Scope Alignment

Aligning a traditional GEM without any self alignment capabilities has got to be one of the most difficult things to do properly. Of couse now a days there are GOTO systems that do just about everything for you, but what to do when the mount standing in the livingroom corner is not one of those. There are a few ways of aligning the mount RA to the North Celestial Pole (NCP). Which way works for you depends on what your intetions for the observin session are. Below are described a few ways of doing the alignment.

You can just simply use the finder and the telescope to do the alignment to the NCP . It is actually easy to align the scope for visual observation this way. However, there is a catch, as always, if your mount RA and Dec axis are not perfectly perpendicular and your OTA mounted parallel to the to the RA axis, your RA axis is NOT pointing where it should. Only your OTA is. When this happens your target will eventually drift from view even with RA tracking.

You might have a mount with a Polar Alignment Scope fitted to the end of the RA axis. Using the PA Scope is easy and you get good alignment for visual work . Using this method you don't have to worry about the orientation of the OTA at all. The Polar Alignment scope views differ slightly for different manufacturers, but the basic idea is to adjust the Altitude and Azimuth adjustments so that Polaris, and possibly some other reference star, are located in marked positions in the PA scope field of view.

 The third way of aligning is called drift alignment. Drift alignment is usually used to align permanently fixed mounts and movable mounts for astrophotography. There are different variations of drift aligning, but here I will concentrate on the traditional way and a simplified way that can be used to perform the alignment using a scope mounted DSLR.

Traditional drift align
The traditional dift align method can be performed either visually or using a webcam. When using a webcam, there are some software tools available to help with the procedure. One such piece of software is EQAlign, which is also free. The basic idea behind the drift align process is to adjust the Azimuth or Altitude while tracking a star at the local Meridian or due East. If you are doing the alignment visually you need an eyepiece with crosshairs (preferably double).

1. The first thing to do is to level your mount and then perform a rough polar alignment of the mount by using a compass and setting the altitude to your latitude or by some other means at your disposal.
2. Next you should point the scope at a star due South (local meridian) and stop the tracking motor if running. Note the direction of the motion of the star and rotate the eyepiece so that the star moves along one of the crosshairs. The direction of travel is West.
3. Recenter the star on the crosshairs and start tracking. Monitor the drift of the star in the eyepiece. If the star drifts North, move Azimuth towards East and if it drifts South, move the Azimuth towards West. The longer you monitor the star, the better the accuracy.
4. Next you need to find a star due east and perform the same routine there except you are adjusting the Altitude. If the star drifts North, adjust the altitude down and if the star drifts South adjust the altitude up.
5. Repeat steps 2-4 until there is no more drift.

If you are using a webcam and a piece of software like EQAlign, the software guides you through the process. Also due to the smaller field of view offered by the webcam the drifting of the reference star is more obvious.

DSLR drift align
A variation of the drift align procedure can be performed with a DSLR connected to a computer. You will need software to control and view the images taken with the camera. There are various software tools that enable you to do this; Nebulosity, BackyardEOS and Astro Photography Tool to name a few (and MaximDL of course).

• Anyway, the basic idea is the same as with the traditional drift align method, but the DSRL align method utilizes the long exposure capability of the camera to get a good visual indication of the alignment error.The first thing to do is to level your mount and then perform a rough polar alignment of the mount by using a compass and setting the altitude to your latitude or by some other means at your disposal.
• Next you need to point the scope due South (local meridian) and keep the tracking running. You don't necessarily need bright stars since you will be using the camera to capture the photons so even faint stars are fine. You should set your camera to take at least 2 min exposures and use ISO setting of 1600 or more (these don't need to be pretty)
• Start the exposure: 

1. Let the mount track for the first 15s (this will give you a starting point)
2. After 15s stop tracking or set tracking speed to lowest possible
3. After 1 min of exposure set tracking to fastest possible

•  As a result of the exposure you will get a star trail image that most likely looks like the letter "V".  The orientation depends on your camera angle, but it doesn't matter. The purpose is to get the "V" in to a single line. You need to adjust the Azimuth either East or West depending on which way the "V" opens. 

In this scenario my star trails drifted North, so I need to adjust my Azimuth East. (If you are wondering about the orientation of the trails; it's due to my camera angle)

•  After adjusting the Azimuth redo the exposure. If the "V" spreads you adjusted in the wrong direction. If it closes you adjusted in the correct direction. Continue repeating the exposures until you get a single line as a result. 

After two adjustments of the Azimuth, the star trail is a single line (don't worry you'll soon figure out how much to move axis).

