

Josh
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Old 19-06-2013, 10:56 AM
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Hi Joshua & Alistair,

I can't imagine refraction causing any elongation over such a short period. My understanding is that refraction is absent at the zenith but results in about half a degree of position change at the horizon. And I guess that the amount of refraction might follow the sine rule for anywhere in between (based on degrees above the horizon).

Thinking about this for a minute or two more, the formula for the relative diffraction might be of the form:

Refraction is proportional to 1-Sin(theta) where theta is the angle between the target and the horizon (either Western or Eastern).

For an elevation of 30 degrees (sin(30) = 0.5), the amount of diffraction would be about (1-0.5), or half, that of the refraction observed on the horizon, that is, about 0.25 degrees.

Let's assume a 1 minute exposure for our target star at 30 degrees elevation. Due to the Earth's rotation, the star will move about 0.25 deg across the sky. In that time the amount of refraction will have changed slightly due to the 1-Sin(theta) factor. Using a calculator, I estimated that the angular change due to refraction will be 0.001885 deg, or 6.8 arc-seconds. For a shorter exposure, the change will be approximately proportionally less. So for a 300 ms exposure, the change would be about 0.03 arc-seconds; not something that would cause elongated stars.

The above working is very much a quick estimation, so happy to be corrected!

Chris
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Old 19-06-2013, 11:22 AM
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Hi Chris,

Thanks for your feedback.

The exposures i was taking were 30 or 40 sec, and i saw noticable elongation. Your 6.8 arc-sec figure looks about right compared to what i saw. I will try again when i get a chance. Maybe using a red filter will cut out the effects of refraction.

Josh
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Old 19-06-2013, 01:26 PM
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Josh,

Was your scope unguided?

If I have recalled correctly, the red end of the spectrum gets refracted less than shorter wavelengths.

Cheers
Chris
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Old 19-06-2013, 01:48 PM
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Quote:
Originally Posted by ChrisM View Post

Was your scope unguided?
Dont remember, but this shot in question was one of many test shots and i would have made sure the guiding was not a factor Guided or not. Im using a PME so it should not be a concern to go 30 sec unguided. In any case, i need to try a gain and post a picture here.

thanks
Josh
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Old 19-06-2013, 05:45 PM
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you will certainly get severe vertical elongation near the horizon due to differential dispersion smearing the image - it is independent of tracking and will be there even in very short exposures, as Alistair was getting at.

test if that is what you are seeing by using a narrow band filter (Ha would be best) and seeing if it is still there - if dispersion, it should be very much reduced

ref: http://www.paquettefamily.ca/astro/star_study/

Last edited by Shiraz; 19-06-2013 at 08:19 PM.
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Old 20-06-2013, 12:28 AM
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Thanks heaps Ray. I will certainly test it when i get a chance and report the result. Great link, even the scope is the same .

Josh
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Old 20-06-2013, 07:44 AM
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I agree that is a very good article on dispersion. For us low altitude dwellers, it makes sense to aim high in the sky and not use a OSC camera for NF imaging.

Chris
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Old 20-06-2013, 08:28 AM
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Quote:
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I agree that is a very good article on dispersion. For us low altitude dwellers, it makes sense to aim high in the sky and not use a OSC camera for NF imaging.

Chris
yes, messes up OSC a bit, but its a problem for broadband lum as well at lower angles.

A couple of us used dispersion correctors for high resolution imaging of Jupiter last year - it stayed below 35 degrees down here and the differential dispersion was quite noticeable, particularly in the blue channel. regards Ray

Last edited by Shiraz; 22-06-2013 at 05:37 PM.
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Old 20-06-2013, 09:40 AM
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Optical refraction and dispersion in the atmosphere are very old topics recognised by the professional observatories. Simple explanation here http://www.astrosurf.com/prostjp/Dispersion_en.html and you will find useful articles in the NASA ADS such as this one:

http://articles.adsabs.harvard.edu/c...IF&classic=YES

... which in table II lists the atmospheric refraction calculated using a theoretical model, vs actual measurements made at a couple of observatories. The agreement is good - 2" - down to 86 degrees zenith distance. On page 193 the article concludes that optical dispersion (red-blue) is typically 1% or less of the refraction, and isn't strongly influenced by the observers altitude, temperature or humidity.

A couple of more recent (amateur) references:
http://www.damianpeach.com/images/ar...on%20Peach.pdf
http://www.farhorizons.nl/articles/A...ctor%201_3.htm


There is also a means to correct it - a variable thin prism is inserted in the optical path in front of the focus, such that the prism is always perpendicular to the horizon (regardless of where the telescope is pointing) and the prism can be adjusted to introduce a small amount of dispersion compensating for that of the atmosphere, based on the zenith distance of the telescope. For high-resolution colour imaging and accurate spectroscopy this compensation is essential on a large telescope. It is possible to include a compensator on a small telescope at f/10 or slower, if you really wanted to, complete with the controls to rotate it and adjust it as the telescope moves. Examples:

http://www.astrosystems.nl/projects_...ie_correct.htm or
http://www.cloudynights.com/ubbthrea...1/Main/4572731

These are also useful for the keen lunar & planetary observers.

Last edited by Wavytone; 20-06-2013 at 10:01 AM.
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Old 21-06-2013, 12:13 AM
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Thankyou Wavytone.
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