Bahtinov Mask

Auto focus software can significantly improve image focus; especially when images are captured over several hours during an imaging session.  One of the biggest challenges of Astrophotography is making sure images are in focus.

Manual Focus Method

Most astrophotographers do their initial focus using a Bahtinov mask to perform the initial focus.  The Bahtimov mask causes a de-fraction pattern to appear in the image of the stars.

The Bahtinov mask is placed over the front of the telescope while the telescope is pointed at a bright star.  Short exposure images are then taken to assess the accuracy of the focus.

The following images show the defraction patterns for a star that is out of focus and the same star in focus.

The difference is subtle.  If you carefully examine the images you will notice that in the left image the vertical diffraction line passes to the right of the center of the X formed by the other two lines.  In the right image it passes to the left of the center of the X.  If you examine the center image you can see that it passes directly through the middle of the X.  A Bahtinov mask is considered the Gold standard for establishing focus when doing astrophotography.  Without the mask is is very difficult to tell if an image is at the best focus point.

Why Auto Focus?

Unfortunately, the correct focus point changes throughout the night because of the changing outdoor temperature.  The only way to keep the images in focus is redo the focus several times throughout the night.  This can be very time consuming because it usually requires moving the scope away from your target, pointing it to a bright star for focusing and then moving the scope back to the object you are imaging.  Therefore a lot of time can be saved by using auto focus software.

Implementing Auto Focus

In order to use auto focus software the telescope focuser has to be controllable by your computer.  Therefore the telescope need to have a software controllable motorized focuser.  There are many ways to add this capability to a telescope.  One way is to add a motor drive to your existing telescope focuser.  Several manufactures sell kits to motorize various types of manual focusers.  Another Approach is to replace the existing focuser with a new motorized focuser.

My Auto Focus Setup

Moonlight Motorized Focuser

My telescope uses a Moonlight focuser like the one shown here.  It has a precision stepper motor which enables the software to adjust the position of the focuser in extremely fine increments during the autofocus process.

I use a program called Sequence Generator Pro to collect my astro images. It can be used to fully automate a night of astrophotography, which can include multiple targets.  One of the tools it includes is an autofocus routine.

In order to use the autofocus routine, the telescope must be fairly close to focus before the autofocus routine is run.  This may seem counter-intuitive, but the primary purpose of the autofocus routine is to keep the telescope in focus throughout the night.  It is not designed to focus an unfocused telescope.

How Auto Focus Works

The autofocus process requires some initial setup before it will work with a specific telescope and focuser.  The user must specify the number of steps the stepper motor will move for each move for each focus adjustment.  The user also specifies the number of points that they would like included in the focus curve (typically 9).  The routine will then move the focuser out 5x the specified step change to collect the first point.  It will then take a short exposure with the focuser at this position and make a calculation to determine the accuracy of the focus.  The value calculated is called the Half-Flux-Radius, HFR.  The larger the value of HFR the poorer the focus.  The software will then move the focuser in by the specified step size, take another image and calculate the value of HFR.  This process continues until the number of specified points have been collected.

Plot of HFR Focus Curve

After the data has been analyzed two reqression lines are are plotted forming what is called a “V” curve.  The point where these two lines intersect is the best focus point.  The software will then move the focuser to this position and take another exposure to verify its result.  If the calculated HFR is less than or equal to the lowest value of HFR measured during the autofocus run the focuser will be left at this position and the software will begin collecting images.  If the value is greater than the minimum measured the routine will either repeat or utilize its sophisticated algorithms to select the best focus point. This can happen occasionally occur when the sky transparency is less than ideal.

The autofocus curve shown is from a recent imaging session of NGC 869 (Double Cluster).

The following image is the confirmation image from the Auto Focus routine.  Click on the image to see the full size image.

NGC 869 Raw Image (Click for full size image)

The next step in the construction of our Roll-off-Roof was the construction of the building.  We chose a design from Back Yard Observatories, BYO.

Scott Horstman contacted me a few days prior to the planned start date, to let me know that a local building supply house would be making a delivery the following day.  Here is a photo of the large pile of material that was delivered.

Building Materials Arrive

With the help of the delivery crew, I was able to identify and count all of the different items before I signed off on the delivery.  This was harder than I thought it would be because there were many types of materials with names that were new to me.  I know things like 2×4, 2×8 etc, but names like OSB and five quarter were new to me.

When Scott and his crew arrived the next day, they inventoried all of the materials before starting construction.   Next, they measured the size and squareness of the concrete pad. Apparently, he has run into pads which were several inches from square.  Scott quipped that it is still possible to build a square building on a trapezoidal foundation, but not ideal!  Our foundation was square to within 1/8″ thanks to our excellent concrete contractor.

Sorting Material and Checking the Pad

Once the pad had been checked, they began framing the walls.  They were able to use the observatory’s concrete floor to assemble the walls.  Since I already had power to the observatory pad, they were able to install a temporary outlet to run their saws and compressors.  It was amazing for me was to not see Scott or any of his crew refer to a single drawing during the construction.  He and his expert crew build so many of these observatories (although they are all different sizes) they automatically know the dimensions for every part!

Assembling One of the Walls

Once each wall was completed it was leveled and anchored in place.

First Wall Up, 3 to Go

Here you can see that three walls have been completed.  In the above photo you can see that the south wall is shorter.  The gap in the top of the wall will be filled once the hinged drop- down south wall has been installed.  Also, you can see the beefed up header on the right (West) wall.  The additional header is needed to support the weight of the roll-off-roof.  Below is photo looking from the South.

