Astrophotography - Tips and tricks please.

[theandies]

So I finally got a good consumer DLSR, the T6i.  My T mount adaptor comes in friday.  I didn't get a shutter release yet but with the T6i I think the remote control through my cell phone will keep the vibrations down.......I guess.  I've always wanted to get into astrophotography but never had a camera that could do it.  I've read quite a bit about it over the years but I'll be new to it just as I'm new to anything other than a point and shoot and cell phone cameras.

 I have an old school (no electronics but I know how to use it very well) Meade 8" Schmidt–Cassegrain that I've had for years.  I do have an off axis corrector and a very good tripod.   I'm just looking for some hints and tips from the astrophotography buffs here if there are any.  What I think I'll try first is planetary pictures and pictures of the moon. After I get the hang of the basics I'll get more ambitious.  Anything would be much appreciated. 

Thanks

John

[TCampbell · Accepted solution]

Most LX200 scopes are f/10, but there were some f/6.3 models and the new LX600 and LX850 models are f/8.  You'll want to know what focal ratio you have because when you use a camera with a telescope there's no adjustable f-stops like there is for a lens.  You'll be doing manual exposures and you'll want to know what your native focal ratio is.

When you connect the camera, connect it "straight through" (no 90º diagonal).   I have a 14" LX200 ACF -- which has longer forks and I have a LOT of clearance behind the optical tube.  My scope can point straight up through the fork arms with an electric focus and my Canon 60Da attached and still have plenty of room so that there's no danger of the camera hitting the base if the optical tube tries to point straight up.  

Check the clearance on your scope (with camera attached) to make sure you are comfortable knowing that you have enough clearance -- as this will avoid potential damage if you slew to a new target in the dark and the camera crashes into the base.

My LX200 has an electric focus with visual back (standard 2" tube opening for a 2" eyepiece or 2" diagonal) and so my camera uses a 2" diameter nosepiece with the EOS t-ring.  I don't use the SCT T-mount.  

When focusing the scope, always "finish" turning the focuser into focus in the counter-clockwise direction.  The LX200 has mirror-shift when you reverse the direction of focus.  This is because the focus knob either pushes or pulls on the primary mirror at the back of the scope and the mirror rides on a central baffle that has to allow at least a tiny bit of slop so that it can slide easily.  When you "push" the mirror forward, it gets the best support from the bottom and also forces the mirror to rest in a properly collimaged position.  When you "pull" the mirror back, it slightly de-collimates your optics and ALSO the mirror may eventually settle on you in mid-exposure (that would ruin the image being captured at the time).   By finishing focus while rotating the knob in the counter-clockwise direction, you give the primary mirror the best support, collimation, and it reduces the potential for the mirror to settle and move after you finish the focus.

The LX200 has a mirror-lock knob and when I do visual astronomy I don't use it.  But when I do imaging, I DO use it.  My scope has an electric focuser on the visual back.  That means I can adjust the main focus, lock the mirror, then use the electric focuser to fine-focus the telescope to my satisfaction.  

I prefer to point the scope to a pinpoint star to focus.  Focusing on the moon or a planet is is not quite as easy to see if you achieved perfect focus.  But when you've minimized the size of a pinpoint star, you'll have more accurate focus.  

There is an application called "Backyard EOS" which is built for astronomy astro-imaging using Canon EOS cameras.  It has focus aids as well as both planetary imaging modes and deep-sky imaging modes.  It controls the camera and performs image acquisition for you (it's basically "tethered" remote shooting control of the camera.)

I normally use a Bahtinov focusing mask on my scope to achieve fine focus for deep-sky images.  But a lot of imagers I know have electric focusers and use a program called FocusMax which is exceptionally accurate focus.  FocusMax deliberately de-focuses a star (you see the donut shape) and starts focusing and imaging and focusing and imaging.  It measure the size of the donut and does several runs creating plots.  It mathematicaly determines where the perfect focus point must be located (even if sky conditions are horrible.)

Both "Backyard EOS" and "FocusMax" require Windows (I use a Mac so I don't use them, but they are popular.)

The moon is a VERY easy target... the correct exposure for the moon follows the "Loony 11 Rule".  That rule says that at f/11 (and it only works at f/11) you can set the shutter speed to the inverse of the ISO sensitivity.  So at f/11 and ISO 100, it'd be 1/100th sec.  At ISO 400 it would be a 1/400th sec exposure, etc.  

If your scope is really an f/10, it's close enough... f/10 is approximately 1/3rd stop more light than f/11 (it's not a significant difference) but it means the moon will be slightly bright.  You can adjust the shutter speed 1/3rd stop faster to compensate.  E.g. instead of 1/100th sec you should shoot at 1/125th sec. and be bang-on the accurate exposure again.  If you have an f/6.3 scope then you are 1.6 stops faster (almost, but not quite 2 stops).  That means insead of ISO 100 and 1/100th, you'd want to shoot at ISO 100 and 1/320 sec.

