Astrophotography Primer

CCD vs Film? Lots of time vs no patience? Alright, this is your place to discuss all the astrophotography what's and what's not. You can discuss about techniques, accessories, cameras, whatever....just make sure you also post some nice photos here too!
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Meng Lee
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Astrophotography Primer

Post by Meng Lee »

Hi,

For all who are interested to take the (expensive) plunge into astrophotography, I write a primer so that you can get started into this venture (almost immediately). I will take on an operational description. Those who wants to know the reasons behind the description can PM me. Those who disagree with my methodology can argue in a different thread.

Introduction

There are 3 branches to this venture:
1) Planetary (Webcam)
2) Simple Deep Sky (DSLR)
3) Sophisticated Deep Sky (Proper Cooled CCD)

Each branch has 3 stages:
1) Equipment
2) Taking the pictures
3) Processing the pictures

Branch 1: Planetary (Webcam)

Since now is the Mars and Saturn season, let me give some tips for taking nice pictures of the planets and the moon.

1) Equipment

a) Mount
This time, the mount need not be excellent. You do not need autoguiding and all that. The mount only needs to have proper capacity to hold the scope, can withstand a certain degree of wind, and can track fairly well will do. You can use the controller to keep the planet in the webcam's view, so bad tracking does not hurt.

b) Scope
Planetary scopes is a controversial topic, but essentially I personally think these are the points: Big aperture (resolution depends on aperture), and good quality optics. Big APOs are excellent, but the budget user can get very good results with long focus Newtonians (like the Orion 150mm f8), SCTs are also good. For any scope, it must reach thermal equilibrium with the environment, in general, an 8 inch should take about 1 hr. Collimation is absolutely critical, at least good collimation must be achieved IN-FOCUS at 50x per inch of aperture magnification.

c) Seeing
This refers to the turbulence conditions. There are 2 types: local seeing and atmospheric seeing.

Local seeing: hot air rising over concrete will create local turbulence, essentially, do not image when the planet is just over a roof or building. Dome observatory is bad when the dome is too small, the scope is simply looking at turbulence at the opening of the dome. People in the observatory will create warm air and it is almost impossible to get thermal equilibrium between the dome and outside.

Atmospheric seeing: If the stars are twinkling like mad as seen with the naked eye, forget about imaging. Look at a star at a similar magnification to the imaging magnification and if you do not see nice diffraction rings at moments of good seeing, you may want to take a lower magnification for imaging. You can determine an optimum magnification for imaging the planet in this way. Of course, this is done after the scope has achieved thermal equilibrium. Image when the planet is at a high altitude also reduces atmospheric effects.

2) Picture taking

a)Roughly polar alignment the equatorial mount will do. Centre the planet with an eyepiece then replace the eyepiece with the webcam. Keep the planet in the webcam view throughout the exposure using the motor controller.

b) Focussing
You see a fuzzy planet with a lot of "snow". To get good focus, you do not focus until the edge of the planet is sharp. You focus until you get the sharpest details on the surface of the planet in moments of good seeing! Do adjust the focus and take a few AVIs, you will have a greater chance of getting a well focused pic.

c) Settings
The object must not be overexposed, especially the moon. Once overexposed, you can never recover the data. So lower the gain until the "highest number" is not 255 and the "lowest number" is not zero. This refers to the histogram values. K3CCDv1 shows you live feedback of the values. For Toucam, use 10 fps, thats all.

The more frames taken the merrier it is. But for Jupiter, do not exceed 1min 45s as the features on the surface will smear. For other planets, do not exceed 3 mins as your computer cannot take it.

3) Processing

a) Use Registax for the processing, here are some tips

i) The better the seeing, the smaller the alignment box.
ii) For moon, use overlapping multialignment boxes for best results, point i) applies.
iii) Default settings is fine to start with.
iv) After sorting the frames, do a manual cut-off, you rather stack less sharp frames together than allowing the blurer frames to get into the stacking.
v) Typically there is only 1 wavelet slider that gives you vast improvement. So find which slider gives you the best improvement and tweak very little for the other sliders.
vi) Stretch the pic a bit to give more tones to the pic so that the pic appears more 3D like, works especially well for the moon.

Branch 2: Simple Deep Sky (DSLR)
I will only cover 2) Simple Deep Sky (DSLR). Maybe one day I will cover all the sections. For DSLR, these are the 3 stages:

1) Equipment

a) Mount

This is the single MOST important factor!! Must have precision gears. Must have autoguider port. Must be stable, meaning your total setup is at most 3/4 of the full loading capacity of the mount.

Recommendations:
GPD2 is very good, but very expensive to make it have an autoguider port. Celestron CG-5 goto and Meade LXD 75 and Skyview Pro are not recommended unless you only image at less than 700mm focal length. HEQ-5 and EQ-6 recommended, GM-8, G11 recommended, and of course Takahashi mounts and astrophysics mounts.

