Astrophotography with Small Refractors

How to Get Great Images with Modest Telescopes!

When it comes to astrophotography, I really like small ED refractors. They are affordable, easy to set up and in combination with a solid equatorial mount, most of them can take great photographs of the night sky. The shorter focal lengths also make for a wider field and therefore less critical tracking.

In this tutorial, we will focus on three viable candidates for astrophotography:

orion-ed-80mm Orion 80mm ED (been around for many years, great optics, about $300 on the used market) f = 600mm, F/7.5
orion-eon-80mm Orion 80mm EON (a more recent upgrade, better mechanics, especially the focuser) f = 500mm, F/6.25
orion-st-80mm Famous little Orion 80mm ST (short tube achromat, very inexpensive, light and simple) f = 400mm, F/5

 

I will also briefly cover the Celestron 80mm ED, which is an excellent telescope (only available on the used market) at the very end of this article.

It goes without saying that the first two telescopes are far superior to the short tube, but I was curious to see what could still be achieved with such a lightweight and affordable scope. Unfortunately, by the time I was ready to shoot with it, the clouds started to come in, resulting in only two useable sub-exposures. So it is not a completely balanced comparison – however, it was clear that the short tube could not compete with the other two models anyway. It makes a great guide scope, and because of the fast optics (F/5) it is quite possible to shoot nebulas and galaxies with it – but only in the early stages of learning how to properly track the stars and how to process the digital files. I used one myself for the first year of my hobby and was very excited when I saw how my initial efforts were rewarded with presentable images of the Orion Nebula, the Andromeda Galaxy and the Pleiades (see also my previous article: Astrophotography on a Budget). It is always best to start modestly and grow from there.

Dark Site, Dark Site, Dark Site

I cannot emphasize enough what a difference it makes to shoot at the darkest available site you can get to. Driving for an hour or more to get away from city lights should really not be an issue, and for that reason the portability of your equipment becomes an important factor. Bigger is not always better! For these tests I went a little further than my usual spot on Piuma Road in the hills above Malibu: I drove up to 2000 feet in the mountains near the Ventura County line. Being away from Los Angeles, the skies were darker and the core of the Milky Way was clearly visible. The Lagoon Nebula (M8), my target for the evening, could be seen with the naked eye.

For this comparative test, I decided to keep all sub-exposures at 60 seconds. I use a Skywatcher EQ-5 mount (much like the Celestron CG-5). After carefully polar aligning the mount, I first did a visual comparison of the three scopes. Right away, it was obvious how much better the image looked in the ED and the EON: not only were stars sharper and more pinpoint, but the background was noticeably darker and the image more contrasty in the two apochromats than in the short tube achromat. Between the 80 ED and the EON, the differences were slighter: the EON has a wider field and is more compact. The dual-speed focuser on the EON is more precise and does not shift upon locking it, but both scopes present a tack-sharp image.

The Shootout

The first scope to shoot with was my 80mm ED. It’s a solid piece of equipment and my only gripe is the focuser: it is very smooth but tends to shift whenever you lock focus. It took a few tries, but using live view – a very important asset for AP – at its 10x magnification on my astro-modified Canon 1000D, I was able to anticipate the shift during focusing on Antares so that after locking, the focus was perfect. I always choose a bright star not too far from my desired target to focus on.

In order to get a flatter field and faster ratio, I used a 0.8x William Optics Focal Reducer/Field Flattener (version 3). Thus, the focal length became 480mm and the ratio went down from F/7.5 to F/6. I found this combination works very well: it is fast enough to get by with shorter exposures and still have plenty of detail in a field that is sharp from corner to corner. I also used a clip-in light pollution filter (CLS for Canon EOS) right over the sensor, because even at a fairly dark site it still helps the contrast. The only time I will take it out would be at the perfect dark site, which near Los Angeles probably means Death Valley: a five-hour drive. It takes a lot to avoid light pollution if you live in the big city!

With the relatively short 60 sec subs (unguided) at ISO 1600 I was actually surprised to see how much detail was being recorded, visible even on the small display of the camera. Here is one (click on image to see it in higher resolution):

Astrophotography- the Lagoon Nebula (M8)

A fair amount of nebulosity in the Lagoon Nebula (M8) and beautiful pinpoint stars across the field. This is the entire frame and there is no trace of breakdown in the corners. It is really a testimony to the great optical quality of the 80mm ED, and the lucky combination with the William Optics FR/FF v.3. I shot about seven or eight exposures, followed by five dark frames, of course also at one minute each.

