Takahashi NJP RA backlash

Last night I was doing some imaging in my backyard observatory and came to some realizations: my RA backlash needs to be adjusted. After doing a drift alignment via PHD2 guiding with a polar alignment error of only 20 arcseconds, I should be able to get long subs. The NJP mount has been regarded as having extremely low periodic error and I have found that it does track and guide well, but every once in a while my tracking would skip. My image looked just like that, a skip or jump in tracking. I did a 5 minute exposure on M97 and it had this jumped star image. How frustrating! Well I knew something had to be up and remembered if you keep the mount East side heavy, then it would put weight on the gear and track more smoothly. I did this and sure enough! Did a series of 35- 5 minute exposures (unguided) with perfect tracking. So that makes me believe my RA backlash isnt set properly and/or I need to always balance the mount east side heavy. I found this link here to perform this procedure: http://www.trutek-uk.com/takahashi/njp_RAadjust.htm

So hopefully this well help and I have learned more about this mount. I havent used this blog in a while, but I will try to keep it more updated with my astronomy adventures!

Vixen VC200L Ronchi Experiments

The VC200L is a Cassegrain reflector with 3 corrective elements inside the baffle tube. The telescope is suppose to be free of coma, spherical aberration, and field curvature and is therefore a good telescope for astrophotography (has a 42mm imaging circle). There is not much information on the web about the optical design of this telescope, so I decided to test the mirror out myself. From my research the secondary is spherical, primary has an “aspherical” surface applied with VISAC coating technology, and the 3 corrective elements are suppose to fix for coma and probably flatten the field. The optical design characteristics because of the weak curve of the secondary, should make it inherently flatter than a SCT or RC telescope. So this telescope appears to be of the CDK optical design (Corrected Dall-Kirkham).

I removed the secondary/spider assembly from the front of the tube to clean off the secondary. I used my Ronchi tester and took some pictures. I have ground and polished telescope mirrors before successfully so a quick peek with a Ronchi tester tells me what kind of curves I am working with.

Using Ronchi for Windows V2.0, I compare the ronchigram to a theoretical calculation for the K value. My K value of -0.559 was calculated using Cassegrain Calculator. The animated GIF image below shows that the primary mirror is not fully corrected and the actual K value is actually closer to about -0.4 A more quantitative testing using a Couder Mask and Foucault Reduction software would probably lead to more accurate values, but I have figured mirrors in the past using the comparative Ronchi method with fairly accurate results (polished a 1/13 wave mirror that was professionally tested).

The coating on my VC200L is not the best and I know it costs a bundle to ship to Japan to apply the VISAC coating that turns the spherical primary to an aspherical figure, but I would probably try to polish in the elliptical figure into the primary if I ever need to recoat. I just cant justify spending $700-$800 for a recoat, when I can polish in the proper figure for much less than that myself.

General parameters for Dall-Kirkham
Primary mirror diameter 200.0000 mm
Primary mirror focal length 692.0000 mm
System focal length 1800.0000 mm
Back focal length 240.0000 mm
Distance secondary from system focus 673.1942 mm
Distance secondary from prime focus 258.8057 mm
Distance secondary from primary 433.1942 mm
Radius of curvature of secondary mirror 840.8852 mm
Primary mirror conic constant -0.5587
Secondary mirror conic constant 0.0000
Extension factor 2.6011