Best of Film Astrophotography - START HERE!
Posted 23 March 2007 - 04:58 AM
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Posted 23 March 2007 - 07:03 AM
1) your camera
2) a way to attach it to your 'scope
3) a way to focus it
5) a way to guide your telescope (for longer exposures)
6) a way to polar align your telescope
1) The camera
If you've got an SLR, you can take astrophotos with it. Period.
If you've got a point-and-shoot, or a rangefinder, it will be much more difficult to get going. Go get a used SLR!
It MUST have a "B" setting (I think all do). That's it. Mirror lock-up is nice, mechanical "B" (otherwise you'll be going through batteries like M&Ms) is nice, replaceable focus screen is nice, all that stuff is nice, but all you really need is a camera to hold the film and a long term shutter. So, if you have ANY SLR, you're all set. If you don't, the following is a great discussion of which ones to look for from Jerry Lodriguss' website:
2) Attaching it to your scope
You need 2 things, a "T-ring" specific to your camera, and a nosepiece that screws into the "T-ring" and allows you to attach the camera to your eyepiece drawtube.
Get T-rings here:
Drawtubes come in 1.25" and 2" flavors - if you have a 2", get a 2" nosepiece to reduce vignetting, otherwise, well, you have to use a 1.25" nosepiece. Alpine Astro has very nice nosepieces from Baader in Germany (they call them "T2" system components, but they're just regular "T" threads):
If you have a refractor or SCT/MCT, then you'll be probably able to bring your scope to focus right away. If you have a Newt, you might not have enough in-focus to get to focus. You have four choices, easy to hard:
- Use a 2x barlow to get the focal plane "out" closer to the end of the drawtube - the downside is that you've increased magnification through your telescope
- Replace the focuser with a "low profile" focuser intended to allow you to get your camera closer to the focal point
- Move the main mirror forward (usually an inch will do it) in the OTA get the focal point "out"
- Buy a new scope!!!
3) Focusing your camera
You might think that you can just look through the screen and focus your camera, but there are a couple of things working against you.
- The typical "microprism" focusing area goes black at the slow focal ratios typical of telescopes (after all, your telescope is acting like a REALLY long telephoto lens)
- The rest of the focusing screen will probably be very dim
- Your eye "accomodates" meaning it's pretty flexible at making something that's slightly out of focus look IN focus
For initial shots of the moon, etc., you can just eyeball it and you'll probably get close, but if you get serious and want REPEATABLE focus, you need an auxilliary method that bypasses your screen and eliminates the effects of eye accomodation.
I use a Ronchi focuser for my Nikon cameras:
Again, Jerry has a great write-up about focusing:
Kodak Elite Chrome 200 slide film or Fuji Provia 400F slide film. Your lab can't screw up astro slides (other than cutting in the wrong place -- make sure you take a couple of regular daytime shots at the beginning and end of the roll) and since the image is positive, you can visually evaluate your slides without scanners, etc.
You might think faster (ISO800 and up) is better but a lot of faster films have poor reciprocity failure characteristics, so that the Kodak and Fuji end up being effectively as fast because they don’t lose sensitivity over long exposures as much as other films.
5) A way to guide your scope
With the previous 4 steps you’re all set to take pictures of the moon. You don’t have to polar align, or even be equatorially mounted, since the exposures will be short (fractions of a second). You WILL have to deal with “mirror slap” vibrating your telescope. If you have mirror lock-up, use it. Use a cable shutter release. If you still have vibration problems, try the “hat trick”:
For longer (10s of seconds and more) exposures, you need a way to guide your scope.
Since you’re just starting out, an auxilliary guide scope with reticle eyepiece is the easiest way to go. Any scope that can be rigidly mounted next to your imaging scope, of at least half the focal length of your main scope, can be used. Orion sells a relatively inexpensive guide scope setup:
Astro-Physics also makes a more expensive guidescope:
I like the following reticle eyepiece:
There's an excellent discussion of HOW to manually guide (and the tolerances required) in Robert Reeves excellent book "Wide-Field Astrophotography"
6) Polar drift alignment
If you want to take long exposures, you need to be accurately polar aligned to avoid field rotation. Drift alignment is the most accurate way to align to the pole. Here’s my favorite webpage on how to do it:
If you have an STV (then you aren't a beginner!), you can use it to polar align:
Posted 23 March 2007 - 07:31 AM
You need a film with several properties.
1. Small grain. This makes your images sharp, and allows nice enlargements.
2. Excellent RED response. While there are a lot of blue and some green objects in the sky, RED is the one that is most common, and also easily lost with a poor choice of film. This is because the RED spectrum is near the end of the film's sensitivity and film makers often chop it off as unnecessary. They do not care that most of your subjects glow in the deep reds of Hydrogen Alpha. Some films do not even record this all-important spectrum at all! So you need a film that has good blue and green, but also very good response with REDS.
3. Limited Reciprocity Failure. When you shoot a normal object in daylight, if you decrease the amount of light coming in to the camera you can increase the exposure time to compensate. This is reciprocity. However, at long exposures, film tends to FAIL in this regard. This means that if a subject is very dim, a MUCH longer exposure is required to capture it. It also means that what you see in a 10 minute exposure may not be much different than in a 20! On the other hand, if you use a film with LOW reciprocity failure, you will be able to expose longer, AND capture more.
So which films are best?
First I recommend slide film. This requires the least amount of control by an unknowledgeable developer. But more importantly, there just are not many excellent color print films out there. Most have very poor blue and red response in the important wavelengths needed for astrophotography. There are a few color print films that will work, but they are very hard to locate, and require you to HYPERSENSITIZE them by baking them in forming gas (an advanced technique to reduce the reciprocity failure).
- Kodak Elite Chrome 200 (E200). This film has amazing red response, way down to the Hydrogen Alpha line and produces deep cherry red nebula. It's also very fine grain, and has very low RF. It does suffer slightly in blue response.
