Hubble Optics CDK 17 Collimation Instructions 03/27/2012 Hubble Optics

Transcription

1 Hubble Optics CDK 17 Collimation Instructions 03/27/2012 Hubble Optics 1: CDK17 Specification: System Effective Focal Length: mm, (this might be slightly different for different set of optics) Figure 1: CDK17 Specification

2 A: The nominal distance from the Secondary Mirror to the first surface of the first corrector lens, 715.9mm (it will be slightly different for different set of optics). This distance should be adjusted based on the procedure described in this instruction. B: The distance from the first surface of the first corrector lens to the primary mirror, 43.75mm C: The back focus from the primary vertex, mm D: The distance from the first surface of the first corrector lens to the last surface of the last lens: 28.1mm E: The distance from the last surface of last corrector lens to the primary: 15.7 mm. This distance should be set as accurately as possible, and should be fixed. This value maybe different for different set of optics.

3 2: Collimation Instruction The CDK17 optical design has four optical elements shown in the figure1. The primary mirror and the corrector lens should be permanently and precisely mounted and aligned. The correctors, the Primary mirror, the primary baffle tube, the focuser should be all squared to the optical axis. The secondary mirror should be movable to allow fine collimation and to set the spacing between the primary and secondary. To get the best performance out of the CDK17 optical system, the optics must be collimated and the secondary to primary mirror spacing must be set Key Collimation Components Primary-Secondary Spacer Tube Ronchi Eyepiece Focuser Adapter if needed. Figure 2: Tools

4 Step 1, Pre Collimation The primary mirror, primary baffle tube, corrector lens, focuser need to be installed, aligned, squared precisely. In particular, the distance between the primary and the last surface of the last lens need to set to 15.7mm (E) as precisely as possible. Then install the Secondary mirror and set the distance between secondary to primary with a tape measure, as precisely as possible, but not critical at this step. BTW, the Secondary Spacer (Focuser Extension Tube) should be so designed that when the focuser is fully racked in, it will set the distance from the last surface of the last corrector lens to the Ronchi screen to be exactly 341.0mm (C+E), as precisely as possible, better within +/-0.5mm. Step 2, Rough Day Time Collimation: Figure 3: CDK17 OTA Viewed From Front Insert the Secondary Spacer into the focuser. Insert a collimation cap (a cap with a small center hole) Adjust the spider wing thumb screws and the secondary mirror collimation screws to make sure all rings and circles are concentric Remove the collimation cap, and remove your eye from the focuser as far as you can see a bright shadow just appear around the central obstruction. Make sure you eye is on the optical axis, and fine tune the collimation screws to make sure the bright shadow is concentric with the central obstruction

5 You can verify that the scope is well collimated by positioning your eye (One eye!) in front of the OTA as far away from the front, just as a bright shadow appear around the central obstruction. The central obstruction, the bright shadow, the dark shadow, and the mirror should all be concentric Figure 4: CDK`17 OTA Viewed From Front Step 3, Star Test Collimation Insert the Secondary Spacer into the focuser. Now insert a low power 1-1/4 eyepiece into the Secondary Spacer (25mm- 40mm). Point the telescope at a bright star. Defocus the star until it becomes a donut looking ring. If the donut hole looks centered (in the center of the donut), proceed to the next step. If the donut hole is not centered, adjust the secondary mirror collimation screws so the defocused star moves in the direction of the fat side of the donut as shown in the following figure. Re-center the star and repeat this process until the donut hole looks centered as shown in figure 6.