Then I programmed my GoTo to slew to the very nearby Trifid Nebula (M20) and quickly shot five subs of the Trifid as well. Here is one:

Astrophotography- Trifid Nebula (M20)

Again, the detail is very good and there is a fair amount of color in the nebula, without any processing. (Note: since I always shoot in RAW mode as opposed to JPEG, the image has been digitally corrected for color and contrast in Camera Raw 6.4 – but no real enhancements yet.)

Time to change scopes. Slew back to Antares, take off the 80 ED and set up the EON. Right away, it feels like a very well designed instrument: everything moves smoothly, it is compact and after attaching the camera (again with the WO FR/FF v.3 and the CLS clip-in filter) working the dual-speed focuser in live view is just like a dream, super smooth and zero image shift upon locking it.

Hopefully it will be perfect focus – even with the 10x magnification you can sometimes be a hair off. So back to the Lagoon Nebula and start another series of eight subs, which takes less than ten minutes.

We are now shooting at F/5 as opposed to F/6 with the 80 ED – so I am expecting the image to be a little brighter and the field a bit wider. Here is one of the sub-exposures:

Astrophotography- the Lagoon Nebula (M8)

Since the orientation has been corrected, the nebula is better framed and the field is indeed a little wider. Stars again are pinpoint across the field and the image is a hair brighter. Here’s the Trifid:

Astrophotography-Trifid Nebula (M20)

Clearly, it’s a wider field, now with part of the Lagoon showing below. The ED shot looks a little more contrasty, but that could well be a result of the Camera Raw processing, because the images were not developed identically. Anyway, again plenty of detail to work with. I shot five subs of the Trifid. Why eight of the Lagoon and only five of the Trifid? Don’t ask – it was getting late…

Finally, testing the 80 mm short tube. As I mentioned earlier, clouds were starting to come in and I had to move fast. I took five subs of each nebula and only one of each turned out to be (borderline) usable. The focal reducer/field flattener could not be used on the 80 ST because it is too big for the 1.25” eyepiece holder, but I left the CLS filter in the camera. Here is the best exposure of the Lagoon:

Astrophotography- Lagoon Nebula (M8)

There is simply no comparison. Stars in the corners are horribly bloated and deformed, the field curvature is totally distracting. BUT at F/5 and 60 seconds, the nebula is definitely showing!

I did another series of dark frames, just in case the temperature had changed, and we packed up. By the time we left, most of the sky was covered in clouds. I did not get to shoot my flat frames, but I confess I often don’t even bother.

Processing the RAW Files

It is always a wonderful challenge to get the most out of your shots. I have found that at times even a single exposure (without using Deep Sky Stacker, just Photoshop) can be processed and enhanced to a surprising degree as long as you keep the noise under control. Therefore, the first step is to get rid of as much noise as possible without sacrificing sharpness and detail. After opening all your subs in Camera Raw (working from Adobe Bridge) you will see several icons in the top right corner: the third icon from the left has two triangles and if you move your mouse over it, it will say “Detail”. Click on it.

Lagoon Nebula Processing

You want to see the files at 100% (indicated in the lower left corner) and move the levers for noise reduction (right side) to a point that all the noise is gone. For these particular sub-exposures I set the Luminance bar at 54 and the Color bar at 68. Then I brought the Sharpening up to 61. This gave me a nice balance with an almost noise-free image. The other levers are at their default settings. Make sure to click on “Select All” (top left corner) before you make the adjustments, so you do it to all the exposures at the same time. You may notice that the image (this is from the EON) actually looks better and more detailed here than in the sub-exposure that was shown earlier. That is because I could not help myself: once I brought up the subs up in Camera Raw to take a screen shot I played with them some more!

It’s a real good idea to work the files as much as you can in before saving the images (as uncompressed TIFF files would be my recommendation). For those who prefer to use Adobe Lightroom (another great software for RAW processing) I’d give the same advice: get rid of the noise and work the files before saving and stacking them in Deep Sky Stacker.

Please take a look at the values displayed below which is the first (Basic) template that comes up when you open the subs in Camera Raw: it will give you a good idea how much messing around is needed to get the subs to look their best.

