DSO Astrophotography - Pitfalls & Misconceptions

[Shibby]

As I've now been imaging for a couple of years, I started thinking about all the hiccups I've encountered along the way as a beginner, so I started writing this list today. However, I have come to a point where I'm not sure if I'm just repeating advice that's available elsewhere. What do you guys think, is this useful? Is it pitched correctly at beginners? If not, please let me know - I won't be offended! If you do think it's a good idea, then please suggest some more pitfalls and I'll add them to the list; perhaps some you have stumbled into yourself!

------------------------------------------------

This is certainly not a comprehensive guide to DSO imaging, but a list of a few common pitfalls that beginners can find themselves, well, falling in to. DSO imaging can be frustratingly slippery slope, so my hope is that this list will offer a little traction to some. For more advice on DSO imaging & equipment, take a look around the forum - you will find a great wealth of information and helpful members waiting for you!

1) Buying the "wrong" telescope.

Many beginners (me included) buy their first telescope for visual astronomy with the hope to use it for astrophotography in the future. Like me, many of these beginners later discover that the scope they bought is far from ideal for astrophotography. Here a few things to watch out for:

Focal ratio

Large apertures are great, particularly for visual astronomy. But when it comes to DSO imaging, f/ratio is crucially important. Without going into all the details, a low f/number is preferable because it tends to produce a brighter image of DSOs, meaning less exposure time is required to capture all that lovely, faint, nebulosity. My recommendation would be to look for telescopes at around the f/5 mark.

NB: This is not a reflector vs refractor discussion!

Reflectors

Many Newtonian reflectors will NOT reach focus with a DSLR camera, due to not having enough "back focus" (This is a mistake that yours truly made). This essentially means that you cannot rack the focuser far enough inwards for the focal plane of your telescope to meet the sensor on a DSLR camera. If buying a Newtonian with DSLR astrophotography in mind, ensure that the particular model you are buying can reach "prime focus" with a DSLR. The Skywatcher PDS range of Newtonians have been specifically designed to alleviate this problem.

Another, optical issue common to (most) Newtonian reflectors is coma, a type of field distortion causing "V" shaped stars towards the edges of the telescope's field of view. However, there are several coma correctors available so beginners should factor in the cost of a coma corrector when it comes to imaging.

Refractors

Refractor telescopes at the budget end of the scale are usually achromatic. This means that they use a pair of lenses to correct chromatic aberration (different wavelengths of light coming to focus at different distances). While good for visual use, the colour correction is not as good as more expensive refractors, which are often apochromatic. The typical result of using an achromat for astrophotography is large, purple halos around stars - this can often disappoint a budding astrophotographer.

2) Not budgeting enough for the mount.

If you're serious about trying your hand at astrophotography, the single biggest allocation of your budget should be to the mount. Equatorial is a must. The beefier the better - it should be able to cope with everything you might load it up with in the future. For more advice on which mount to buy, have a good read around the forums.

3) Capturing in jpeg format

When you start taking exposures with a DSLR, make sure you are saving images in RAW format. Jpegs are not only compressed, but also have less colour depth (a smaller range of colour values). So by saving images as jpegs, you will be wiping out valuable, faint data, lost forever...

4) Not enough dark subs

Dark calibration frames are used to remove dark signal (e.g. hot pixels / amp glow) from your images. A common misconception is that they remove noise, whereas they actually *add* noise when subtracted from your light subs! For this reason, you should capture as many dark subs as you can in order to reduce the amount of noise added. 20 is a good minimum number to aim for. Too few darks is often worse than no darks at all!

5) Not taking flats

In my opinion, flats are even more important than darks. Images are extremely awkward to process without taking flat calibration frames. One particular pitfall is to move the camera before taking your flat frames. For more information on flats and how to take them, please explore the forum.

6) Polar scope reticle alignment

If you're considering astrophotography, you may already know that your equatorial mount needs to be nicely polar aligned for accurate tracking. A common pitfall is to not align the reticle of your polar scope first - this is of utmost importance! There are several polar alignment guides available, please take a look around the forum for advice & links.

[DaveT]

Thanks Lewis -- some really good advice i wish i had had when i went buying. Some more info on flats would be good mate, as i have only taken darks. Im guessing flats are when you take it with a complete white?? but some advice on how to get the best way to do it would be fantastic

thanks again

Dave

[Mansnake]

Yep some nice info.

I would also like a dummies guide explaination of flats, darks and lights.

Also its nice to meet a fellow Lewis from East Anglia !

[themos]

I would also like a dummies guide explaination of flats, darks and lights.

OK: You need to shoot flats and darks.

[Shibby]

Hi guys, you should be able to find lots of info around the forum about calibration frames. e.g. here's just one about flats: http://stargazerslounge.com/imaging-tips-tricks-techniques/174858-darks-lights-flats-etc.html

If you use DSS, I recommend reading the manual as it has a great explanation of the various different types of calibration frames.

