Camera lens as telescope

Yep, you are reading this right. You can use camera lens as a telescope. Normally, astrophotographers do it the other way around – use the telescope instead of camera lens. So why did I bother? Two reasons mainly:

  1. Pure curiosity. I found several posts and pages by people who have done it before and I was curious how well does it work. Whether it is any good for visual observations of planets and deep space objects, etc.
  2. Getting myself and airplane-portable telescope without having to buy one. Since I already have several telephoto lens and I pack into my carry-on bag anyway and since none of my telescopes (save for Orion Short-Tube 80) are carry-on luggage friendly, having a way to attach eyepiece to my lens would allow me to have 2-in-1 camera lens and apochromatic refractor telescope with me during vacations.

Attaching the eyepiece to camera lens is not that hard actually. You need something that would securely attach to your camera lens (most likely a bayonet lens mount) and something that accepts telescope eyepieces. For first part people commonly use macro extension rings. For second – eyepiece holders, barlow pieces, parts from tube extenders, etc.

Macro-rings are normally used in macro-photography to allow normal lens to focus beyond their near focusing limit by moving the lens away from the camera sensor or film. For our purposes all we care about is that these extenders have the bayonet connector that we need. I had a set of Kenko tubes laying around that I no longer needed so I ended up using 12mm tube for the eyepiece attachment project.

A cheap and simple alternative is to use a rear-lens cap. The downside of this solution is less secure connection between the camera and the eyepiece since the cap doesn’t have a spring-locking mechanism and is generally softer and less rigid.

I also had a 1.25″ T-thread adapter that also dubbed as 1.25″ extension tube. Initially it was too long but that wasn’t something a few minutes with metal Dremel disk couldn’t fix. I cut it so that it would securely insert into the Kenko extension tube opening but would only protrude forward no more than 1mm. Some lens have the back element extremely close to the camera connector so anything protruding forward from the EOS EF lens adapter can scratch these and ruin your lens.

I removed all of the electrical connectors from the Kenko tube so that they wouldn’t get in the way and made sure that everything fits together. First I used superglue to get the initial connection between the Kenko macro extension tube and sawed-off T-adapter. Once the pieces were securely connected, I glued them with epoxy for good measure.

When building such an adapter you should try to make it as thin as possible. Camera lenses don’t have much back-focus because DSLR sensor is very close to the lens mount. Make the adapter too thick and you will not be able to focus on infinity. Even with thin adapters using star diagonals would be impossible.

Once the epoxy cured, I used my new EOS EF to 1.25″ adapter to connect Baader Hyperion 8-24mm Clickstop Zoom Mark II eyepiece to all of the EF lenses that I had. All of them worked, except for 50mm f/1.2L lens that wasn’t able to achieve infinity focus. My test involved holding the lens by hand and focusing on Vega. There wasn’t much else I could do given severe light pollution coming from Seattle.

  • Canon 300mm f/2.8L – probably the most worthwhile lens in terms of telescope conversion. Easily focused on Vega on all eyepiece zoom levels. At eyepiece at 8mm, it provides 37.5x magnification and brightness factor of 0.12. Theoretical limiting magnitude for this configuration is 12.8. At 12mm eyepiece FL, the system will produce the same 25x magnification as Zumell Tachyon binoculars but with slightly higher brightness.
  • Canon 300mm f/2.8L with 1.4x tele-extender. No problems focusing. 53x magnification at 8mm EP.
  • Canon 100mm f/2.8L macro. Focused without any problems. 13x magnification. As I used it for terrestrial viewing, there was a very noticeable pincushion distortion.
  • Canon 50mm f/1.2L. Theoretically 6x magnification, but couldn’t focus on infinity.

PS: After typing this post I did come out once more and was able to visually find M31 with 300mm lens and 24mm eyepiece.

PPS: Since M42 T-threads are still very much usable, I can now use EF lens with astrophotography CCD imagers like Orion G3 monochrome imager and autoguider and narrowband filters allowing me to do narrowband wide-field astrophotography.

Satellites astrophoto

As I was setting up for the NGC6960 imaging session I snapped a few quick pictures to evaluate LP filter performance and make sure that my framing was right. As luck would have it I caught a couple of satellites in the 4 second frame.