Operational:
Level the mount and align roughly north and then do some rough drift alignment. The rest will be taken care by the autoguider and stacking with translation-rotation.

b) Scope

Good to start with refractor 500-650mm APO. Focal ratio is recommended to be f5 or f6. Not too fast otherwise vignetting is severe and not too slow otherwise the exposure is unnecessarily too long. Focuser must be strong and smooth. Crayford focusers must be tightened to prevent slipping.

Guidescopes preferably refractor (can be APO or achro) to reduce movable elements. Guidescope must be mount FIRMLY to prevent any differential flexure. Use Losmandy dovetail mounting style. Same for the mounting the scope to the mount.

The scope+guidescope+dslr+autoguiding camera must be balanced carefully in both RA and DEC so that autoguiding can work well!! Never be lazy about this step.

c) Autoguiding

my combination ST80 + Meade DSI + PhD + GPUSB works up to 1250mm imaging focal length, simple to use and highly recommended for DSLR imaging.

2) Picture taking

a) Focussing

Buy ImagesPlus or MaxDSLR for your focussing, acquitision of pictures and stacking and processing. Personally I like ImagesPlus a lot. Focus VERY CAREFULLY using the FWHM setting or half-flux diameter setting on an unsaturated star. The "auto" feature in ImagesPlus can get you to good focus in less than 10 mins.

b) Optimum ISO

In general, set ISO between 200-800. Rule of thumb is: Objects with large range of brightness, use lower ISO. Objects with small range of brightness, use higher ISO.

c) Modified DSLR

Use a modified DSLR if you want to take nebulae, if not, then just expose 4-6 times longer will do also for nebulae. If the camera is modified, then the colour balance is totally off, then use a Macbeth Card for colour balancing.

d) Optimum Exposure length

Shoot in Raw+Large format, and examine the Large JPEG file in any program that shows you the histogram. In general, all 3 PEAKS, red, green, blue should all lie between 30-50% of the dynamic range. Then just carry on and shoot as many frames as you can get.

(I do not describe dark frames here because I just want the reader to get a nice pic easily. The reader can learn about dark frames later. Learn fundamentals first.)

3) Processing

My workflow is usually like this:
a) Use ImagesPlus to (calibrate the frames) debayer
b) Stack in imagesplus using extended add if you want the highest signal to noise ratio possible. We talk about other variations next time.
c) Photoshop: set daylight white balance using macbeth card.
d)Photoshop: set black point
e) Photoshop: Pull the curves a little bit
f) Photoshop: Use magic wand and run Croman's tool at fine detail and high aggressiveness. Croman's tool will balance backgound colour also.
g) Photoshop: Run 2-3 rounds of curves to stretch out the histogram nicely
h) Photoshop: Run 1 round of highpass filter to sharpen the image a bit.

Tutorial ends here. I hope you can get nice pics now.
Last edited by Meng Lee on Thu Dec 20, 2007 4:00 pm, edited 1 time in total.
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jermng
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Post by jermng »

Thank you Meng Lee! :)
Jeremy Ng
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
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mch3898
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Post by mch3898 »

This primer is a real practical help. Thanks Meng Lee for sharing. Just bought Images Plus and the camera controls, including the focusing aid, are great. :)
christopher
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Meng Lee
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Post by Meng Lee »

Hi Chris, long time no see. In fact you will be imaging at 1260mm with the C8 which is the same as I will be doing with my VC200L, so lets work together in the 1000-1500mm regime. After that, we can try to embark on even higher resolution, then our mounts, HEQ-5 and EQ-6 will have to be replaced as well.
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mch3898
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Post by mch3898 »

Hey, I am still very much a newbie!! Still trying hard to learn the ropes. The EQ-G mount you recommended is great and I fully appreciate what you experienced astrophotographers mean when you said it is important to invest in a good mount more than any other equipment.

By the way, just placed an order for EQDIR from Shoestring to help me get more from the Sirius mount. Hopefully with the EQMOD, getting 3-star alignment will be easier.
:|
christopher
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kayheem
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Post by kayheem »

Hmm... can the mods make Meng Lee's write-up a 'sticky' somewhere?It would definitely come in useful to those starting out in astrophotography.
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Meng Lee
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Post by Meng Lee »

Personally I think the EQDIR serves to integrate the system as a whole, with EQDIR, the Synscan controller cannot be used. GOTO is not really important in astrophotography, as long as the mount can track, has autoguider port, and can adjust autoguider speeds, that should be enough.
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Tachyon
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Post by Tachyon »

I have printed it out already... thanks!
[80% Steve, 20% Alfred] ------- Probability of Clear Skies = (Age of newest equipment in days) / [(Number of observers) * (Total Aperture of all telescopes present in mm)]
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Meng Lee
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Post by Meng Lee »

Er, Andrew, I don't think you need this primer, its too simple for you already.
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mrngbss
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Post by mrngbss »

Thanks! Simple enough for me. :)

OT: ImagePlus has Version 3 now. Supports SER files.
Wee Nghee the Pooh
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