Also its nice to meet a fellow Lewis from East Anglia !

Well, now I don't feel so original!

[Russe]

What about BIAS?

How bright do flats have to be? Is a white computer screen enough?

[themos]

has this question never been asked and answered on this forum?

If I post an answer it will only be the 20th time I've done so. It's not going to sound original so i won't.

[ollypenrice]

I think the advice is sound, though if you specify F5 rather than a range you steer people towards Newts since only fairly posh apos get down that far. My own rule is 'not slower than F8 under any circumstances' and as fast as possible after that.

I know you don't want to turn it into a reflector-refractor thing but beginners will always find refractors easier. I don't say better (though I may think it!!!) There are the issues of wind and collimation to consider.

I would try to avoid perpetuating the domination of scope weight in choosing a mount. It is vitally important to understand that tracking accuracy requirements rise with focal length and that just having a big mount and an autoguider doesn't mean you'll get good enough guiding. An NEQ6 will carry a 10 inch SCT but will it deliver round stars at F10? Mine certainly won't.

The most common beginners' mistake is, to be honest, buying an SCT. Health warnings about focal length would reduce the number of people caught out in this way.

Olly

[Shibby]

Thanks very much for the feedback & tips Olly. (I forgot to ask for corrections!) I'd like to update the list but can't seem to edit the post any more...

Mods, is there a way to give access to editing single posts??

w.r.t. buying the mount, shall I just leave it as "prioritise budget for the best mount you can get. The longer the focal lengths you want to be able to image at, the more accurate the tracking or guiding needs to be" do you think?

[themos]

It's a pretty brutal effect because with longer focal length comes greater width and mass of optics requiring an even beefier mount.

I'd say the one thing that you can't compromise on is optics. If your mount is not up to scratch, you'll go for shorter sub-exposures. If your camera is not sensitive enough or is only 8-bit or the sensor is too small, then you will have to shoot a lot more frames. But if your optics can't push photons to the right place, you're finished and there's no way to untangle them.

[DaveT]

Thanks Shibby -- good link. reckon i will need to read it 3 or 4 times to take it in lol

Dave

[SlipperySquid]

As with any new hobby, things ARE going to go wrong.

It just seems that with astrophotography there is so much that DOES go wrong that it can push you away before you even get started

Perseverance and patience is my mantra and when things aren't going according to plan and I seem to have exhausted every avenue I just pack up and get ready to fight another day.

If I started things over again I would definitely have gone for an EQ6 mount! I went eq5 then heq5 now saving for eq6!

Great write up by the way!

Clear skies,

Matt.

[ollypenrice]

It's a pretty brutal effect because with longer focal length comes greater width and mass of optics requiring an even beefier mount.

I'd say the one thing that you can't compromise on is optics. If your mount is not up to scratch, you'll go for shorter sub-exposures. If your camera is not sensitive enough or is only 8-bit or the sensor is too small, then you will have to shoot a lot more frames. But if your optics can't push photons to the right place, you're finished and there's no way to untangle them.

Of course, ideally, it all needs to be good! However, I don't really agree on the priority list if we look at what is available and at what price. I would say that, with a wise choice of modern products, the order should be mount-camera-optics. I say this because there are budget optics out there which run the best pretty close. (There is also a lot of rubbish but why buy that at a similar price?) Budget autoguided mounts can keep to a sub pixel error on short to moderate FL imaging scopes. But the camera... ah the camera! A decent CCD will blow a decent DSLR out of the water. While a run of the mill guided HEQ5 can do 30 minute subs a DSLR can't.. But a cooled CCD can. Short subs just don't suffice on faint targets. You never get over the read noise. So for me, as I say, my budget would be mount-camera-scope.

Olly

[themos]

! A decent CCD will blow a decent DSLR out of the water.

I don't doubt that a mono, cooled CCD is better. However, the physics of the situation says that the difference between these can be made up by investing more time. Read noise is actually not that different, I see QHY cameras, for instance, with a quoted read error of about 7-10 electrons. My 450D does 6 electrons and costs 1/6 of the QHY12. And you can cool DSLRs with a diy solution costing less than £150, say.

The wildcard in this equation, I'd say, is the weather. You can't buy good weather in the UK so it makes sense to have a "fast" mono CCD and perhaps not pay top money for the best optics you can afford if you're going to suffer from bad seeing most of the time.

[Shibby]

Just heard from Helen, she recommends posting a new thread at the end of the discussion.

I think I'll chop down the scope choice advice, and just present the individual pitfalls in the list.

Anyone else got any pitfalls to add to the list?

[dph1nm]

Short subs just don't suffice on faint targets. You never get over the read noise.

You are spoilt by those French skies! I saw some figures someone on another forum had worked out, and they reckoned that in typical suburban skies with a DSLR on a typical amateur scope you can be over the read noise in as little as 20 sec exposures!

NigelM

[mindburner]

this is great. I have learned 2 new things from it. The advice on darks and Jpeg's