Satellites Astrophoto

Satellites Astrophoto

The one in the bottom right corner is very likely defunct at the dashed line indicates that it is spinning rapidly (slightly less than one revolution per minute).

I hoped to identify the other one by rewinding the time in StarWalk app to the time when picture was taken but it didn’t show any satellites in that portion of the sky.

300mm astrophotography – Veil & Network Nebulae

This night I was finally able to take all of the 300mm astrophotography lens mods for test. Both mods are centered around Canon 300mm f/2.8L prime lens and were described earlier in the blog: adapter for using astronomic 2″ filters with this lens and a contraption to securely attach the lens to telescope mount.

Due to the size of the lens I had to use a side-by-side adapter to fit the lens and Orion mini-guider on the CGEM-DX mount. Since the whole setup is so light compared to 11″ OTA that the mount is meant to handle, I ended up using one of the smaller weights (originally meant for Losmandy G-11 mount, since Celestron doesn’t sell smaller weights for CGEM).

Since it impossible to precisely align stars though DSLR lens I used bulls-eye overlay in PHD to line up alignment stars and then did a drift alignment with PHD.

Once I had the Veil Nebula in the frame I took several quick ISO6400 8 second shots to make sure that the framing was right and to compare images with and without Astronomik CLS-CCD light pollution filter. Since I live in a red light pollution zone near Seattle the difference was huge. See for yourself…

Astronomik CLS light pollution filter

Astronomik CLS light pollution filter

Upper image was taken without the filter and lower – with the filter. In the upper image the red channel is 4x higher than in the lower. With the filter, even with such a short exposure you can clearly see the nebula. The filter and the adapter are definitely keepers.

Below are final results for that imaging session:

ngc6960 Veil Nebula, Network Nebula

ngc6960 Veil Nebula, Network Nebula

 

Open clusters: M39, NGC7082, NGC7062, NGC7067, NGC7071, NGC7093

Open clusters: M39, NGC7082, NGC7062, NGC7067, NGC7071, NGC7093

 

Overall, I’m quite happy with the results as the nebula images came out with the same quality as ones I took about a year ago with the same lens but from blue LP zone over the mountains – http://blog.astrophotographytargets.com/2011/09/ngc6960-veil-nebula-ngc6992-network-nebula-in-one-shot/

NGC6960 – first proper with 0.7x reducer for 1100 HD

This is the first successful picture I’ve taken with the 0.7x focal reducer for Celestron EdgeHD 1100. Given the red light pollution zone close to Seattle, I’m quite happy how it came out. Since stars are nice and round I guess that tinkering with the caliper to get the sensor exactly 146mm from the back-plate from the reducer paid off. This was also the first time I did PDF drift alignment rather than built-in all-star on the mount. Also quite happy with the tracking. Will post more details later.

 

Celestron Reducer Lens .7x for EdgeHD 1100 – Part 1

After very long wait I finally had a chance to take my new f/7 reducer lens for EdgeHD 1100 for a spin. As we say in Russia, “first pancake never comes out right”. This was the case with this reducer lens.

Celestron claims that 0.7x focus reducer retains the same optimal focusing distance as the telescope without the lens – 146mm. This however doesn’t mean that total amount of backfocus is preserved. I use 1.25″ dielectric mirror diagonal with 16mm illuminated reticle eyepiece to do initial mount alignment. I was always able to achieve focus by attaching them straight to the telescope. With focal reducer in place I couldn’t achieve focus even when  the focuser knob hit the limit. I had to take out the diagonal. In retrospect I should have taken out the reducer and put it back on for astrophotography… oh, well…

Once the mount was properly polar aligned. I replaced the eyepiece with DSLR and started snapping 5 minute pictures of NGC6888 (one of my favorite targets) at ISO800. They looked fine on the camera LCD but as I examined them on the computer I had to scrap the session as all of the stars were crescent-shaped even in the center of the image. I’m pretty sure that the scope is properly collimated as I would have noticed asymmetrical star circles as I was focusing though the EP.

Next day with the help of caliper I determined that neither 2″ T-adapter, nor specialized EdgeHD T-adapter provide proper backplate to sensor distance. With 2″ adapter coming up around 40mm short and Celestron EdgeHD adapter being 20mm too long.

