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Imaging with an 8se and DSLR.

LukeSkywatcher

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LukeSkywatcher

LukeSkywatcher

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Ive been thinking on this for a while and i cant see why most say its not a very good way of doing things.

I have an 8se, i have a Canon 450D. I can connect my 450D to my 8se.

My 8se is perfectly capable of tracking and keeping objects in the FOV for as long as i want.

The general thought is that such a scope is not good for AP because its on a single fork Alt/Az mount. 

Ive never tried AP with it, but have tracked objects for several hours. 

So if my scope can track any object i desire.........why is it that most people say that AP cant be done with this scope. I'm probably missing the trick here, so please feel free to enlighten me.

I can track with the scope, so why should i not be able to get the same results as if i was using an EQ mount.

As i have said, i have never tried it...........but i'd love someone to explain the science or mechanics as to why a single forked Alt-Az is thought not suitable for AP.

I really cant see the difference in theory between an EQ mount and a Go-To/tracking Alt-Az mount.

Thanks in advance

Paul

p.s.~~i dont just mean my 8se, i mean ANY scope, Alt-Az mount that has tracking ability.

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Steve Ward

Steve Ward

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Hi Paul ,

Because the mount tracks in steps , up then right , up then right for example it does not smoothly track purely in RA thereby introducing a rotation effect .

Not noticeable when use visually but very prominent when multiple subs are stacked together.

You should be able to get a 'wedge' that fits under the mount that effectively aligns things and turns the Alt/Az into an Equatorial platform .

Celestron show them as discontinued but I'm sure you could find one S/H somewhere ... http://www.celestron.com/browse-shop/uncategorized/wedge,-nexstar-6se-and-8se

Good explanation here ... http://www.astrotx.com/Field%20Rotation.htm

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Alien 13

Alien 13

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The mount in question can track an object in X and Y but unfortunately it will also rotate as it moves across the sky so you are limited to around 30 second exposures before this field rotation shows up.

Alan

P.S if you have the kit anyway its well worth a try either through the scope or with the camera piggy backed with a wide angle lens.

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MarsG76

MarsG76

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You can do AP with a 8SE alt-az setup BUT you do need to consider a few points and limitations...

Firstly the difference with just tracking for viewing and AP is that you wont notice if the object drifts in your eyepiece slightly over time but long exposure on a DSLR or any camera will show up as streaks/trails. Thats said, the 8SE single arm mount does have a Autoguide port, so you can attach a autoguider, such as a OAG or guide scope to your mount and track it accurately.

The Second point is field rotation in a Alt-Az mount tracking compared to a EQ. The EQ mount tracks in only one axis, the RA and the scope and camera orientation rotates with the sky as the night goes on. With alt-az tracking it tracks in both horizontal and vertical steps, keeping the camera orientation the same through out the whole tracking time causing the image in the frame to rotate (field rotation), another thing you wont notice in the eyepiece over time.

You can do astrophotography with a 8SE but you will be limited to how long your subs can be before field rotation stars to show, generally a couple or minutes is the absolute limit, 2-3 if you crop a part of your frame in processing. Another thing you will need is a program that will stack and derotate your subs such as Nebulosity.

If you capture and stack a lot of 90 second subs at ISO1600, you are still able to make awesome images. Even using a EQ mount, I cant image subs for much longer than 5 minutes at ISO800 or 10 minutes at ISO400 in RGB (UV IR Cut) before the sky glow over exposes the subs.

The one thing you will not be able to do with a alt-az mount is narrowband imaging. When I do I find my self doing subs of 15, 20 or 30 minutes, those length exposures will definitely be ruined by field rotation with out equatorial tracking. 

I've attached some files to demonstrate what I mean.

Hope this answered your question.

MG

Mounts and Coordinates-Part2.pdf

post-43662-0-46127200-1449163365.png

post-43662-0-48169000-1449163371.jpg

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D4N

D4N

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One thing to note about wedges is that they move the centre of gravity away from the centre of the tripod. You need to make sure that the tripod cannot tip over when the mount slews.

/Dan

Sent from my iPad using Tapatalk

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LukeSkywatcher

LukeSkywatcher

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Hi Paul ,

Because the mount tracks in steps , up then right , up then right for example it does not smoothly track purely in RA thereby introducing a rotation effect .

