Takahasi Polar Scope

[montewilson]

This question is to anyone who has used the inbuilt Takahashi polar scope. In particular I would be interested hear from Mike Sidonio as I know he uses one regularly on the NJP.

I have an EM-200 which I guess uses a similar polar scope. I tried using it once at Ilford with no luck. Sure I read the instructions but obviously I got something wrong.

My guess is that maybe I got the wrong star for the Simga Octans or I am making a mistake on the time zone offset.

Sigma Octans is a slightly yellow star isn't it? The others in the field are much bluer and fainter.

Ilford is at 149 46' E deg so from what I can tell that places it .25 deg behind the mid point of our time zone. That means I should be offsetting the time zone mius .25 deg. Is that right? Or is is plus .25 deg?

When it works how good is it? I have problems with aligning, in so much as an STL-11000 and FSQ produce a big field and therefore field rotation is a constant issue on long exposures.

If you get it right with the Polar Scope do you have to do any further drift aligning afterwards?

I have tried to train with it in Sydney but the light pollution makes it impossible to see the Sg Oct in the scope so I can only test it in the country and that is not often enough to be proficient with it.

Regards

Monte

[Dennis]

Hi Monte

I believe the NJP PAS is somewhat different from the EM200 PAS. Like you, I found the EM200 Tak manual somewhat terse and testing to read and understand.

For Brisbane at 152° E, I set the offset on the bottom S scale to +2 as the Time Zone Meridian is 150° for Brisbane.

My EM200 has an N hemisphere PAS reticule.
Month/Days run 1 to 12 Clockwise (CW).
Time runs 00 to 23:59 Counter-Clockwise (CCW).

I wrote an MS Excel spreadsheet that uses the current Computer System Date and converts the S hemisphere Month/Day (CCW) to the equivalent N hemisphere (CW) PAS reticule value.
The same spreadsheet also uses the current Computer System Time and converts that S hemisphere Time (CW) to the equivalent N hemisphere (CCW) PAS reticule value.

So, when I fire up the Notebook and open the spreadsheet, the XLS file runs minimized to just display the red window shown in the attached screen print and I can immediately see what values I need to dial into the PAS reticule. Pressing F9 refreshes the spreadsheet with the current time.

I can e-mail you a copy if you would like to use it. I've attached a couple of screen prints from Starry Night Pro that show the EM200 PAS reticule and Sigma Octans, one with the 4 main stars “joined” in a trapezium and an overlay of the Tak Pas reticule and the star chart.

Cheers

Dennis

[montewilson]

Dear Dennis,
Thank you so much for the reply. It is a huge help. This information is basically impossible to come by through official channels and I have otherwise been rather dissapointed with the results with the PAS.

What I find most amazing from your reply is that apart from the reversal of the dates for the Southern Hemisphere on that Tak chart is that the time also has to be reversed, but they don't tell you that in the instruction manual! Its is obvious when I think about it but then again I am in the habit of trusting instructions.

That would go a long way to explaining why I never got the damn thing to align.

Would you be so kind as to send me the .xls?

my email is montewilson at bigpond dot com

This will be a huge help.

Regards

Monte

[montewilson]

Hi Dennis - I got the email and it makes perfect sense. many thanks. Can you tell me, when you use it correctly how good is it? They claim it will work to a couple of minutes of arc. Is that your experience?

Regards

Monte

[Dennis]

Hi Monte

I’ve not measured any drift due to Polar Alignment errors. However, here is a two pane composite image I took recently, showing some offset between the first (red) image (03:15am) and the final (blue) image (05:11am) some 2 hours overall.

The EM200 was only polar aligned with the PAS, using the XLS spreadsheet to provide the alignment data.

The FOV is approximately 26x17 arc mins.
On my screen, I measure the 17 arc min dimension to be approx 130mm.
The difference in registration of the stars over 2 hours is approx 5mm on the same screen.
So, in 2 hours, the drift = (5x17)/130 = 0.65 arc min.
Hmm, that sounds too good to be true though?

Cheers

Dennis

[montewilson]

Dear Dennis - Thank you for the informative example. Is this image a whole frame from the camera? I ask because what I see looks more like some kind of linear drift. I would have expected to see a more circular rotation (is there an other kind? ha ha).

My field of view with the STL and FSQ is about 7.5 times what you have so FR is a big issue. I can get away with 3" of movement before the CCD will register it so I am well boxed in with this issue especially in exposures of 15 to 20 mins.

The example tells me I can expect about 40" of rotation per 15 mins but this is related to the declination too.

Regards

Monte

[Dennis]

Hi Monte

Yes - this is a whole frame from the ST7, 765 x 510 pixels. CCDSoft tells me that the plate scale is 2.02 arc sec / pixel.

Cheers

Dennis

[montewilson]

Dear Dennis - I think what you have in that picture is some kind of shift between the guide scope and the imager. Is that possible with your set up?

All the stars have the same offset angle from their original positions so in my book that isn't field rotation. If it were we should see a rotation of the images as if there were a pin in a middle of the frame and one image had been rotated slightly with respect to the other.

I hope you see where I'm coming from.

[Dennis]

Hi Monte

Yes - I was wondering that myself when I was processing the 50 frames spanning 2 hours. The stars appeared acceptably round in each frame, so initially I figured it may not be shift or flexure, as I understand those fault modes tend to produce trailed stars?

However, at an imaging focal length of 918mm and 2 minute sub-frames, maybe the flexure or shift is below a certain threshold?

This auto guiding is not as easy as it appears eh!

Cheers

Dennis

[montewilson]

Hi Dennis - I guess its up to me to check itout myself. Flex could be your culprit.

[Dennis]

Hi Monte

JohnH had a recent post describing some of the problems he has encountered with auto guiding, which was quite useful, although he appeared to be tearing his hair out trying to fix them!

Shortly I'll be attempting to auto guide at the native F12 of my Tak 180 Mewlon (2160mm fl) so I find all these posts very interesting and helpful when discussing the various approaches, configurations and then sharing the analysis of the results.

Cheers

Dennis

[montewilson]

Hi Dennis - You can see my set up at http://www.iceinspace.com.au/forum/showthread.php?t=65 somewhere down the page there are a few pictures.

The FS-60c is now being held with some proper guide scope rings so I am hoping I will have no flex.

My STV is used with the FS-60c at 355mm so it will give me 5.2"/px this is more than enough to give me suitable guiding for the FSQ at 530mm FL which has an image scale of 3.5"/px

My quick calculations tell me at 2' error on the PAS I should be seeing a field rotation of about 15" at the edge of the FSQ frame. I will do it more accurately later but that seems workable. It is even better if I can get closer than 2' by drift aligning.

[Dennis]

Hi Monte

A very nice set up - excellent cable control and routing! What are the pod shaped bits attached to the dew shield by the dew heater strip?

Cheers

Dennis

[montewilson]

They're scuba diving weights.

Go to http://astronomy.proem.com.au

and you'll see a bit more about my scope. There are some pix from the STV but I haven't as yet got around to putting up some of my STL pictures yet. Will hopefully have some soon.