Planets are slightly more difficult... they are dimmer than the moon so you'll use a higher ISO setting.  But they are typically imaged by shooting about a minute's worth of video frames.  Stacking software is then used to identify the best frames out of the video and those are combined to create a composite image.

If you happen to image Jupiter, note than Jupiter has a fairly fast rotational speed... the surface changes enough in MERELY 10 minutes that if you try to combine images shot more than 10 minutes apart you'll get blur.  All the data you capture for Jupiter needs to be captured within that 10 minute window of time.  There is a program called Registax that is particular popular (and free) for planetary image stacking.  

If you image Saturn, it's low in the sky this year (because it's an "outer" planet and our northern polar axis is pointed toward the sun.  That puts saturn low relative to the horizon for astronomers who live in northern latitudes.)  This means you'll get some atmosphereic dispersion when you view it.  Atmospheric dispersion is a form of chromatic aberration -- except it's caused by our atmosphere working like a lens.  The atmosphere splits "white" light into the rainbow spectra of light.  You'll see a "red fringe" on one edge of the planet and it's rings... and a "blue fringe" on the opposite edge.  Not to worry... one of the features of Registax is that it can separate the single color image into red, green, and blue color channels and it lets you "shift" them back on top of each other.  This greatly improves the focus quality of your image.

Deep sky objects are particularly difficult.  This may cause you to lose all your hair.  You have been warned.  🙂

To detail how to take deep-sky images would take a while... but I can sum up:

The scope needs to be mounted on a "wedge" (e.g. such as a Meade Superwedge).  The wedge is moutned to the tripod and the scope is then mounted to the wedge.  The wedge is tilted so that the tilt angle is adjusted for YOUR viewing latitude.  If the scope is merely on an alt-az mount then you'd get field rotation as you imagine and that would create blurred results.

You mentioned your scope does not have "electronics" but it would need to minimally have an RA drive that can track at sidereal speed.  Do you have this?

The mount needs a "precise polar alignment" (which takes a bit of effort.)  

You take numerous long exposure images (e.g. 5-10 minutes would be typical).  

You also need to capture "dark" frames, and it's also helpful to grab "flat" frames and "bias" frames.  I can explain what these are if you haven't heard of them.

The images are then stacked using image registration and integration stoftware (for deep sky objects there's a free program called "Deep Sky Stacker".  I use something called PixInsight do do my registration & integration (stacking) but PixInsight isn't free.

You mention this is an "old school" LX-200 with "no electronics".  Are the electronics fried?  I've seen a number of LX200 models and some have very old primitive electronics, but I've never seen one that doesn't have any electronics.    There are places that will either repair or refit the scope so that it does have working electronics.  Finding the very old original boards in working order is tough.  There were some bad capacitors used which would dry out over time and then blow.  The guys that service the scopes know which ones blow and they replace them with modern equivalent capacitors that wont blow BUT it's critical that they do it BEFORE the scope has a problem.  If the capacitors blow before being replaced they often take out other electornics on the board and now it's a more serious repair.  Those "more serious" repairs can involve trying to find replacement boards that aren't made anymore, haven't been made in years, and are becoming increasingly rare.

So... now there are services that simply pull the original boards and replace them with completely new boards, but the refit kits give you electronics that work like an LX90 (not an LX200).  LX90's dont' have PEC -- so they aren't as precise as LX200's, but it's still better than nothing.

Moon shots are easy (no elecronics needed.)

Planetary shots are fairly easy but it helps to at least have a working RA drive that can track at sidereal speed (clock-drive) even if there are no computerized go-to electronics.

Deep-sky objects, however... will really need working electronics and an autoguider.  This are the most complicated images by far --- because they require such long exposure times.  And during those very long exposures, you can tolerate any movement or tracking errors -- otherwise it ruins the image.

[bhattvd]

Tim:

Your dedication is amazing!  I dont know you, you dont know me, and still you took time out to carefully discuss my issue in full details!  You went to the details of Field of view of the Galaxy in arc-minutes!

You are very knowledgeable in Astronomy.  You rightly said that Pinwheel Galaxy will be too tiny for a 35 mm lens.  I was planning to use my zoom lens for that.  I have a 6" reflector telescope as well.  Tonight I am planning to pull it out and aim at M101 and see whether it is visible with 1400mm focal length.  Tonight I have clear skies but it is too much windy which could hamper my "seeing" with the telescope.

I will definately work on your tips to focus more minutely and see whether I have more success.  I will let you know the outcome after I try it out couple of times.  I will also receive my more sturdier tripod tomorrow which will help in my Texas hill country terrain's high wind situation.  My house is on top of a hill with a beautiful sunset view.  I have a good visibility of West, South and north skies from our cabana.

I agree with you about the need for the camera modification.  Do you have a recommendation for where can I do that?  Also, a question is that after they take out the native filter and put a new Hydrogen-Alpha filter can I still use my camera for day-today photography needs as well?

Thanks again for your help.  It is getting dark out there.  Its time to work on your tips!

Best Regards and Stay Safe,

Vinay.