When imaging deep-sky objects with my long exposure modified cameras I often have to discard frames because the stars are not nicely round, but have become streaks.
This is caused by the Periodic Error of my TAL-2M mount.
The problem is that I can only assess the quality of an (i.e. no streaks but nice round stars) AFTER the whole exposure time has elapsed and then I am too late :-\

The solution for this is AUTOGUIDING, whereby TWO webcams are used and automatically guiding corrections are made 'on the fly' during the long exposure imaging by the imaging webcam.
So: one webcam does the actual long exposure IMAGING, while the second GUIDING webcam is aimed at a bright star with shorter exposure times [but it is very nice if this Guiding webcam is also modified for long exposure, so that you can use e.g. exposure times of 1 second which makes it MUCH easier to find a good guide star]. For this you need a Splitter Box and by doing so the computer software can determine the shift in pixels and give a message to the (in my case RA only) motor(s) of the telescope in terms of "a little faster/slower please".
This all happens while the long exposure imaging is going on.

Here are my own experiences with my TAL-2M; whereby I image via my modest TAL-2M telescope and guide with a piggybacked 400mm SLR Photolens with 2x Teleconverter.
Of course the other way around is also possible: please browse further down.

One more thing: on my TAL-2M only the RA is motorized and the DEC is NOT motorized, so I can only autoguide on the RA: therefore to 'manage' the DEC it is mandatory for me that the Polar Alignment is just perfect.

On July 14, 2005 I started the experiments to use my piggy-backed 400mm SLR Photolens and 2x Teleconverter for Autoguiding
(Note: the 2x Teleconverter is not on this photo)

1. Prerequisites
For Autoguiding I use the following:

2. Preparing for Autoguiding
Note: I assume you already know how to image without guiding; should you not know this then first go to my Tutorials page, e.g. Beginners or DSO Tutorial

3. Setting up the Guiding instance

4. Setting up the Imaging instance

Here is an example:.
You see the screen print of a 30 second exposure; I imaged with my TAL-2M and focal reducer - focal length ~ 690mm - and autoguided with 400mm SLR Photolens and 2x teleconverter: focal length 800mm.

Example of a 60 second exposure:
TAL-2M in prime focus - focal length 1200mm - and autoguided with 400mm SLR Photolens and 2x teleconverter: focal length 800mm.

5. Guiding with the main (1200mm) scope and imaging with piggy-backed 135mm SLR Photolens

I wanted to image NGC6960 (part of the Veil Nebula) and used (for imaging) my 135mm SLR Photolens with SC3a b/w RAW camera and H-Alpha filter; guiding would be done with my TAL-2M in prime focus and SC3a colour camera.
I found it very hard to find NGC6960 with the 135mm SLR Photolens, because I had not yet lined up my TAL-2M with the piggy backed 135mm SLR Photolens, so they were 'out of sync' with each other.
I aimed my TAL-2M at a bright star (Altair) and put it in the centre of the K3CCDTools 'circles'; after which I tried to find Altair with the piggy backed 135mm SLR Photolens by making adjustments with my Witty.
This took a long time, mainly because of the light absorption of the H-Alpha filter which requires long exposure times and I did not want to remove the H-Alpha filter because that too takes time and I had already focused the 135mm SLR Photolens.
When I finally was ready to image ... the clouds came :-{

I learned my lesson and the next time I have used the sun to synchronize TAL-2M and 135mm SLR Photolens: much easier!
(Of course I used my Solar filters).

This indeed helped a lot: when I aimed my 1200mm focal length scope at Altair, my 135mm SLR Photolens also had Altair on the preview screen.
Then it occurred to me that I could both instances simultaneously with the live video feed on my screen and after having done so I adjusted the 135mm instance, while keeping an eye at the 1200mm instance at the same time.
I tweaked the 135mm instance with my Witty till I was happy: here you see a screen print of what I mean:

My TAL-2M 1200mm focal length and my 135mm SLR Photolens are now in perfect sync with each other.
At this moment the clouds came so I leave it here for when I have clear skies again.

While waiting for clear skies I made an (as yet experimental) Filter Drawer which wil hopefully enable me to remove/insert the H-Alpha filter without disassembling my setup.

I am aware of the fact that there is a difference in focus with/without the H-Alpha Filter, but re-focusing surely will be a lot easier than re-finding the object.

I am experimenting with the following two scenarios, in both of which the Filter Drawer and the Witty play an important role.