Lagoon Nebula Processing

A closer look makes it easier to read:

Lagoon Nebula Processing

What I find most striking is the color correction: temperature all the way down to 3000 K and the tint -84 into the green. That means the original file had a very strong magenta bias and was pretty warm to begin with. Color is usually all over the place in the RAW files and even though I set my color balance on Auto during the shoot (hoping it will correct whatever bias is there), using different filters to reduce light pollution invariably changes the color. Position in the sky also is an important factor: lower in the sky makes the image warmer.

I pushed the blacks up to 12, which is quite high – I could have left more definition in there, even though that would have added a bit of graininess (= noise). Personally, I like the background close to black, it just looks more dramatic and appealing that way. The brightness is pretty high as well at 79, which helps to get more definition in the nebulosity. A bit of fill light is good for the same purpose.

Finally, some extra vibrance and saturation helps the color come alive. The whole point of showing all this is to emphasize the importance of shooting in RAW and how much can be done to the subs before they are stacked. It makes a huge difference in the final result.

Stacking the Images in Deep Sky Stacker

This part of the tutorial can be pretty short, since I do not want to repeat a lot from the previous one. Deep Sky Stacker has its own instructive guidelines on their site (http://deepskystacker.free.fr/english/) and it is totally fine to use their default settings. Dark frames are imperative to shoot, because your sensor will otherwise add its own “stars”. Flat frames really help correcting the fall-off towards the sides and corners of the image. I strongly recommend shooting them even though I don’t always do so myself! Dark flat frames and bias frames will add additional quality to the shot and ultimately it is all a matter of how perfect you need the end result to be. In my opinion, first get your tracking and processing skills in place, then you can start working on the finer details.

Deep Sky Stacker is a miracle and without it, most amateur astronomers would not be able to get the results they are getting. It is an absolutely must to use when shooting Deep Sky Objects with a DSLR.

Final Processing

These days, the first thing I do with the final stacked (but mostly unprocessed) image is go straight to curves in Photoshop (Ctrl – M), grab it in the middle and pull it way up. That will bring out a lot of the nebulosity but at the same time it makes the image look flat and too bright. The background becomes pale and washed out, but that is exactly what we need. The only black left in the shot are the very edges where Deep Sky Stacker had to adjust the frame to make sure that all the sub-exposures were perfectly lined up.

Lagoon Nebula Processing

In order to bring back the density, we go to Levels (Ctrl-L) and move the left cursor further to the right:

Lagoon Nebula Processing

We can even do this a few times: first bring up the curves and then bring back the density by going to Levels and moving the left cursor to the right. To make sure that the highlights (in this case the central part of the nebula) don’t burn out, we can put a feathered lasso around the bright areas, and hit Inverse (Ctrl-Shift-I). That will leave those areas untouched while we apply the corrections to the general image.

Typically, the photo will still have a color shift, in this case magenta – but it can also be green, cyan or red. In order to improve the color, we go to Color Balance (Ctrl-B) and highlight “Shadows” in the lower left corner of the Color Balance window. Then we choose the cursor that corresponds with the color shift and move it in the opposite direction: so in this case towards green. What happens is that the background will go darker and at the same time the color becomes more natural.

Lagoon Nebula Processing

In order to save time, you can try using the automatic corrections: Auto Tone, Auto Contrast, and Auto Color. In my experience, you get lucky sometimes and other times it is way off. You can always undo it or bring the effect down by going to “Edit” (top left) and then “Fade Auto Tone” (or Contrast or Color).

Lagoon Nebula Processing

It is by combining different tools and corrections and playing around until it starts looking really good that you end up with a truly nice image.

Conclusions

All three telescopes have their merit. The 80mm short tube is really not meant for imaging but it makes a great guide scope and a very affordable and portable “grab and go”. I ended up stitching the two shots of the Lagoon and the Trifid Nebulas (the only usable subs) together and the result is not even bad, especially considering the instrument it was shot with:

80mm Short Tube Final

As long as you keep it small, it looks nice and shows both nebulas.

The 80mm ED fully lived up to its reputation of great optics. It may even have slightly better contrast than the more expensive EON, but that is a tough call – they both throw up a beautiful and sharp image. If I were to use the 80 ED on a regular basis, I would probably want to change the focuser.

Here is the final combined image of the 80 ED:

80mm ED Final

It is clearly an excellent imager, and the fact that it sells for only $300 on the used market makes it a very attractive candidate as a first scope for AP.

