Vixen SP Polar Alignment in 2020

[MercianDabbler]

I've been meaning to get around to figuring this out. Having re-read the manual several times I don't think I fully understood how to do it in 1988 when I bought the scope and now there is the additional complication of Polaris having moved beyond the positions shown in the manual.

On the plus side I have an Android tablet and access to modern polar alignment apps but I can't say that I really understand how to use the two together.

I've been struggling to ask this question in a simple way and have failed, so apologies for that. Hopefully someone will stick with me...

The manual is somewhat terse in its descriptions...

Whaaaat??

So line the RA hours and minutes for now (in local, DST or UTC??) up with the current day and month... I think??

Here are the dials...

Skipping the straightforward stuff about levelling and uncovering the polar scope... finally the manual explains what to do with the reticle...

My longitude is 1.5 degrees west. Let's say it is tonight (23 Oct 2020) at 21:00 BST.

I have the option of using a tablet app. Most of these show a view that looks like the reticle, but my reticle was designed for the position of Polaris in 1986.

It seems to me that this is solveable but I'm unsure which bits of the SP instructions are redundant if I use the tablet app.

• The tablet app on its own doesn't seem enough because I still need an accurate way to transfer the setting from the picture on the tablet to my alignment scope.
• One app (PolarFinder) provides a 'HA' number (I guess this is the Hour Angle for Polaris) and a 'P.Scope' number (no idea!) but the help suggests transferring the alignment by eye and does not explain what to do with these numbers.
• Can I rely on 'up' on the tablet being the same as 'up' in the scope?
• The big circle in the SP reticle is (if I understand the diagram correctly) 48 minutes of arc in radius, which isn't right for 2020. I'm guessing that the tablet app shows a circle that is correct for 2020. I think this should be 39 minutes radius given that the decl of Polaris now is +89° 20' 56.9"
• The change in Polaris position shown in the manual seems rather more than other sources suggest - I've read 1 minute in 5 years. Is the manual wrong?

Another option (I hope!!) may be to just use the hour angle for Polaris from Stellarium but I'm unclear how to make this work using the scope dials.

I'm hoping that I can figure out how to position the reticle so that Polaris should be exactly on the line towards the smaller circle but it seems to me that if I follow the instructions to the letter then that would have happened in 1986 but it won't work in 2020. Some bits of the instructions need to be ignored, but which bits?

Then if I get Polaris onto the line I just need to judge where 39 minutes radius would be.

Thanks in advance.

[MercianDabbler]

Answering my own question

I think I've mostly figured out a way to do it now...

1. Look up the RA of Polaris now

2. Look up the RA of Polaris in 1986

3. Follow the Vixen instructions but line the date up with a time on the RA dial equal to UCT less the difference between 1 and 2. The fact that I'm in the UK helps here because I really can't get my head around the longitude part and how you are supposed to dial in the time for other time zones.

5. I have set the dial on the eyepiece to 1.5 degrees West.

6. I think that should put Polaris exactly on the line. The reticle has a bit more than the manual suggests, including a graduation at 40 minutes of arc, so judging 39 minutes is pretty easy.

7. Double check that the view in the reticle roughly matches what the tablet app shows (so far it seems to work!)

Does that seem right?

I have some coordinates for Polaris in 1986 but I'm not sure how accurate they are. Does anyone know a good source for this info?

[Froeng]

Hi MercianDabbler,

Only just saw this thread. I updated my SP-DX with this:

https://astrogarten-shop.de/en/accessories/mounts-tripods/optional-parts-accessories/vixen-polar-axis-plate-year-2005-2025.html
it works perfectly - at least for another 5 years!

You don’t really have to worry about the timezone correction as you are only 1.5 degrees west of Greenwich. Greenwich is the central meridian for UT, so you could set the adjustment scale to 1.5deg West ( the white line on the polar scope against the E/W scale) but I don’t think this is possible accurately. The scale is simply too small. I used to live at around 10deg E (Hamburg), for which the central meridian for the Central European Timezone CET is at 15deg E. So I had to adjust (counter-intuitively) to 5deg WEST.

Hope this helps

Frank

[MercianDabbler]

Thanks Frank. So far my technique seems to be working and agrees with the tablet app but I'm only using short exposures.

Maybe I'll wait to see if there is a new version of the plate one day... that one has 3/4 of its useful life behind it. I'm quite glad that my longitude is simple because... as you say the way to set it for other places can be very counter intuitive.