Not noticeable when use visually but very prominent when multiple subs are stacked together.

You should be able to get a 'wedge' that fits under the mount that effectively aligns things and turns the Alt/Az into an Equatorial platform .

Celestron show them as discontinued but I'm sure you could find one S/H somewhere ... http://www.celestron.com/browse-shop/uncategorized/wedge,-nexstar-6se-and-8se

Good explanation here ... http://www.astrotx.com/Field%20Rotation.htm

I absolutely thought that this would be the only reason why an Alt-Az was not suitable............but i had to ask. I have no plans to do AP so a wedge is not on my shopping list.

Thanks for confirming my thoughts.

Brilliant replies. If i didnt ask.........i'd never know. 

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MarsG76

MarsG76

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One thing to note about wedges is that they move the centre of gravity away from the centre of the tripod. You need to make sure that the tripod cannot tip over when the mount slews.

/Dan

Sent from my iPad using Tapatalk

I think putting the OTA on a EQ mount is less hassle, more reliable and looking at the cost, it's not much more expensive for a eq mount that would be just as, if not more accurate than the wedge addition option.

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MarsG76

MarsG76

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A wedge isn't the only option for imaging on Alt/Az mounts  you can always consider using  a rotator...

Peter...

Looking at price of a derotator, you might as well get a AVX mount.

Having just bought an 8SE I found these answers very helpful too

Thanks

Bob

Very good OTA.

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ollypenrice

ollypenrice

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Don't worry about it. You can do image DSO on an AltAz just fine.
Meade LT6 ACF
Canon 70D

You can do limited DSO imaging on an Alt Az but lots of targets need long exposures which are not possible in Alt-Az unless you are imaging from the North or South Pole!  :grin:

Paul, alt-az versus EQ aside the thing you need to consider is that accurate tracking for visual observing and accurate tracking for imaging are quite different things. All my mounts will hold Saturn in the middle of the EP all night without being touched. None of them can do unguided imaging for long exposures. The setup you propose images at 0.53 arcseconds per pixel. So a tracking error of half an arcsecond (not much!! ) will blur your image. That is to say, an error of about a two thousandth of a moon moon diameter will cause loss of resolution. OK, you can lose some resolution and still have an image but I'm trying to give you an idea of the nature of the beast. The other thing to consider is that each pixel in your camera is being illuminated by a measly half an arcsecond of object. Half an arcsecond of object doesn't often give off much light.

Our Mesu, under autoguiding, runs an accuracy of about half an arcsecond. 

For all that, put the camera in, get it focused (Bahtinov mask), pick a bright target and see what you get. It could give you quite a buzz.

Olly

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MarsG76

MarsG76

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Wonder if you could make a derotator using a raspberry pi and a bit (lot) of maths...

Of course, why not. You would need to determine the declination of your object to calculate the speed of rotation for you de rotator aperatus to perform.... Still seems like a eq mount is less hassle.

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jimbo747

jimbo747

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A friend of ours has an alt az reflector with field de-rotator just down the road. Make sure you've put your coffee down before you click. :grin:

http://a405.idata.over-blog.com/4/15/06/42/DSCN7169.JPG

Put your hand in your pocket for one of these, Paul. Six million Euros...

Olly

I've got one of those!!

(The aluminium flight case in the background that is)

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jimbo747

jimbo747

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Of course, why not. You would need to determine the declination of your object to calculate the speed of rotation for you de rotator aperatus to perform.... Still seems like a eq mount is less hassle.

Trust me, spending £4 on a pi zero a stepper motor and some bits is a lot less hassle than telling the wife I've spent the best part of a grand on a mount that will be used once or twice a month around these parts!

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dph1nm

dph1nm

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You can do limited DSO imaging on an Alt Az but lots of targets need long exposures which are not possible in Alt-Az unless you are imaging from the North or South Pole!  :grin:

I think this was the case a few years ago, but with modern cameras now having extremely low read noise short exposure astrophotography is becoming much more efficient. Now whether a typical commercial alt-az can track for 30secs at 0.5"/pixel is quite a different matter!

NigelM

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