•  Next target either East or West and repeat steps 3-5 except this time adjust the Altitude.  

Pointing East. My star trail has drifted North, so I need to adjust the altitude down.

In the end the star trails are again lined up.

• You should retry the Southern orientation to make sure that the Azimuth setting is still correct. If you have not leveled the mount with a bubble level or some other means, you should repeat the South and East/West adjustments as many times as needed to get the startrails to stay in line.

Try to target stars as close to to 0 degrees declination as possible. If you want more accurate alignment you can extend the exposure time as much as you wish.

At least for me this method is easier and faster to do than the traditional way and I don't need to bring the webcam along. It can be performed with the imaging setup.

Clear Skies

Focuser Project

It's been some time since I last posted and even today the topic is an on going focuser update project. The purpose is to make a crayford style focuser to replace the rack and pinion focuser of the AstroMaster. The reason for this focuser update is that the stock focuser does not have enough backfocus to allow for DSLR primefocus photography as it is. In order to take the photos I've posted in my blog, I had to do a quick and dirty modification of the stock focuser and it has left me with some slack between the drawtube and the focuser frame.

In order for prime focus photography to work, the telescope needs to have enough back focus to allow for the DSLR imaging sensor to move to the focal plane. The amount of back focus needed depends on your setup (camera+adapter+acessories). Quite often with the "budget" newtonians this is not possible without doing some modifications to the telescope. There are a few possibilities to get the focal plane to hit the sensor of the camera with a newtonian:

1. Move the main mirror inwards in the tube. This often means shortening the telescope tube and also there is the possibility that the secondary does not catch all the light coming from the main mirror, depending on the size of the secondary.
2. Get the focuser to move inward the necessary amount. This can be achieved by getting a lower profile focuser or modifying the original.
3. Move the focal plane with the use of a barlow lens or other optical means. This however means that there are more optical elements in the light path, amounting to additional light loss and possibly more coma due to extended focal length.

In my case I started off with modifying the original focuser. The DSLR I'm using (EOS 1000D) has the sensor 44mm from the front mounting flange + additional 15mm for the T-adapter and the adapter for the 1 1/4 inch eyepiece holder, for a total of 59mm backfocus. After doing some measurements I needed some 30mm of additional backfocus for the camera to achieve focus. What I did was first of all remove the top portion of the drawtube frame. The top of the drawtube frame is what keeps the drawtube from wobbling, so when I removedit, I had to improvise with some small screws and teflon strips. That got me some 15 mm and the other 15mm comes from attaching the camera directly in to the drawtube, without the eyepiece holder, using a self made adaptor.

This setup has gotten me this far, but I started thinking about a rugged re-usable focuser. Meaning that if I switch scopes I can use the same focuser  with small changes. The basic idea is to have a straight base and then have an adapter plate to fit to the shape of the telescope tube. Also I want to have the possibility to adjust the tilt of the whole focuser.

Rummaging through the metal scrap at work, I found a nice piece of aluminium to act as the starting point for the focuser frame. A 40mm diameter hole was drilled in the center (to be finetuned later).

I machined the rough dimensions at work with a larger milling machine, the fine tuning and smaller work will be performed with a smaller milling machine.

After some hours (and lots of aluminium chips) later the main components were finished. From left to right the components are;

• The drawtube
• Main frame of the focuser (one piece)
• Focuser axel
• Holder for the focuser axel
• Eyepiece holder

The pros at work were nice enough to make me the draw tube and eyepiece holder with the lathe there. The height of the focuser frame is 50 mm and together with the drawtube+eyepiece holder is a total of 70mm. Height is 10mm less than the current, quick and dirty, modified stock focuser on my AstroMaster.

Currently there are no bearings in the main frame of the focuser for the drawtube. The mechanism seems to work fine since the sides polished to some degree. The white part in the focuser axel holder is teflon, which is used as a bearing for the focuser axel. The screws (one visible in the photo) protruding from the backside are used for setting the correct pressure to the axel-drawtube contact.

The drawtube axel is made from acidproof steel, which makes for a "sticky" contact between it and the drawtube. No additional friction providing material is needed.

The four large hex screws are used for adjusting the tilt of the focuser frame (they are at a staright agnle to the base even though in the photo they don't seem to be). The smaller screw on the right side of the frame is used for locking the drawtube if needed. The frame also has some spare to acommodate for a larger drawtube, for 2 inch accessories, if necessary.

All in all the focuser turned out fine. Now what is needed is some black coating on the inside of the drawtube and possibly on the outside (it's a bit too shiny) and the actual knobs for the focuser axel.


Clear Skies