Some Astrophotography Questions

[fizzy123]

Hi to all gurus, I have some astrophotography questions that had me puzzled for quite some time, hope to get some answers for them. I have decided to accquire a Celestron Nexstar 5 SCT for visual use and I hope to dabble with alittle deep sky photography.

1) Why is it necessary to get a EQ wedge for this setup? The telescope is already trackking the DSO across the night sky already so why is there a need to even polar align it? Shouldn't the object be dead in the middle of the camera's CCD chip, say for at least 30++ mins without any external interferrence?

2) I am aware that some website say that the scope in alt-azi mode during astrophotography, can only have reliable trackking for 30s, after that, the image will degrade due to the ciruclar motion of earth. So why is it 30s and not more or less?

3) Supposedly, one utilised a off axid guider, to constantly ensure that the DSO is fixed within the FOV of the guiding eps, then can a scope in alt-azi be used for DSO astrophotography?

4) For the recticle eyepiece, why is there a need for one, when say a normal eyepiece can be used instead?

[orly_andico]

Any fork-mounted scope will not be useful for astrophotography in equatorial regions like Singapore, because the polar axis (azimuth) would be almost parallel to the ground.

An alt-az scope does track the object, but it does so by moving both the alt and az axes at the same time (a polar-mounted scope would track by using only the polar axis). The result is that an alt-az scope with tracking would see "field rotation" which is that the object moves in a little circle. Which would spoil your photography.

To counter field rotation, Meade used to make a device called a field derotator, which would be placed on the focuser and which would rotate the camera to compensate. Note that the amount of field rotation varies depending on where in the sky the object you are tracking is located. So it's not just a dumb motor but must be controlled by the same GoTo computer which is driving the alt and az axes.

Meade's Envisage software can also do field derotation in software (e.g. take a lot of 30-second exposures, derotate each and then stack them). Needless to say, Envisage only works on Meade telescopes. I believe there is some other software to do this generically.

Finally, at f/10 the Nexstar 5 is quite slow photographically. You probably want to purchase a focal reducer to bring it down to f/6.3. But another problem you will have is that at certain points in the sky, the camera + reducer will hit the fork base.

Personally, I think if you want to do more than trivial dabbling in astrophotography, you should consider an EQ-mounted scope and not a fork-mounted one.

[weixing]

Hi,

Any fork-mounted scope will not be useful for astrophotography in equatorial regions like Singapore, because the polar axis (azimuth) would be almost parallel to the ground.

I saw one in action before... quite scary...

Meade's Envisage software can also do field derotation in software (e.g. take a lot of 30-second exposures, derotate each and then stack them). Needless to say, Envisage only works on Meade telescopes. I believe there is some other software to do this generically.

Derotation in software?? Once rotation(star trails) appear in your single image, I don't think there is anything you can do.

Anyway, the effect of field rotation is basically same as star trails. For example, when you point your camera at Polaris, Polaris is "always" in the centre (like your Alt-Az GOTO mount tracking a star accurately), but after long exposure, star trails appear around it. If you mount your camera on an EQ mount and point at the Polaris, the star trails (field rotation) won't appear as the EQ mount will follow the stars around the pole.

Hope this help you understand.

Have a nice day.

[timatworksg]

Here's my 2c..hehe!

A wedge won't be useful for any single/double fork mounted scope as the ones on sale won't go to near 90degrees, which we need. You would have to DIY one. Even if they did make one, your scope will be mounted 90degrees down and that will put lots of stress on the mount head anchor bolt and Azimuth gears. For visual, normal Alt AZ fork mounted scopes work with no probs.

There are some nice images done on a ALt AZ,...like the IOptron Mini Tower. But yes, only 30second exposures before you start to get field rotation as mentioned. Stacking software will rotate and align but will work with short subs. So alot depends on how far one wishes to take his/her Astrophotography adventures to. Also, stars lower in Altitude can probably go a little longer in exposure but as they get higher, your exposure time reduces for ALt Az shooting.

The EQ mount, though cumbersome and expensive (depending on set up) tracks the star at an Arc rotating along the North/South Celestial pole (not magnetic North/South) and keeps the subject as well as it's surrounding stars continuously in your EP FOV. Hence why long exposures where as the camera is exposing, there is no fear of the subject moving away from it's position in the EP.

This lastly brings about the reticle EP. The lines within are used to basically observe a star chosen to help with aligning the mount on it's RA and Dec axis. In a nutshell, one places a guide star (one in the south for RA and one from the East for Alt/Dec) on/between the red lines and checks if the star drifts away from the lines. If the star stays within for at least 5mins, the mount is aligned and long exposures are possible for 1 min. Some use CCD drift methods, or just allow software to help in drift aligning.

Guiding software will help but it benefits more from a properly aligned mount as well. Meade have their own system and anything Meade works hand in hand. Their DSI cams communicate with their Fork mounts and Software best. And as you can see Meade have a world of their own...lol!!

Hope that helps.