South Wall Showing Cutout for Drop Down Section

Once all four walls were in place, the framing for the roll-off-roof support structure was added. In addition, they are starting to frame the deck we had them add under the roll-off-roof supports.  The deck turned out to be a very nice addition to the observatory.  It makes a very nice place to sit and relax when we are waiting for it to get dark prior to a summer observing session.

Construction of Deck and Roll-off_Roof Supports

With the framing for the deck completed it was time to start adding the decking.
We chose treated five-quarter decking, but in retrospect I would have probably gone with the more costly composite material to reduce future maintenance.

Deck Framing Complete

All of the decking was nailed down it was cut to length in a single cut.  That is a pretty neat trick to make sure that all of the boards are exactly flush at the visible end of the deck.  The other thing I realized is that doing it this way saves considerable time that would be required to measure and individually cut each board.   Being the weekend hacker that I am, I don’t think I would have thought to do it this way.

Deck Planking

Once the walls and roll-off-roof support structure were completed, fabrication of the two rolling beams that support the roof trusses were assembled.  There are four heavy duty steel “V” rollers used on each beam.  The beam is also strengthened with a treated 2 x 8.  The 2 x 8 also covers the gap between the top of the wall and the bottom of the roof.

Construction of Roof Rollers

The following photo shows the roof support beams sitting on the angle iron rail which supports the roof and guides the rollers.

Roof Support and Rollers Installed on Wall and Roof Rail

Finally the roof was completed and ready to be rolled back if the rain would stop!  We were lucky we were only getting sprinkles instead of a “hoosier” downpour, which would have stopped the work.  Once the roof was on, the roof and walls were covered with felt paper before the metal roofing and cedar siding were installed.

Roof Completed

In this photo the cedar siding has been installed and the metal roof is being completed.

Metal Roofing and Cedar Siding Being Installed

The final photo shows what the observatory looks like with the roof rolled back!  Our friends and family are always amazed when they seen the roof rolled back for the first time.

Observatory with Roof Rolled Back

In the next installment, I’ll move inside the observatory to show some of the interior details.

Indiana weather does not offer an abundance of clear nights for observing or astrophotography.  However, last winter we had a lot of clear nights, but the time to setup and teardown my astrophotography setup kept my wife and I from taking advantage of most of them. We found it really hard to get motivated to setup and teardown our gear with snow on the ground and frigid temperatures.

As our frustration grew, we decided we should build a “simple” observatory.  Our initial plan was to construct a small concrete pad with a permanent pier; which could be covered with a small garden shed which could be rolled over the pier to protect the telescope and mount.  Of course, as our planning progressed, the scale of our  “simple” observatory began to expand! After nearly 9 months of planning we decided to build a medium sized roll-off-roof observatory.

We settled on a 15′ x 15′ Roll-off-Roof design with two piers and an area to setup a third telescope inside the observatory.  One of the piers will be dedicated to astrophotography and the other will be used for visual astronomy.  The advantage of an observatory is that anytime there are clear night skies it is easy to open the observatory and begin observing; eliminating most of the long setup time.  The elimination of 3 hours of setup and teardown will substantially increase our motivation to take advantage of the clear and cold winter skies.

Framing of Observatory Pad

Construction started in early November. The pad forms were constructed the day before the concrete pour.  The pad included two cutouts where the piers will be installed.

Completed Forms Showing Pier Locations and Power and Data Conduits

Conduit was added to handle the electrical and data cables that will be needed at each pier.

The Concrete Pour Ballet

On the following day It was time to pour the concrete.  After watching this expert crew work I was relieved that I hadn’t tried to do this myself.  Watching their choreographed ballet I was impressed with their skill and organization. Things were happening so fast that there was no time to stop and decide who was to do each job or discuss how something was to be done.

Completed Observatory Foundation

Here is a photo of my wife admiring the completed pad.

Check back in soon to see the observatory construction begin.

Fred

Saturn

On Tuesday night the skies were clear so my wife and I got with some others from the Indiana Astronomical Society to do some observing.  I took our Meade LX80 mount and 6 inch Astro Tech refractor to give it a run.  During the winter I added a StarGPS to the LX80 to make the setup a bit easier.  I highly recommend the StarGPS system.  It eliminates many of the potential errors that can lead to poor scope alignment.  If the scope is carefully leveled and set to its home position, the GPS will enable you to get a near perfect alignment every time.

After I assembled  and leveled the LX80 I installed the Astro Tech scope and turned on the power.  In about 2 minutes the StarGPS found our location.  It loaded the correct time and location into the handset and then put the hand controller into the “Easy” align mode.  I did a two star alignment and was pleased to see a “Alignment Successful” message on the hand controller.

The first thing I did was slew to Jupiter to catch it before it dropped below the trees.  I was rewarded with a nice view of Jupiter and four moons. Jupiter’s bands were visible but the Great Red Spot was not visible.

My wife and I spent the rest of the night viewing a number of deep space objects.  One of the nicest DSOs we saw was when my wife got M81 and M82 in the eyepiece at the same time.  We also had a nice view the Sombrero Galaxy (M104).  Later in the evening I tried out Meade’s “What’s Up Tonight” feature to take  a guided tour of the nights best objects.  The hand-controller selected a few objects that were new to me;  so it was time well spent.

We finished up the evening by getting our first view of Saturn this year.  I always enjoy looking at Saturn because it has such a striking appearance.

Fred,  KC9QQ

Cube Sats being Launched from ISS

I don’t know how many of you saw this photo on the today’s Astronomy Photo of the day, so I am showing it hear in case you missed it.  The cube-sats are near and dear to we hams since they often carry Ham Radio beacons or repeaters.

This photo shows three cube-sats being released from the International Space Station last November.

73,

Fred, KC9QQ