- Fujichrome 400F Provia (or Fujichrome Sensia II 400). These have very good color response in blue and red! It also has fine grain, not as fine as E200, but very good. The film also has low RF and can capture below magnitude 18 in 25 minutes at f/4. This is my all-time favorite film.
The best way to stay on top of the changing world of astrophotography films, subscribe to the Astrophotography Mailing List Archives.
Clear Sky to yah!
Posted 23 March 2007 - 07:33 AM
DESIRABLE FILM CAMERA QUALITIES
TIME EXPPOSURES: This is probably the most important feature of all. The camera must have some way to hold the shutter open for as long as you desire, whether that is 1 second or 1 hour! Many cameras have either a "T" setting for "time exposure" or a "B" setting for "bulb setting", a hold-over from the use of old flash bulbs. Either of these settings will keep the shutter open for as long as you hold the shutter button down.
LOW POWER: Most cameras use electric power (battery) to operate and/or hold the shutter open. Keeping the shutter open for a for long time can drain a battery in those cameras, making them unsuitable for astrophotography. While some of these cameras can be adapted with an external battery pack or AC power, a better solution would be to have a camera that does not require any power to operate the shutter.
MOUNTING: The ability to securely fasten the camera to a mount is very important. During a time exposure the camera must remain in perfect position for sometimes over an hour. The slightest breeze or even telescope movement can shift the camera and ruin your photos. Many cameras use a 1/4" threaded hole so you can screw the camera onto a tripod or mount. This makes securing the camera for easy!
CABLE RELEASE: A cable release allows you to operate the shutter from a distance. This can be several centimeters to a meter or more. This also allows you to operate the shutter without touching the camera and causing vibrations. For time exposures, many cameras require you to hold the shutter down manually. The cable release should have a locking mechanism to let you push the shutter down, and then lock it in place during the exposure.
FOCUSING SCREENS: The view the a camera is critical to making sure you have framed your subject well, but many objects in astronomy are extremely faint. Most cameras that use a ground glass focusing screen absorb so much light that it can render the view useless! Focusing is extremely difficult if not impossible with the standard focusing screen. The ability to change that screen to one that is more suitable is a big plus for astrophotographers.
LENSES: While you can certainly attach your camera to your telescope without a camera lens, there are wonderful shots of the sky that REQUIRE a regular camera lens due to the huge field of view needed. Examples include photographs of constellations, star trails, comets, meteors, and many large nebula such as the North American Nebula do very well with a regular telephoto lens. The ability to select the best focal length for your subject is very important.
VIEWFINDER: The standard viewfinder in most cameras is not ideal for astrophotography. The image is too dim, and difficult to focus properly. The ability to remove the viewfinder so you can attach a magnifier is a big plus! However, even if you can't remove the viewfinder, some cameras allow the attachment of a magnifier to the standard viewfinder and this helps a great deal too. So while having a removable viewfinder is a benefit, it's not the most important thing to worry about.
MIRROR LOCK: When using a Single Lens Reflex (SLR) camera, the mirror must flip up quickly to allow incoming light to strike the film instead of passing up through the viewfinder. This causes some shaking, and in cases where you are using a short exposure (<1 minute) you will most likely have some blurring in your photograph. The ability to lock the mirror up BEFORE the exposure is a wonderful additional feature to look for.
WEIGHT: Astrophotography already demands a lot from a normal telescope mount. Adding your camera is just one more additional weight to make things more difficult! When all else is equal, the lighter camera is always the best. Remember, you will have to counter the weight of your camera with additional weights on your mount, and this can make a huge difference in the ability of your mount to take a good photograph. Try to get the lightest camera you can.
The one type of camera that satisfies all of these demands is the Single Lens Reflex (SLR) camera. While not every SLR has ALL of the above features, many do, and with the advent of digital cameras it is very easy to find used film cameras very inexpensively in used camera shops and on-line auctions. This is one big advantage of the digital revolution... we get less expensive cameras and lenses!
Basic 35mm SLRS
The common 35mm Single Lens Reflex (SLR) camera is an ideal camera for astrophotography, but some brands and models stand out above the rest. Here are a few cameras that make ideal astrophotography cameras. Note that there are many others!
Nikon F2 and F3
Olympus OM-1 and OM-2
(email me to add others)
I use the Olympus OM-1
The OM-1 is the workhorse of film astrophotography. The camera was practically designed from the ground up with microphotography and astrophotography in mind. It is extremely lightweight, uses no electric power for the shutter, has mirror lock, replaceable focusing screens, and a prism magnifier is available! It's an outstanding camera.
The OM-1 has many lenses available for it, from very wide angle lenses to very long telephotos. And best of all, you can find these cameras on-line from $50 to $100 US.
MEDIUM FORMAT CAMERAS
Medium format cameras are becoming more popular among astrophotographers because good deals can be found on-line and in used camera stores. These cameras can take in a huge swath of sky, and are ideal for capturing large nebula. You often do not shoot at prime focus, but instead use a good quality lens and shoot piggyback (attach the camera on top of your telescope for the ride, while using the scope as a guidescope).
As far as quality.. I think the work by veteran astrophotographer Wang Ho Wa should put an end to any doubt about medium format camera capabilities. He uses a Pentax 67 for some of the most stunning wide-field work I have ever seen.
Wei-Hao Wang Gallery
Very few cameras have the model longevity of the Pentax 67. First introduced in 1969, the camera was little changed until 1998 when it was updated to the 67II. It's very heavy, so a strong mount is required.
Posted 23 March 2007 - 07:38 AM
First the basics: The difference between TRACKING and GUIDING.
TRACKING is what your scope's mount does to compensate for Earths rotation. It's what keeps your objects in view. Most scopes track. It's accurate for visual use, but not photographically. The scope may not be aligned well, gears are not perfect, the sky refracts light, wind pushes your scope, etc... There is always a way that ERROR gets into your tracking and causes it to drift from where it should be. For visual use, you may never notice it.
GUIDING is the process of making SURE your scope is tracking well. Somehow you have to MONITOR what your mount is doing and CORRECT problems before they get to the point that the error is visible on your photos.