5.1 Scenario A: first find the object and then a guide star

  1. Setup K3CCDTools for guiding (see above 3. Setting up the Guiding instance and 4. Setting up the Imaging instance)
  2. Aim main scope (in prime focus) at a bright star and focus to perfection
  3. Use the Filter Drawer to remove the H-Alpha filter from the (imaging) SLR Photolens optical train
  4. Cover the Filter Drawer (no filter inserted yet)
  5. Focus the 135mm SLR Photolens rather well
  6. Line up 135mm SLR Photolens at the bright star, so that main (guiding) scope and (imaging) SLR Photolens are in sync
  7. Push the whole setup to the desired object (I use my setting circles) and make sure it is on the preview screen of the imaging 135mm SLR Photolens instance
  8. Have a good look at the star pattern on the preview screen
  9. Put both windows of the guiding and the imaging instance on the computer screen (see above example)
  10. Gently push the main scope with piggy backed SLR Photolens left/right/up/down till a nice guide star is on the preview screen. While doing so keep a watchful eye on the preview screen of the imaging instance and use the Witty to keep the object (or star pattern) in the centre of the video feed.
  11. When a nice guide star has been found: use the Filter Drawer to insert the H-Alpha filter into the optical train of the SLR Photolens
  12. Adjust the exposure time of the imaging instance
  13. Focus the 135mm SLR Photolens to perfection
  14. Tweak the imaging camera settings to perfection
  15. Use the Drift Explorer to start and control the Autoguiding

The above scenario worked out marvellous: I found NGC6960 immediately with the 135mm SLR Photolens and did not loose it while searching for a guide star with my main scope: success!

5.2 Scenario B: first find a guide star and then the object

  1. Setup K3CCDTools for guiding (see above 3. Setting up the Guiding instance and 4. Setting up the Imaging instance)
  2. Aim main scope (in prime focus) at a bright star and focus to perfection
  3. Use the Filter Drawer to remove the H-Alpha filter from the (imaging) SLR Photolens optical train
  4. Cover the Filter Drawer (no filter inserted yet)
  5. Focus the 135mm SLR Photolens rather well
  6. Line up 135mm SLR Photolens at the bright star, so that main (guiding) scope and (imaging) SLR Photolens are in sync
  7. Push the whole setup to the desired object (I use my setting circles) and find a guide star in that area
  8. Now put both windows of the guiding and the imaging instance on the computer screen (see above example)
  9. Next use the fine controls of the Witty to get the object on the Preview screen (it is a great help to have a detailed star chart (such as Cartes du Ciel or Megastar) available with the field-of-view of the imaging camera-with-lens superimposed.
  10. When the object has been found: use the Filter Drawer to insert the H-Alpha filter into the optical train of the SLR Photolens
  11. Adjust the exposure time of the imaging instance
  12. Focus the 135mm SLR Photolens to perfection (hint: use a white marker pen to put marks on the SLR Photolens)
  13. Tweak the imaging camera settings to perfection
  14. Use the Drift Explorer to start and control the Autoguiding

This scenario worked also very nice, as I departed from a guide star and the star hopped to the object: the fine controls of the Witty made that very easy.

Here are some Filter Drawer and Autoguiding test results.

August 11, 2005. First test results. The object is NGC6960.
Exposure 60 seconds, H-Alpha filter inserted in filter drawer, autoguided.
Seeing: fog building up rapidly, but I just wanted to do the tests.
August 11, 2005. First test results. The object is NGC6960.
Exposure 75 seconds, H-Alpha filter inserted in filter drawer, autoguided.
Seeing: fog building up rapidly, but I just wanted to do the tests.
August 11, 2005. First test result of dark frame, to see if any light was leaking. It seems no further action is required.
Exposure 75 seconds, H-Alpha filter inserted in filter drawer.
August 12, 2005. First real result: exposure 240 seconds, H-Alpha filter inserted in filter drawer.
Imaging: 135mm SLR Photolens with SC3a b/w RAW camera, Baader Infra Red Blocking Filter and Astronomik H-Alpha filter.
Guiding was done via my TAL-2M 15cm f/8 in prime focus with my SC3a colour camera.
Settings of the imaging camera: Brightness 50, Gamma 100, White Balance: auto, Gain 100.
Seeing: 8/10, urban light pollution.
August 16, 2005.
This animation shows the difference between a guided and an unguided 90 second exposure with my 135mm SLR Photolens with SC3a b/w RAW camera and Astronomik H-Alpha filter.
The guiding was done via my TAL-2M 15cm f/8 1200mm focal length with my SC3a colour camera.