[fizzy123]

Well, lets say I want to take M31, M45, M35, M42, which are relatively bright and mag >5. Can these create images just by exposing for only 30s?

[timatworksg]

Fizzy123,...

Here's a link to Steve Forbe's Astro Blog

http://trapeziumtelescopes.com/blog/

He Mini tower upgrades and if you surf thru his blog you will see some pretty impressive shots done on the mini tower at 30s.
Personally it's not impossible. Alot comes down seeing conditions and LP,...then lots and lots of exposures to stack for detail. I think you can get some decent pics....though not Hubble quality but pretty keepsakes!

In the end you can take shots AltAz or EQ. It comes down to whats achievable and end results. Longer exposures give you lots and lots of stars with nebulosity thats even hard to find. Plus longer exposures give you more detail of your view. With shorter exposures, you may not capture much of the nebulosity but sometimes with good seeing and some photoshop tweaking you can pull it out. M42 is possible on short exposure but you won't capture the full, if not almost full nebulosity as you see in most pics. Maybe just a small crescent glow. Some like it some want more. Clusters will be difficult as you may not be able to grab all the stars in the globular.

Just my thoughts. In the end, if your happy with what you achieve Have Fun!!

[cloud_cover]

Hi Fizzy!
I'm no guru (far from it!) but here's my experience to date:
It firstly depends on your focal length. If you're doing relatively short FL, any mount you use will be more forgiving. If the FL is longer, then any deficiencies (including field rotation in Alt/az) becomes more evident quickly. So you need to firstly consider what's your focal length (divided by the CCD size, of course for crop factor).
Next it depends on the speed of your exposure. Saying 30secs is not very meaningful in this respect: While the length of exposure determines how accurate your tracking needs to be, the time needed for the exposure depends on both your f ratio as well as your ISO speed. So someone using ISO 12500 with an f 2.8 lens will be VERY VERY different from another person using (for example) ISO 1600 on an f/10 lens (about 6 stops down - the eqivalent exposure time being 30 x 64 = 1920s). So you really will want to consider your f ratio as well as imaging camera. In this respect what Orly says is absolutely right: consider a faster (lower f number) system.
Having said that, the C5 should do well imaging the moon and planets since these tend to be very bright subjects requiring only fraction-of-second exposures.
To answer your question, it is most certianly possible to image M31 using sub 30s shots. My picture in the thread: http://www.singastro.org/viewtopic.php?t=8545&start=15 was taken at f/8, ISO 6400, 20s subs. Multiple stacks , of course. It is also possible to image nebula using sub 2s shots. I'm in the middle of processing the Lagoon/Trifid nebulae, taken at 1.3 (or 1.6s) subs at ISO 12500 without a tracking mount (i.e on a camera tripod). I'll upload the poorly processed take when I can (its in my lappy). Its not pretty, but its doable. (It'll be prettier with even more subs, of course)
I've seen some very nice DSOs using Obsession Dobs (yep, a premium telescope, but it is alt/az) so shooting nice dim subjects is a possibility but you'll run into technical challenges (not insurmountable, but challenges nonetheless)
Since you're considering a Nexstar 5SE, may I suggest you consider the Celestron C6NGT? Its a 6" Newt on a CG5NGT mount, cost USD $100 more than the C5 (on optcorp.com) but has a tracking (and autoguide capable, with guider, of course) equatorial mount, has a focal length of 750mm (about 2/3 of the C5) and is f/5. If you can accept the newtonian design it may be a better imaging platform.
Finally, I googled deconvoluting software. Try these links
http://science.niuz.biz/field-t158175.h ... dcfd19f799&
http://www.bialith.com/
http://www.cloudbait.com/science/shiftproc.html

Hope it helps!

[weixing]

Hi,
Is it possible to use Alt-Az mount for AP, yes it's possible... is it practical, that's another question. There are lots of factors involve, here are some:
1) Signal-to-Noise Ratio: A very low noise sensor will help a lot as using shorter exposure mean lower signal. To maintain the same signal-to-noise ratio as a longer exposure image, you need a very low noise, high sensitivity and efficient sensor.

2) Sensor pixel size: the larger the pixel size, the more light it can collect.

3) Focal length of your scope: the longer the focal length, the more obvious the star trail will be, so will limit your exposure time.

4) The object location: if the object is near to the pole, your exposure per shot can be longer and still without star trail.

5) Object surface brightness: Object with brighter surface brightness will be easier.

So if you want to AP using Alt-Az mount, you properly will need:
1) A very low noise, high sensitivity and efficient sensor with large pixel... this usually mean lower resolution.
2) Choose a bright surface brightness object near the pole.
3) Use shorter focal length.
4) Shoot a lots of shot.

The disadvantage will be you'll need lots of hard disk space and took a longer time to process.

Just my $0.02.

Have a nice day.

[fizzy123]

Anyone got a picture of a DSO taken with a computerised Alt-Azi mount without any EQ setting? Wish to get a gd first hand understanding behind the short-comings.