Guiding is done either with your eyes (manual or hand guiding) or a small CCD camera and computer (autoguiding). I hand guide exclusively, but most people these days prefer to Autoguide.
While the new rave in astrophotography is to use an autoguider, and let a computer make small corrections in your telescope's tracking, I am old-fashioned and still make the corrections myself.
These days, you use the mount's hand controller to make guiding corrections. You really need to set your scope up with as perfect polar alignment as possible, this way you should only have to make Right Ascension (RA) corrections and no (or very few) Declination (DEC) corrections.
For me, I choose to hand guide for two reasons. One is so I can monitor the exposure and sky conditions, and fix other problems that may occur. Since I am out there, if a plane flies near, I can block the scope to prevent a trail etc.. But the MAIN reason I hand guide is to be 100% responsible for TAKING the photo. I feel distant to the craft if I let a computer guide the shot.
This posting will deal with HAND GUIDING only, as it is the most demanding and is what I am most familiar with. I'll let someone else describe Autoguiding.
You can GUIDE your scope one of two ways..
OFF-AXIS: You attach a small device called an off-axis guider between your camera and your scope. It has a tiny prism to grab a small piece of light that would never reach your camera anyway. You look through it with an eyepiece and focus on a star NEAR the object you are photographing. This way you can watch that star and detect any error in time. Here is a photo of Meade's Off-Axis guider:
GUIDESCOPE: You attach a separate telescope to your main scope and use one for the camera and one for guiding. This is called a GUIDESCOPE and it allows you to find many more stars to guide on, as well as giving you brighter stars. The downside is that IF this guidescope and your camera SLIP in respect to one another you may think there is error when in fact there isn't. This is called FLEXURE. With care, you can reduce the chance of this. I, and most astrophotographers prefer the GUIIDESCOPE method over the off-axis guider. Here is a photo of my scope with my Orion guidescope attached:
In both cases you need to see the star you are guiding on (GUIDESTAR) and detect any movement. We use an ILLUMINATED RETICLE EYEPIECE to do this. This is an eyepiece that has dual crosshairs which make a small BOX in the center of the view. This is called a GUIDEBOX. The lines are illuminated with faint light from a battery so you can see them against the dark sky. You then put a star in the box and watch for movement. If the scope is TRACKING well, you will see no movement. If there is error, you will note the star moving within the box. This is GUIDING ERROR and you must correct it before the error is large enough to show up on film. The amount of error you can let slide is called GUDING TOLERANCE and that amount depends on the main scope's focal length, the guidescope's focal length, and the eyepiece focal length. I will describe the way to figure this all out later in this lesson.
The #1 reason for error when GUIDING is inaccurate Polar alignment of the scope. This alignment must be very precise, and is usually done with the DRIFT METHOD. If you detect any error in DECLINATION then the alignment is not good, and should be fixed. Any error in RIGHT ASSENSION can be CORRECTED with your telescope's hand controller by speeding up the scope a tiny amount, or stopping the scope's tracking depending on if you need to correct forward or back.
To guide by hand requires three things..
1. A way to watch a guidestar. The guidestar is any star near your target that you can see easily and monitor. I use a separate GUIDESCOPE, while others use a radial OFF-AXIS GUIDER. The guidescope allows a larger selection of stars, and allows you to guide on fainter stars.
2. A way to MONITOR the guidestar. We all use an illuminated reticle eyepiece. This is eyepiece (I use a 5mm) that has duel cross hairs that are illuminated against the dark background of space by a red LED. You can center your guidestar inside the tiny "box" made by the intersection of the duel cross-hairs and quickly detect if the star "MOVES" from it's stable position. As you see the movement you can correct it, to prevent any noticeable effect on your photo.
The QUESTION is how much movement is too much? I'll address that in detail below.
3. A way to make corrections to the tracking of your scope. Most everyone uses a hand controller - the one that controls most modern scopes. If your mount allows, and you have the system, you can set your hand controller to make very tiny corrections, so instead of the right button slewing your scope forward in RA, it simple doubles the very slow RA speed so you get a very small correction. The LEFT button will simply STOP the RA motor and allow the Earth to rotate under the stars, thus giving you the ability to effectively back up.
So while guiding, just how much error is too much?
The goal of guiding is to quickly spot any errors in your mount's tracking and correct it before it shows up on your photograph.
First you need to understand that you can only CORRECT errors in RA. Errors in DEC will accumulate over time - no matter how well you correct them. DEC errors are POLAR ALIGNMENT ERRORS and the only way to fix them is to readjust your mount and start again. Use the DRIFT method to gain as much accuracy you can. Ideally, you want to shoot your entire exposure without a single DEC correction - although in practice you can often get by with 1 DEC correction every 10 minutes or so. We'll get back to this later.
RA corrections can be corrected as often as you need to.. in fact, you can consider your mount's drive as continuously correcting for RA - so adding a few of your own won't make any difference.
The KEY is to:
-- 1. Detect the error BEFORE it can show up on film (or CCd chip) and blur your image (star trails)
-- 2. Correct it
-- 3. And NOT do this any more than necessary so you don't go blind or destroy your neck and back.
SIDE NOTE: Non-astrophotographers often ask me how difficult astrophotography is. I point to a sink tap and say "Go over and twist and bend until you can look UP into the tap. Now stay there for an hour without touching the tap or the sink!" God I love this hobby!
OK, so now you have perfect polar alignment.
You have a camera piggybacked or attached to the main scope.
You have your guidescope or off-axis guider centered on a bright star near your target.
Everything is LOCKED down, camera focused and ready.
As you look in the guidescope you will see the guiding box. This box is only a guide. I never use it; instead I place my guidestar on the intersection of two cross-hairs and keep it there.
How much can the star drift in the guidescope before the error shows up?