My personal favorite of the evening was the 80mm EON. It is a little shorter than the ED, beautifully finished and really easy to work with. There is nothing I would change about the design and at some point I’d actually like to play with its bigger brother, the Orion 120mm EON.

With its fast optics (F/6.25) and wide field the 80mm EON makes a perfect imaging scope that is still very portable.

Here is the final combined image, shot with the EON:

80mm EON Final

Finally, I would like to add a few words about the Celestron 80mm ED, which is optically identical to the Orion 80mm ED. However, it has a lighter and more elegant looking build. The copy I used had an upgraded focuser (a dual-speed Crayford) which made working with it very comfortable. Also, I used a better camera: a modified Canon 5D Mark II. As opposed to the Canon EOS 1000D, it has a full frame sensor. I shot at ISO 1600, taking about ten unguided sub-exposures of 1’42” each. Even though there was a fair amount of noise, due to the warm weather and some vignetting because of the larger sensor, the end result was better than any shots I had taken so far. I used an AT 0.8x reducer/flattener which worked very well in combination with the scope, giving me an effective focal length of 480mm at F/6. I followed the same processing steps as I did with the other images.

Here is the final image: it comes from only one frame rather than combining two, since the larger sensor produces a significantly wider field. I highly recommend this excellent little scope as well as the beautiful 5D Mark II.

Astrophotography- Celestron 80mm ED and Canon 5D Mark II

Clear skies!!

* * *

View Martin Cohen’s astrophotography website here for more of his outstanding work!

Comments

  1. Martin, this is great! Exactly what I needed–which was an idea of just how much I could do with an 80mm scope. And a reasonable mount. And which software, and how to manipulate the images, and whether I’d need to drive out to the desert. I’ll be checking out your other articles for sure. Thanks for taking the time to write this, and for making it all so clear and orderly. It was very easy to follow and very informative for a guy looking to begin astrophotography. Many thanks.

    – Frank

    1. careful of the objective lense used…….Hoya FPL51-equivalent ED element ……on the Explorer Scientific ED80 …..that word equivalent gets me nervous.

  2. Hello! i was just wondering if they make the clip on filters for Nikon cameras? I just bought the ED80 and am trying to figure out how/where to use filters. The scope came with a 2″ broadband filter (that screws into the diagonal) and a 1.25″ Narrow band filter (screws into the 1.25″ eyepieces) .. .but if I were to add a camera instead of an eye piece, where would the filter go?

    1. Hey Mike,

      Unfortunately, there are no clip-in filters for Nikon DSLRs, or I would be using them myself! So best is to screw the filter into the threaded nosepiece (T-adapter) that connects your camera to the focuser. Usually it will be 2″ wide (48mm) but with a small sensor you can get away with a 1.25″ nosepiece and filter, though you may end up with some vignetting.

      Best,

      Martin

  3. I am a little confused about your processing workflow. By digitally removing noise from each sub aren’t you sacrificing a lot of useful photons that could be far more efficiently managed through dark frame subtraction during stacking? Dark frame subtraction relies on the average noise within the light frames and dark frames being the same so that they cancel each other out, if the light frame noise has been digitally altered first then dark frame subtraction risks adding negative noise back into the image. I was certainly under the impression that when stacking and using calibration frames that the best results come when all calibration and stacking is done using untouched raw image files. Then final digital manipulation of levels and noise can happen on the stacked output image. Having said that the final images using your technique are quite beautiful so I’m not sure whether your way is better or at least no worse.

    1. Hi Rob,

      Your question is very important and it is one that I asked myself as well. As a matter of fact, at some point an experienced astro-photographer told me I was doing it wrong! So I started experimenting in Deep Sky Stacker, importing the original unaltered RAW files. And the results were not as good. So I went back to removing noise in my subs before stacking and importing the altered TIFF files. Perhaps with a lot of subs (thirty or more) the outcome would be different, not sure about that. But anywhere between three and twenty, it looks like my method bears fruit. One very important detail: MAKE SURE THAT WHATEVER YOU DO TO THE RAW LIGHT FRAMES, YOU ALSO DO TO THE DARK FRAMES, THE FLAT FRAMES AND THE BIAS FRAMES. They need to be processed exactly the same way otherwise you are absolutely right: it will produce weird effects once DSS starts subtracting.

      Thanks so much for addressing this issue, and please share your own experience – there is never only one way and we can always learn from each other.