Today I intend to double check scope collimation, and do another session with 2″ adapter + 36mm EOS EF extension ring.

Hopefully, this next “pancake”, will be better.

DSLR Solar Astrophotography

Sun though Coronado PST, Baader Hyperion Zoom and DSLRToday I took some shots of the Sun while it’s not covered by clouds. To really see details on surface and solar prominence, special H-a (Hydrogen Alpha) telescope is needed. Most affordable one is Coronado P.S.T. (Personal solar telescope). Unfortunately it doesn’t allow to take prime focus photographs because of extremely  short focus.

It is possible to modify a Meade barlow to achieve  focus with DSLRs and I have done so. However I wasn’t happy with image quality and ended up using positive projection though Baader Hyperion Zoom Eyepiece. After some experimenting I ended up shooting at 20mm with resolution of about 1.5 arcseconds per pixel which is roughly equivalent to 880mm telephoto lens.

I’d have to say that H-a filter does eat up a lot of sunlight. At ISO400 my shutter speed was mere 1/125s.  I had to take the additional H-alpha etalon off the telescope because despite providing better picture for visual observing it ate up even more light, bringing shutter speed down to 1/25s, making it very hard to focus and degrading image quality overall.

Here’s the photo of my setup.

My solar imaging setup

M3 – Globular Cluster In Canes Venatici

Messier 3 (NGC5272)

Messier 3 (NGC5272)

After several months of futile attempts to eliminate flexure hidden somewhere in my new setup I decided to give up and go with off-axis guider setup instead. I had Orion Deluxe Off-Axis Guider laying around gathering dust for quite a while already and I finally got around to using it as outlined in the previous post. As I was shooting from my front-yard I decided to pick an easier target to test out the new setup. Here’s how it came out. It  definitely can (and will) be improved upon. M3 was discovered by Messier in 1764. It is one of the brightest and largest globulars, containing aroudn 500,000 stars at a about 34,000 light-years away from Earth.

Off-Axis Guider Setup

Setting up off-axis guider for the first time can be tricky. It certainly was so for me. I spent several hours in the dark trying to make it work with no results to show for it and ended up giving up.

The reason it is hard is that guide-camera has to be precisely in focus for it to stand a chance of seeing guidestars. If it is slightly out of focus, what little light was there would be smeared all over the sensor and become completely undetectable. Trying to find focus in the night is a futile endeavor because most guide stars that make it in the guide-cam though average f/10 scope are so faint that 2 second exposures are required to see them. Worse yet, most prism positions will yield frames with no detectable guidestars at all.

So it pays to be prepared and roughly focus your guide-cam during daylight. Setup your telescope on a mount but don’t power it on. The mount should not move. Using clutches point the telescope at something far enough that it can be focused on but not too far. Ideally it should have sharp easily detectable pattern. In my case I used a broom. Attach your camera to the scope and focus it on the object. Since you are doing this in daylight, use shortest exposure your camera supports. I used 0.001s on my Orion G3 Monochrome.

Now you can adjust your off-axis guider to focus the guide camera. Don’t forget to adjust the prism tilt screw if your guider has one.

This will give your a rough focus that would be good enough for the guide-cam to detect brightest stars (ones that you can see with unaided eye) on exposures around 0.5s. Find such bright star with your telescope and focus your imaging camera precisely using a Bachtinov mask. Notice where your guide-cam prism is respective to the picture and move the mount slowly in that direction until you see the star in your guide-camera. It should be bright enough for Bachtinov pattern to be visible. Use it to precisely focus your guide camera.

You are now set. Find your target, setup the guider and shot away.

Several things to keep in mind:

  1. Finding a nice guide star can be tough. Use longer exposures on the guide-cam (1.5s..3s) and rotate the prism slowly.
  2. Calibrate your guiding software each time. Even slight adjustment of the prism will mess up RA/Dec orientation.
  3. Remember nice calm graphs you had when guiding through 162mm FL scope with RMS of less than 0.1? You can forget about them. Graphs will jump all over the place. But worry not – there will be very little blur and definitely zero drift. I used Orion G3 Monochrome Imager as a guide camera and Canon 5D Mark II as imaging camera on 2800mm FL scope. G3 pixel is 1.3X larger than pixel in the Canon. This means that with 2 RMS you will have 2..3 pixel blur on the image. It is quite good. Remember that there’s still atmospheric seeing that will still blur your image somewhat regardless of the quality of your guiding.