A nice formula by noted astrophotographer Michael Covington is:
Guiding Tolerance= 2 arc tan * 1/40 F
- Where F is the CAMERA focal length in millimeters. Also, this assumes that an acceptable error on film is 1/40 mm. CAMERA can be a telephoto lens, prime focus or eyepiece projection. Note that in eyepiece projection you must know the new effective focal length of the camera system.
I have included a table of focal lengths versus Guiding Tolerances (in arc sec), and how many seconds in TIME that tolerance looks like with an undriven telescope at the equator.
Lens----- Tolerance(arc sec)---- Drift Time (seconds)
18 mm---- 290"------------------ 20
28 mm---- 185"------------------ 12
50 mm---- 105"------------------- 7
135 mm---- 40"----------------- 2.5
200 mm---- 25"----------------- 1.7
800 mm----- 6.5"---------------- .4
2500 mm-----2.1"---------------- 1/10th of a second!
In the above example, with a 50 mm lens, you could let your guidestar be 105 arc seconds off, and still have good star images on your final print. Looking at the table, this is how far a star would drift in SEVEN seconds of real time if the telescope drive were turned off.
If shooting through a 2500 mm scope--typical 10 inch SCT, it could only be 2.1 arc seconds off--less than 1/10 of a second of untracked drift!!
Now all you have to do is figure out what 2.1 arc seconds (for example) looks like in your guidescope.
Find your typical setup. Let's say you use a LXD75 8" SN.
This scope about 800mm F/L (812mm to be exact).
- Now you do the calculation and find that you can be off by no more than 6.5 seconds of arc.
- 6.5 seconds of arc is .4 seconds of real TIME.
- Look in your guidescope and watch for about a 1/2 second with the drive OFF. That tiny amount of movement is slightly MORE than you should tolerate (because the actual calculation is .4 not .5 seconds).
- Note what that error looks like in your guiding setup. Is it 1/2 a guide-box? 1/4? 1/8? If you find it's 1/5 or less, you will have a hard time catching the error before you could do anything about it! So in this case you would want to INCREASE the magnification of the guidescope (Barlow or smaller F/L guiding eyepiece) or get a longer F/L guidescope.
Note: The guiding error will usually not show up opn your final image, if you catch it fast enough and correct the error immediately. If you are shooting something very faint, you may get away with lots of small errors that you quickly catch and put back on track. If you have bright stars in the field, that may not be the case though!
Keep on guiding, making small corrections as necessary to keep the guidestar within your tolerance. If you start with the star in the lower left of the box, keep it there. Do not decide it will be easier now to switch and move the star to the upper right side or whatever in the middle of your exposure! Wherever the guidestar is at the beginning of the exposure, that’s where it shall remain!
Note your error in DEC. After 5 to 10 minutes you may see if your mount is not perfectly aligned. As you correct the DEC error (see I told you I would get back to this!) you should make a mental note of how much error you have ACCUMULATED.
For example. You see ¼ of guidebox worth of error. You correct it. Then 5 minutes later you make another ¼ guidebox correction. That’s now ½ a guidebox worth of FIELD ROTATION you have just allowed.
This means your entire photo will show signs of rotated stars around the guidestar in the photo – to the amount of whatever ½ a guidebox worth of drift is. If you can tolerate that much, keep going. If not, stop the exposure, readjust the polar alignment, and begin again.
Understand that after some experience this will be easy. Since you now know exactly how much drift you are getting in DEC, you will be able to quickly make the proper polar alignment correction.
Let's say you note an error after 20 minutes of guiding. That should require only the tiniest amount of polar alignment adjustment vs. if you saw the error after only the first 3 minutes!
--- NOTES on guiding magnification ---
The more magnification you guide at, the larger tracking errors appear in your guiding reticle. This boils down to comfort vs tolerance. Higher magnification allows you to allow small errors occur (what we call guiding tolerance), but it's not a good habit to get into. On the other hand, I do not guide as accurately when piggybacking a 135mm lens vs prime focus at 812mm. There's no need.
Some try to find a guiding magnification to let them stay comfortably within the guiding reticle's box, allowing any movement of the guidestar within the box to go without correction. At the 11X ratio with my setup, I try to keep the guidestar behind a line the whole time.. never letting it peek out for more than a split second. I probably could go 1/4 of a box without the error showing up, but I like to err on the side of over-correcting vs under.
Adding more magnification also means less comfort at the eyepiece. You end up with a dimmer star, and of course a narrower field of view - which makes finding a guidestar that much more difficult. Also, eye relief suffers. Like I said.. it boils down to comfort and tolerance.
Seeing plays an important part too. You can only guide as well as seeing will allow. When the star is jumping all over the place, give it up and try later in the evening or another night.
The argument to support higher magnification is that the better your guiding, the finer the resolution of your image will be. Small tracking errors destroy resolution your tiny stars blur and very fine detail is lost.
I like to think of my guidescope as a MICROSCOPE. I am peering down into a microscope at my film, watching the tiny white spec of light burning an image into a single grain of film. The more the star is allowed to move, the more blur I am allowing.
The 11X ratio works for me. Each astrophotographer has to find the ratio that works best for him/her.
---- GUIESCOPES VS OFF-AXIS GUIDERS ----
For beginning imagers, which means those using WAY less than 2000mm of focal length, use a guidescope. It's a LOT easier. The rest of this is WHY.
The caveat is that if you're using an SCT without a mirror-lock, the shifting of the mirror can force you into using an OAG. And if you're starting film imaging with an SCT you're asking for a lot of frustration so I don't recommend it. Why? Vignetting, mirror flop, focus flop, long focal length, small FOV, slow (f10) optics, etc. etc. etc.
What are the fundamental decision criteria?
It comes down to two things, ease-of-use versus freedom from differential flexure.
Below a certain focal length, say around 2000mm to 3000mm, the decision can be based on ease of use, which leads to guidescopes, above that range you're forced into off-axis guiders because OAGs essentially eliminate differential flexure.
What's differential flexure?