      Clear skies,

      Martin

  4. Hi Martin, this article is awesome, I’m just starting out and until I get the scope I want, I’ve been imaging with an ST-80 – eventually my guide scope as you mention on my CGEM DX. I was disappointed with my first light shots but now I see the difference that you get here too. Why is the ST-80 so bad? Is it the bad focuser or lack of a FR/FF? It is almost like a challenge to get it to take better photos!

    OH, also – before I forget, I was interested in how you reduce noise before you go into DSS. Is that what most APers do? I actually saw that and it seems more intuitive.

    clear skies
    Marty

    1. Hi Marty,

      There is just no way you can expect the same quality from a $90 ST-80 as from a $300 80mm ED refactor. I love those little short tubes because they are light and you can take them anywhere, but for imaging you need the best lens you can get. An APO-Triplet is excellent but a well corrected ED lens will do the job just fine. It is all about how precisely corrected the lens is – also, the eye is more forgiving than the sensor of your camera.
      As far as reducing noise before going into DSS: yes, that’s how I do it and some AP-ers disagree! But I tried both ways and it give me the better results. See also my answer to Rob S. who addressed the same issue.

      Best,

      Martin

  5. Can you comment or do an article on how you achieve focus? Are you using BY-EOS and FWHM in live view and/or a bahtinov1 mask?
    thanks
    Marty again

    1. Hi Marty,

      I do have a Bahtinov mask and I actually rarely use it. I always do my focus using live view (10x for best detail), but if you don’t have live view I believe a Bahtinov mask is a great way to focus. I recommend checking out some of Ray Shore tutorials, he uses the Bahtinov mask a lot.

      🙂

  6. I regret reading this well-written article. I have been content being a visual-only observer with a 120mm APO. The simplicity, minimal gear, and less time needed per viewing session are what keep me away from imaging. I searched for good 80mm APO on Google and stumbled upon your site. Your pictures are glorious, but even better are your explanations showing exactly what to do. Imaging doesn’t appear TOO bad. My wallet is going to hate me.

  7. Marty,

    I have been reading many web articles as my son and I are starting to gain interest in jumping into the astrophotography hobby. I really enjoyed the article and found your approach to be more clear and practical than most others. So, you seem like the right person to ask this question – Will using an unmodifide DSLR dissapoint us when we are just starting out?

    1. Hi Joe,

      An unmodified DSLR will be great for the moon, for star clusters and even galaxies. I was working last night on some new images of the Andromeda Galaxy (M31) and I used an unmodified Nikon D600. The only times that you really need a modified DSLR is when you shoot emission nebulae that are rich in Hydrogen Alpha, such as the Lagoon Nebula, the Eagle Nebula and many others. The deep red wavelengths of H-alpha are blocked by the regular filters in your DSLR and for that reason a modified DSLR (or a CCD astro camera) is highly recommended. Hope that helps!

      Martin

      1. Marty,

        Thank you. That really helps. Is it fair to say that a modified DSLR would no longer be usable for normal terrestrial photos?

        Joe

  8. Hi,
    im trying to use my Canon 5d mark 3 with an Explorer Scientific ED 80, and i have an adapter for the canon. But when i attach the adapter my 5d3 says i have to update the lens firmware? please help

  9. Hi Julius,

    I’m a professional photographer and normally shoot Canon gear. Are you talking about a T-mount for your EF-mount body? There should not be any electronic contacts on a T-mount adapter that could cause your Canon body to interact with the DSLR body. A T-mount for astro use is a simple mechanical only adapter. That said the latest EOS 5D mkIII firmware is 1.3.3 released January 28, 2015.

    HTH

  10. I find it a bit unfair concerning the price of the OTAs. BUT I wonder how the $100 short tube would have performed when you used a flattener for that too. Or just not using a flattener/reducer for the APO or ED? I mean these flatteners/reducers are expensive so that even widened the price gap. But on the other side it is quite strange to add a $300 flattener to a $100 OTA.

  11. Hi Carsten, you can get a decent reducer/flattener for less than $120 new, like the Orion 0.8x reducer. The William Optics v.3 that I used for the above photographs is around $140 on the used market. I imagine that you can improve the photographic image of the Orion short tube with a flattener, but it will never be as good as with an 80mm ED or APO. Again, it was not designed to be an imaging scope! For the money, it is still a wonderful telescope that I recommend to anybody who likes to get started in the hobby.

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