6 Figure 5: Poorly Collimated and Defocused Star Figure 6: Collimated and Defocused Star

7 Step 4, Setting the Primary to Secondary Spacing: For your CDK17 optical system to perform as designed, the spacing between the primary mirror and secondary mirror should be, and can be set to an accuracy of +/- 1mm with a very reasonable effort! When the primary to secondary spacing is set correctly, the focal plane will be positioned at the designed distance behind the primary mirror. The special designed secondary spacer places the Ronchi screen of the Ronchi Ocular exactly at the designed focal plane. In order to set the primary to secondary distance, you will be moving the secondary mirror. By adjusting the secondary mirror distance, you will be moving the focal plane relative to that Ronchi screen. The goal is to adjust the secondary mirror to make the focal plane to coincide with the Ronchi screen on the Ronchi Eyepiece. When you have done this, you have accurately set the primary to secondary spacing. Once the collimation is roughly done and a bright star is centered, replace the low power eyepiece with the Ronchi Eyepiece. Rack the focuser all the way in, as far in as it goes, and make sure the Secondary Spacer is seated all the way into the focuser and the Ronchi Eyepiece is seated well in to the 1 1/4 adapter. If the Ronchi is nulled (you can see no lines, or one Ronchi line covers the entire aperture), see figure 8. This means that your spacing is already set so you can proceed to Step 5. If you see two or more lines you should move the secondary toward or away from the primary. Step 5: Adjusting the Secondary Spacing on the CDK17 Figure 7: Collimation Screws

8 The instruction used is specifically for the design described in the above picture, however the principle should be the same for other designs. 1. Loosen the four collimation screws. Loosen them in equal amounts to maintain your collimation. 2. Hold the secondary housing with one hand. This is to keep the housing from rotating. (The four screws sit in shallow detents and we would like them to go back in the detents when the procedure is complete). 3. Take a flat head screw driver and place the tip in the central bolt of the secondary assembly. Rotate the central bolt in a clockwise direction. For CDK17, move the secondary 0.4mm will move the focus 3mm. 4. Take the four collimation screws and rotate them in equal amounts until the assembly is tight again. 5. Check the Ronchi screen and repeat process as necessary until you get a desired null. To move the secondary away from the primary: 1. Loosen the four collimation knobs. Loosen them in equal amounts to maintain your collimation. 2. Hold the secondary housing with one hand. This is to keep the housing from rotating. (The four screws sit in shallow detents and we would like them to go back in the detents when the procedure is complete). 3. Take a flat head screw driver and place the tip in the central bolt of the secondary assembly. Rotate the central bolt in a counter-clockwise direction. For CDK17, move the secondary about 0.4mm will move the focus 3mm. 4. Take the four collimation knobs and rotate them in equal amounts until the assembly is tight again. 5. Check the Ronchi screen and repeat process as necessary until you get a desired null. Determining the Direction to Move the Secondary To determine which way to move the secondary, gently slide the Ronchi Ocular out of the spacer tube. If as you do this, the Ronchi reaches null, the focal plane is too far back and you need to move the secondary mirror away from the primary. If as you slide the Ronchi out of the spacer tube, the number of lines grows, the focal plane is too far forward and you need to move the secondary toward the primary. For every one unit the secondary spacing is changed on the CDK17, the focal plane moves 7.6 units. You want to keep the error at the focal plane to less than 5mm or just fewer than two lines on the Ronchi image, and it equals to an error in primary to secondary spacing of about +/- 0.66mm. The next several figures shows a series of Ronchi simulation for the CDK17 with the focal plane 0mm, 1mm, 2mm, 5mm and 10mm away from the Ronchi screen. Remember you want a better than 1mm Secondary-Primary spacing accuracy, or a less than 5mm focal plane accuracy for the CDK17.

9 Figure 8: The image on the left is what you will see if the primary to secondary spacing is perfect. The image on the right is what you will see if the focus is off by about 1mm, or the primary to secondary is off 0.13mm. Figure 9: The image on the left is what you will see if the focus is off by about 2 mm (or the primary to secondary is off 0.25 mm). The image on the right is what you will see if the focus is off by about 4mm (or the primary to secondary is off 0.51 mm).

10 Figure 10: The image on the left is what you will see if the focus is off by about 5mm (or the primary to secondary is off 0.66 mm). The image on the right is what you will see if the focus is off by about 10mm (or the primary to secondary is off 1.32mm). Step 6, Fine Collimation: Use a high power eyepiece (5mm or less) and follow the same procedure described in Step 3 (adjust the secondary collimation screws so that the donut hole is in the center of the donut). You may want to switch to low power when making adjustments and re-centering the star and back to high power to check the collimation. Finally, use the Ronchi eyepiece to recheck the spacing. As long as only small adjustments were made in Step 6, the spacing should still be fine. If it is not, repeat Step 5 and Step 6.