Differential flexure is where your guidescope and imaging scope physically move/flex with respect to each other. The effect can be severe where there's so much jiggling that the corrections being made for tracking errors are wrong (because the guidescope is moving w.r.t. the imaging scope. Or worse, it can be extremely subtle, showing up as a very slow differential movement caused by the two scopes slowly changing alignment as the scopes track across the sky. In this case you can have perfect guiding, but still get trailed stars. This latter case is particularly important for film imaging where typically you take very long (30 min to multiple hour) exposures.
Differential flexure can happen ALL OVER the place. In the mounting of the guidescope, in the guidescope rings themselves, in the focuser itself. Above 3000mm it can take heroic efforts to debugt and cure differential flexure.
By contrast, an OAG is a physically very solid unit, with the off-axis pick-off mirror mounted very close, and solidly to the imaging camera. Even if the OAG moves, the guide-port (where you look through or stick in your autoguider) and camera port will move together, so no more differential flexure. But there are some real downsides, which I'll get to.
Advantages of using guidescopes below around 2000/3000 mm
The major advantage of using a guidescope is ease of use:
- you can guide directly on your imaging subject (with an OAG by definition you have to be guiding off of something near but slightly away from your subject)
- the image is bright (OAGs are dim because the pick-off mirror is typically very small)
- guide stars are ROUND (With an OAG, you're picking up rays off-axis. With some telescope designs (SCTS, Mewlons, Mak Cass) the off-axis rays are highly comatic. Which means your guidescope can be a nasty little seagull which makes guiding or autoguiding a judgement call - "where did I decide the center is???"
Disadvantages of using a guidescope below 2000/3000mm
It's added weight. So if you're getting close to the imaging load limit of your mount, you may need to go to a heavier mount or image very carefully under light wind conditions.
How to mount a guidescope - side by side or over/under
You can mount a guidescope side by side: http://22.214.171.124/...pment/M250.html
or over/under: http://www.astropix....I0705/I0705.HTM
Side-by-side is typically heavy because of the need to have a very solid mounting plate. But over/under is probably as much load on the mount because the guidescope is farther away from the rotational axis of the mount. In terms of mount loading, side-by-side vs over/under is probably a wash.
There's a very subtle disadvantage to side-by-side. Since the two scope are offset laterally from the DEC axis, the scopes will actually track very slightly different arcs through the sky as the guide scope makes corrections in DEC. BUT, you say, my polar alignment is perfect, so I'll have no DEC corrections. Nooo.... Tracking virtually ALWAYS needs slight and continuous DEC corrections as the star traverses the sky because of refraction in the atmosphere. As the star gets lower in the sky, it "bends" off of the theoretically perfect arc due to atmospheric refraction. So you have to make DEC corrections. But the axes of the two scopes are offset from the DEC axis so you'll get a very slightly different arc for the imaging scope. And for very long exposures that will show up as trailing that can look like differential flexure, but is actually due to atmospheric refraction.
The effect gets more pronounced as the spacing of the scopes gets larger, and as the focal length of the imaging scope gets larger.
Over/under completely eliminates this effect, and the effect is virtually invisible for short exposures (< 30 minutes) and short focal lengths (< 1500mm).
So which? If you do a lot of camera with short lens work, side-by-side is pretty flexible because of the ability to mount all sorts of weird things (like large format cameras!), especially with a flexible system like the Losmandy DSBS.
If you're doing prime focal imaging with refractors < 2000mm, over/under is extremely convenient.
How to mount a guidescope - rings
No matter whether side-by-side or over/under, you need guidescope rings with alignment pins that allow you to align the guidescope with the imaging scope. More grief can come in here.
You want the pins to make a solid connection to the guidescope so the guidescope can't move around. You don't want pins with squishy soft plastic tips (meant to keep from scratching up your guidescope) because they'll let the guidescope move around. The very pretty Megrez80 guidescope rings are an example of a poor design.
A better solution is the Losmandy guiderings with hard derlin plastic tips. You can really crank down (enough that I've slightly dimpled my Megrez80) on the tips without them deforming. Some people have reported problems with these tips but I've never experienced any.
The best, ideal, solution, is the guidescope offered by Astro-Physics. The tips are METAL, and the guidescope has strong metal reinforcing rings on the OTA that the tips fit into, so that you can REALLY crank down and yet not deform the guidescope.
Finally, make sure your guidescope has a really solid focuser that doesn't have any play, and can lock. After all this work you don't want the focuser introducing flexure.
Into the wild blue - beyond 3000mm
Above 3000mm things can get pretty hairy. Everything flexes/moves. The guidescope mount. The guidescope focuser. The imaging scope focuser. The imaging scope OTA itself. The mirror for SCTs or Newts. It can be virtually impossible to get rid of differential flexure (I gave up once I got to 2400mm). The way out is the OAG, but it's a challenge.
Challenges of using an OAG
- Stars are dim. The off-axis pick-off mirror is small, which means that stars are very dim, limiting your choice of guidestars
- Finding a guidestar can be hard. You typically can't see the guidestar in the imaging scope. You may have to rotate the OAG body to find a guidestar, compromising image composition.
- The guidestar can be very comatic (as discussed above), making deciding where the centroid is tricky.
- Just getting to focus the first time with an OAG is a challenge. First you focus your main scope on a nice bright star. You peer through the off-axis port with your reticle eyepiece. It's dim. It's out of focus. Typically you can't see anything. You have to find a bright star -- gradually move the scope over until the bright star you originally focused the image scope on is kind of visible in the off-axis port. You focus your reticle eyepiece in your off-axis port by fiddling with its placement in the port. Yay! You put in your camera, refocus on the bright star and... oh, you changed focus, which for a Cassegrain of any kind means that the focal point in the off-axis port also changed appreciably compared to the on-axis image because the focal plane is curved, which means your guide-port is out of focus again. ARG!!!!!
I had an extremely nice, extremely expensive OAG that was simply too much work/too frustrating to use. Why don't I sell it in Shop and Swap? I smashed it to smithereens in frustration. (who me, violent? :o)
Some people use OAGs very successfully, but they're clearly manlier men than me!!!