 

Canon 300mm f/2.8L astrophotograhy accessories

In the past I did some astrophotography with Canon 300mm lens (NGC6960, NGC6888, Andromeda Galaxy). There were some minor problems though:

  1. Canon 300mm F/2.8L is a huge lens and with tripod shoe being the only point of support point it easily shifts around and can mess up the picture.
  2. It is impossible to use any kind of standard filters with this lens as it is. So even something as simple as light pollution filter is not an option.

I’ve tried to address both of these problems and it seems like I’ve succeeded.

1) After some tinkering in Google SketchUp I came up with a model of a part that would complement ADM Accessories vixen camera mount and secure the 140mm lens shade to the vixen dovetail. The model is freely available on shapeways.com. Perhaps it is a bit thicker than necessary, but better safe than sorry.

canon-300mm-f28l-lens-holder-closeup

canon-300mm-f28l-lens-holder-side

2) I got my hands on 52mm drop-in filter holder for Canon 300mm lens and modded it with a adapter ring so that it will take 2″ astronomical filters (corresponds to 48mm)

Now I just need a clear dark night to test this out. If the filter experiment works well, the grand plan is to use H-alpha, OIII and SII filters and then combine the images to achive effect similar to Hubble Space Telescope.

You can download the original 3D model here.

Canon 5D Mark II 12V power adapter

Standard Canon 5D Mark II Li-Ion battery pack (LP-E6) has 1800mAh capacity at 7.2V. This translates into roughly 13Wh of stored power. When shooting 10 minute exposures, the camera draws 3W of power on average. When shorter exposures are used, power usage goes up since more power is required every time image is read from the sesor, processed and written to the memory card. This translates into roughtly 4 hours of shooting before the battery goes out.

This may seem like a lot. However if you take a 90 minute trip (one way and not counding packing/unpacking) to a dark location, you may want to shoot for the whole night. Having a spare battery may of may not be enough. There is a nice Hanken ACK-E6 Replacement AC Power Adapter  that costs under $20 that can be used to power your Canon 5D Mark II DSLR. The down side is that it runs off 110V AC. Initially I ran it off inverter plugged into my main 60Ah deep-cycle battery. However inverter draws around 4Wh by itself. Small things add up and when you add heaters to the equation, even 66Ah battery may not last until dawn.

With no 12V power solution seemingly available from Canon or other companies, I’ve decided to put one together myself. I used the battery connector pack from the Hanken adapter, 12V car power plug and 25W adjusable switching regulator to put in between then. 25W should be enough to account even for peak power usage when camera processes and stores taken image.

While switching power adapters are effecient, the downside is that they produce slight high-frequency noise in the output power. I was afraid that this noise may adversely affect image quality so I decided to run some tests. I’ve put the camera into dark room, covered the lens and set it to shoot 6 10 minute darks with 1 minute delay in between to compare noise level when running on battery and on 12V switching power supply. I then used ImageMagick to compute image histogram and analyze frequency components of the image to see if PSU introduced some regular noise patterns.

Initially I tuned the regulator to output 8V just like the Hanken. However after I ran the dark tests it turned out that the camera produced measurably more noise when running on PSU versus the battery. There weren’t any regular pattern to the extra noise. There was just more of it.

I tried to lower the output voltate to 7.2V to match the battery, but the camera wouldn’t power on at all. I kept gradually increasing the voltage until the camera came to life at 7.4V. I did another set of measurements and found that noise amount and patterns didn’t have any detectable differences between PSU and battery shots.

So I can conclude that using switching regulator doesn’t introduce detectable amount of noise into long exposure photography with Canon 5D Mark II. In the initial test excess voltage from the PSU caused additional heating of the sensor that resulted in more noise. It should be noted that even in that case the amount was minor: 1.3% average frame luminance versus 1.14% in battery and 7.4 volt PSU shots. In all likelyhood it will not matter for daytime photography and have only minor impact on long-exposure astrophotography.

UPDATE: Since Canon 5D Mark II and Canon 5D Mark III share the same battery, the same adapter works in Canon 5D Mark III DSLR without any trouble.