So that's it. A long winded answer with a short simple conclusion - if you can get away with it, use a guidescope!
Suk + Clownfish
Posted 23 March 2007 - 07:56 AM
1. Falling asleep while guiding and hitting your head on the scope, knocking it way off target.
2. Getting so sleepy you actually see a tracking error, but ignore it in your sleepy bliss. Then you come to your senses and realize too late.
3. Guiding perfectly with a guidescope while the main scope dews up.
4. Guiding for an hour with 100% accuracy only to realize you forgot to open the shutter.
5. Breathing on the guiding eyepiece and fogging it up! Quick, waft some cool air to remove the dew before an error occurs!
6. Start guiding a long exposure with your Mountain Dew or other caffein beverage out of reach.
7. Forgetting to pee before starting a long exposure.
8. Having your illuminated eyepiece batteries die off in the middle of an exposure!
9. Forgetting that you switched the guiding speed on the hand-controller to a fast slew - Now you attempt to guide and end up moving the scope halfway across the sky!
10. Setting the "Hemisphere" switch from "N" to "S" and wondering why you're having to correct ALL the time. (Suk)
Posted 29 March 2007 - 09:52 AM
Building a Simple Mount to Track the Sky
In order to capture stunning views of the cosmos without trailing or blurry images you must track the sky to prevent the sky’s apparent motion from ruining your images.
The skies apparent motion doesn’t move in a simple left to right motion. It rotates overhead in a large arc once per day. These stars moved around a fixed point, and your camera must move in that exact same motion in order to prevent the stars from streaking. So let’s look at that motion in detail.
First, this motion is constant. The movement of the sky is one continuous movement around a fixed point. Since we know the Earth rotates once per day, which means one full 360° in 24 hours. This translates to 15° per hour (360/24=15) or 1/4° per minute (360/24/60=.25). This is called SIDEREAL TIME.
Second, the sky moves in a near perfect arc around one single axis.
Third, the arc is precisely centered on a fixed point in the sky, called the CELESTIAL POLE. For those who live above the equator, this point is very close to the North Star (Polaris) which makes it easy to locate. For those who live below the equator it is much more difficult to locate, as there are no bright stars right over the Southern celestial pole, but a good star chart can help you find this point.
A camera platform capable of long exposures of the night sky must track the camera at the sidereal rate, in a perfect arc, and aligned to the celestial pole. That’s it. Sounds hard, but it isn’t!
Introducing the Hinged Astrophotographic Tracker (HAT). This simple device will follow the sky at the correct angle, correct alignment, and at the correct speed to match the sky with such accuracy that you can use it to capture stunning views of the night sky. The basic design of this mount was first designed by George Haig of Glasgow, Scotland in the April 1975 issue of Sky & Telescope. He called it the Scotch Mount, and it was capable of tracking the sky for up to 30 minutes with a 50mm lens without noticeable error. Over the years, the mount has become known as the Barn Door Tracker, since the hing swngs the two boards open like a barn door. The version detailed here is more accurate then the Scotch Mount but very easy and inexpensive to build. Let’s break down the details:
- TRACKING RATE:
In order to move your camera at the correct rate to track the sky, we have to move our camera at the Sidereal Rate. Remember that the sidereal rate is one revolution per 24 hours. This rate can be easy to accomplish! We can use a small gear to push our mount at the correct rate. Ideally, we would want a gear that moves only once per day, but to try and turn a gear accurately one revolution per 24 hours by hand is impossible. It is just too tiny of a movement. So we’ll use a gear that spins much faster, and find a way to accurately maintain a constant rate.
If I asked you to spin a gear exactly 1440 times in one day, you may wonder how you could do that accurately. But if you rotate the gear one turn every minute, that still translates to a rate of 60 revolutions per hour, or 1440 turns per day! Add a simple stop watch and a small mark on the object and anyone can move it accurately one revolution per minute. It’s even easier to turn it one quarter-turn every 15 seconds, which is still one revolution per minute. So we only need to design something that allows us to maintain the average sidereal rate of 1 complete revolution every minute.
To move our tracking platform at the sidereal rate, we will use a threaded steel rod.
This is like a simple bolt, except much longer. In fact, you can buy these in lengths of a meter (3 feet) or more. Also, threaded rod comes in many diameters and different thread pitch. Choose the best quality rod you can, as the accuracy of your tracking will be influenced by any manufacturing defects in the rod. When completed, you will want your mount to move at the sidereal rate, based on the movement of this rod. Be sure you know the exact thread count (pitch) of your rod, so you can make any calculations needed.
So how do you get movement from just a rod? If you place a nut on the stationary rod and turn a nut at a constant rate, the nut moves up or down the rod at a constant rate. It is also true that if you turn the nut while keeping the nut stationary, the rod will move up or down. This is how we will make our mount move!
- TRACKING ARC:
Many of the very faintest objects in the sky, can be captured on film in only 30 minutes of exposure. The brighter objects need even less, and if you use a digital camera you need only a few minutes! But let's assume you want a long ½ hour exposure - which is probably more than you will need for a fast lens.
Remember that Sidereal time is 15° per 1 HOUR so you only need half of that, or 7.5° for 30 minutes. That's quite a short distance! A very nice arc can be accomplished with a simple hinge.
A hinge opens up in a sweeping arc, creating a perfect circle. Imagine the hinge is at the center of an imaginary clock, and so the attached board rotates around the center, which now becomes a giant hour hand on a 24-hour clock! Since we only want 30 minutes of exposure, that's just 7.5°. This is a very small portion of that 24-hour clock. If you use two pieces of wood only 11” long and you open it to the required 7.5°, then the distance they will travel is only about 1.5 inches. That's how short of a distance we are talking about!
- TRACKING ALIGNMENT:
In order for your camera to rotate correctly, you must make sure your hinge is pointed exactly at the Celestial pole. This is a critical step that must be done with a very high degree of accuracy. As the board swivels around the hinge, it will now follow the perfect arc of the sky. Once aligned, all you have to do is move the mount in the same direction that the sky moves. In the Northern hemisphere that is counter clockwise around the celestial pole. In the Southern hemisphere it moves clockwise.
In the northern sky, you can locate the Celestial Pole by finding the bright star Polaris, the brightest star in the constellation Ursa Minor. To locate Polaris you can use two other common constellations to help guide you.
The drawing here shows how a line from Ursa Major ("Big Dipper") and Cassiopeia goes right through Polaris. Depending on your location, and the date and time, you may not see both Ursa Major and Cassiopeia at the same time, but you should be able to find Polaris using just one of those constellations and the chart here. Once you locate Polaris, note where in the sky it is, as it will not move from that spot as long as you do not travel too far away. Once you locate Polaris, you will be very close to the Celestial Pole.
The actual contruction of he HAT is very simple. The first step is to take two pieces of wood and attach a hinge to create a movable platform that can swing open. You mount your camera on one board and secure the other to a solid tripod. The second step is to point the hinge axis very accurately at the Celestial Pole, which enables the camera to sweep in a perfect arc around the pole. The third step is to create a way to move the camera platform at the correct sidereal rate.
The movement of our mount can be accomplished with a simple threaded rod. But there are a lot of things to consider when using a rod to push our hinged boards apart. The main issue is accuracy, because a straight, fixed rod, turning at a constant rate will not allow two hinged boards to swing open at a constant rate! Here's why:
As the rod pushes the top board apart, the contact point where the rod hits the platform will not stay at the same place. As the separation angle of the two boards increase, the rod's contact point will slide up the platform, increasing the distance from the hinge.
This new point of contact means that the platform will open more slowly as the contact point slides further up the board. Once the top board is 90° straight up, the rod will no longer make contact and the platform will fail to move at all! This is called Tangent Error. To correct this tangent error one of three things can be done:
A. Curve the rod so it matches the radius of the contact point relative to the hinge. Since the rod is curved, it will always stay in contact with the same spot.
B. Allow the rod to tilt so that it remains in contact with the same spot on the top platform.
C: Adjust the speed at which the rod moves. If you increase the speed of the rod as the platform swings open, or decrease the speed as the platform swings closed, it will allow the mount to move at a constant rate.
You can also correct Tangent Error by building a rod drive mount that uses two or three moving arms (boards). These are called a double arm or triple arm trackers.
This artcle only describes a simple single arm drive, and so the simplest way to solve this error is with a curved rod as illustrated in diagram “A” above.
• But how much of a curve is needed?
• How long of a rod?
• Where must it be mounted?
All these questions can be solved very easily if we consider that a curved threaded rod becomes a simple gear in our “clock” motor, and the threads can be considered teeth in our “gear”!
THE CLOCK GEAR DRIVE
We will need to know:
• The size of the gear (radius)
• How far from the hinge to place it
Since we already know the thread count of our rod (teeth in our gear) we can make a calculation to solve the above questions!
Consider: The Earth rotates once per 24 hours. Actually we need to be a little more accurate than that, since it rotates once per 23 hours 56 minutes. This equals a total of 1436 minutes. If we want to keep things simple, we want our gear to move at the rate of one tooth (thread) per minute, thus our gear will have 1436 teeth. But how big of a gear will it be? This depends on how fine our threads (or gear teeth) are on the threaded rod. Let's use a rod that has 20 threads per inch. This means that for every inch of rod, there are 20 threads so that a nut will turn 20 times to cover that 1 inch. You can also say that our “gear” has twenty “teeth” per inch.
So.. if a gear has a total of 1436 teeth and there are 20 teeth per inch, then the gear must be 71.8 inches around (circumference). (1436 / 20" = 71.8")
This means that the gear's diameter is 22.855 inches. That's a very big gear.. and very accurate too!
Diameter = circumference / pi (71.8 / 3.14 = 22.855)
But remember, we fortunately only need a small portion of that gear! If the maximum duration of an exposure is 1 hour, then we only need 1/24th of our gear; which is about 3 inches (71.8” / 24hrs = 2.99”). To give us some room to work, we will triple that, or a 9” piece of threaded rod.
So we now must bend our rod to match a curve with a 22.855 inch diameter.
Since a steel rod has spring to it, you should use something with a diameter a lot smaller than 22 inches. You can use a tree or another round object with a large enough diameter that you can smoothly bend the rod around.
To make sure you get exactly the correct diameter, trace out an arc on a piece of large paper with a 22.855 diameter.
You do not need to complete a full circle; just a half circle will do nicely. Be very careful, and try to get as close to the correct arc as you can. Keep checking with the paper arc frequently. Any error here may show up on your photographs as a tracking error. BE SURE NOT TO BEND ANY THREADS!
Now select the most accurate portion of the rod, and cut a 9” length to use on your mount.
- Gear Location?
The last question now is how far from the hinge do you place this curved rod, so it will open the platform at the correct speed? It is accurate to imagine this rod is part of a large clock gear, with the axis at the hinge.
Since we know the size of the gear (22.855” diameter) and you want to have the gear rotate about the hinge, our curved rod must be located exactly at the radius, or 11.43” from the hinge (22.855/2). Be sure you measure precisely from the hinge pivot pin.
The above photo shows the curved rod in place on our Barn Door Mount. It is bolted to the top, and runs throw a hole in the bottom. Both holes are exactly located at the radius or exactly 11.43" from the hinge.
The next step is to create an easy way to slide this curved rod up at the exact rate as the sky appears to move (the Sidereal rate of 15 degrees per hour). Since we made this “gear” with 1436 teeth to match the 1436 minutes in a day, the rod needs to push the platform up at the rate of 1 thread (tooth) per minute. If you place a nut on your threaded rod and turn it at the rate of one revolution per minute, the “gear” (rod) will move at the rate of 1 tooth (thread) per minute, exactly matching the sky!
If you attach a nut to a large disk, you will be able to turn it very easily. You can then draw some lines on the disk to allow you to keep track of its position and make it easier to maintain the accurate rate. You can cut a disk from plastic, or fashion one from an old CD disk as shown here.
Clock Gear Frequency
You need to turn the disk one full revolution per minute, but how frequently must you turn the disk? Can you simply rotate it quickly once, and then wait a full minute before doing it again? Or do you need to make tiny movements every second?
Well that depends on the focal length of your camera lens. If you are using a very short focal length lens you may simply turn the disk ½ a turn every 30 seconds. But if you want to use a very long lens, like a 300mm telephoto lens, you must turn the disk every 5 seconds or you will capture some trailing between movements. Here's why…
Consider a camera with no tracking movement at all. The higher the focal length of the camera lens, the faster it will detect any star movement from the Earth's rotation. Remember, this is how you are able to get the Star Trail photos.
The formula 1000 / FL * (cos D) = EXP will give you the maximum length of time you can expose your film before star trails will become visible. FL = Focal Length of your camera lens, D = the declination of the area of sky you are pointing at (degrees), and EXP = the exposure time (seconds). The declination is how far from the celestial equator you are pointing at, where zero is at the equator and 90 is at the pole. The closer you point to the pole, the less star trailing you will record. To the right is a table showing how quickly your camera will show star movement based on varying lens focal lengths and declination. It shows the frequency to turn the drive to prevent star trails (minutes and seconds).
As you can see from this table, with a common 50mm lens you must move the mount at least every 20 seconds in order to prevent any star trailing from being recorded if you are pointing near the equator. However, if you are pointing near the pole, you can turn the drive nut once every minute! With this knowledge you can mark your spinning disk with even time-increments and use these marks to help maintain the correct tracking frequency.
Note: Be sure to use the correct frequency based on the fastest moving portion of your field of view, not just where you are pointing at. That is, you may be pointing at the pole which moves slowly, but using a wide angle lens that captures stars all the way down to 60 degrees. In this case, use the time calculated for 60 degrees. Now that we have a platform that will move at the correct arc as the sky, and a drive system that will move the mount at the exact rate as the sky's apparent movement, once it is accurately aligned to the Celetial Pole (as detailed below - see Polar Alignment)
To attach the camera, you can find lots of ways that will work. Just be sure you have the camera secured well, and that it can be rotated to any view of the sky. You can't move the mount once it's aligned!
In my example here, I attached the top of a cheap mini-tripod head that I had laying around. I simply cut the legs off, and then epoxies them to the wooden platform.
You can also make an easy do-it-yourself camera support with a 2" PVC compression coupler, 2" wooden ball, and a 1" length of the same 1/4" threaded rod left over from the tracker! The coupler is cut and glued to the wooden platform, and the rod is inserted and glued into the wooden ball. You then want to add another ball inside, or a piece of plastic pipe that will hold the ball up so that the ball will move freely after the cap is added. To adjust the direction, you simply twist the cap on the PVC coupler and rotate the ball to the desired position. Tighten the cap and take your picture!
So now you have an accurate platform to track the sky at the exact rate, angle and arc! Drill a hole in the base, to mount on a stable camera tripod, or construct a home-made platform that will allow you to move the mount to the correct position. Standard camera tripods use a 1/4" 20 thread nut as an attachment bolt. You can simply drill a hole in your mount's base large enough to hold the nut flush with the bottom of the mount. Then glue it in place with epoxy. Be certain that the nut is very secure - you do not want to have it fall out and drop your camera!
- POLAR ALIGNMENT
The #1 cause of tracking error with this mount is poor alignment to the Celestial Pole. In order for your mount to compensate for the Earth's rotation, you need to make sure your tracker is precisely aligned with the celestial pole.
So how do you find the exact pole? Fortunately for those who live in the Northern Hemisphere, it is very close to the North Star - Polaris. Polaris is fairly bright star that makes up the last star in the handle of the Little Dipper – Ursa Minor- as we discussed in the tracking alignment section above.
Once you locate Polaris, point the hinge axis directly at it.
This will put you very close to the Celestial Pole, but not close enough for our use. Now you must make one final adjustment to get the accuracy needed for long exposure astrophotography.
The North Celestial Pole is located ¾ of a degree away from Polaris, which is a very tiny amount. The full moon is 1/2 a degree across, so you need to adjust your mount by slightly more than the diameter of the moon. But which way do you move the mount? This is also very easy!
The north Celestial Pole lies 3/4 of a degree away from Polaris in the direction of the bright star Kochab, or Beta Ursa Minor is.
You simply point the mount at Polaris, and then shift it slightly (3/4 of a degree) towards Kochab. Now you will be VERY closely aligned with the Celestial Pole, and your mount will track very accurately!
You can add a very small telescope (finder scope or gun scope) to help you locate the exact celestial pole to increase accuracy. The closer you get to perfect alignment with the pole, the longer you can expose before the trailing or blurring will occur.
In this photo I have attached a simple 6X finderscope from a commercial telescope to the top board. This allows you to get very accurate Polar Alignment vs. what you can get only using your eyes.
You should first look at the moon in order to learn what 3/4 of a degree looks like. Remember, the moon is 1/2 degree across, so once you see what 1/2 a degree looks like, you can easily see what an additional 1/4 of a degree would be.
To use a finderscope, it is absolutely essential that the finderscope is perfectly aligned with the HINGE . It will be much easier if you do this in daylight. First, secure the tracker on a tripod and then sight on a distant object in the finder, such as a telephone pole or church steeple, while looking through the finderscope, rotate the top board as far as you can. You should see the sighted object simply rotate in the field of view. If, however, the object swings out of view, then the finder must be carefully moved to improve the alignment. Keep repeating these steps until the object simply rotates around the center of the finder, and you will have it aligned perfectly with the hinge.
To keep cost down, many astrophotographers use a simple straw or small pipe to sight through. I have seen one tracker sight through the hinge itself, by removing the hinge pin during alignment!
Enjoy and Clear Skies to you!
PS: I would LOVE to see YOUR photos taken with this tool!