Roundtable Platforms

[Canopus Lim]

Ok...so the 3 dob guys in Singastro (me, Rlow and ACC) have received our tracking platforms for our dobs. ACC has not collected it yet..

I only managed to test the platform 2 times and I am still tweaking my rough polar alignment and vibration issues. The roundtable (.... actually for Rlow and I..it became square table.. ), is single axis and has to be polar aligned. I am a newbie in polar alignment, and my 2nd night I managed to only keep Jupiter in the 70 percent of the field (as I ony looked within 70 percent of the field as eyepieces and telescope performs better) in my 5mm Baader Ortho (320x) for about 2 min. The true field (at 70 percent of it the ortho's true field) is probably 6 arc min; Hence 2min for 6 arc min field is that good rough polar alignment? Anyway, 2 min is many times longer than without the platform as it takes less than 20 seconds to go out of the field and I need to adjust the scope. I do find it easier to see details with the platform as the object does not move too fast and it can stay in the sweet spot (around the centre of field) much longer..

The roundtable platform has no way (practically speaking) of adjusting its position once the telescope is placed on it. There is a risk of toppling the telescope if I were to push the platform with the scope on it and there is no way to adjust the leveling pads with the telescope on. Hence, the initial compass pointing and leveling at 1 degree has to be accurate.

Any ideas of how to do a better rough polar alignment? I hope to get at least 5min of viewing time for my 6 arc min field (with the 5mm ortho). Do you guys point exactly north or take into account of magnetic declination?

Here are some pictures:

1. The platform and its speed controller
http://img259.imageshack.us/img259/16/img0246pg5.jpg

2. 12.5" Obsession dob with the platform. My dob mirror box is way smaller than the platform....
http://img228.imageshack.us/img228/5015/img0244bh7.jpg

3. Front View
http://img383.imageshack.us/img383/5898/img0247or8.jpg

[Tachyon]

Actually, it looked pretty scary - like it's about to topple over!

[rcj]

is this from Johnsonian?

[kayheem]

Hence, the initial compass pointing and leveling at 1 degree has to be accurate.

Any ideas of how to do a better rough polar alignment? I hope to get at least 5min of viewing time for my 6 arc min field (with the 5mm ortho). Do you guys point exactly north or take into account of magnetic declination?

Hi,

How do you do you align to the north? This may be affected if the compass is put too near the motors as the magnets inside them may affect the accuracy.

I think the magnetic declination in Singapore is pretty small and does not vary by a lot. I think it is roughly 0.1 deg declination.

Kay Heem

[Canopus Lim]

Tachyon,

It will not topple even at max travel. The platform is for the equator so it is slightly inclined only. I have tried it without the friction pads and it will not topple. There is no fear of it toppling on its own.

Kayheem,

The magnetic compass is placed far away from the motor so I do not think it will be affected.

To ALL,

This post is just to ask about suggestion on how to do a better rough polar alignment. How good the alignment depends on how I align it. That is why I am asking for suggestions on how do you guys do better rough polar alignment with your scopes. For example, how accurate do you point north with reference to the magnetic compass and do you guys include magnetic declination, or how accurate is your leveling of the tripod and placing the GEM at 1 degree lattitude? When you guys have done that, how good is that alignment already? I have stated 6 arc min field for 2 min doing a rough polar alignment. Is that considered good or good enough?

This is especially so since I am a newbie in polar alignment. That is why I have asked these questions.

This is NOT a review. Like all things, there will be needed tweaks. If you want reviews of Roundtable platforms you can find them in Cloudy Nights.

http://www.cloudynights.com/item.php?item_id=1097

[rcj]

Hi Canopus,

This may or may not be relevant to you but polar aligning of any typical GEM in my experience involves performing the drift alignment of the mount after alignment to compass north (aka magnetic north). I would notice that the drift alignment would eventually make corrections to the initial compass alignment that are variable during different parts of the year, but is slight, and how much time you spend on the drift alignment procedure would depend on how accurate you need the mount to be aligned. Perhaps doing visual observations at powers below 200X would not be as critical as say magnifications above 300-400X, where the object would move quicker out of view. My current typical drift alignment accuracy is 3min (no drifting) at 250X. Last time, I used to spend more time (5 minutes no drift at 300X) drift aligning. It would usually not take more than 10 minutes nowadays, especially on concrete ground which makes the job easier.

[weixing]

Hi,

The magnetic compass is placed far away from the motor so I do not think it will be affected.

You'll be surprise how easily the compass will be affected by it surrounding including hand phone and counter-weight.

For example, how accurate do you point north with reference to the magnetic compass and do you guys include magnetic declination, or how accurate is your leveling of the tripod and placing the GEM at 1 degree lattitude?

Just level and point to north as accurate as possible lor... Not practical to include the magnetic declination because the compass itself got an error which needed to be calibrated... adding the magnetic declination without calibrating the compass might make thing worst.

I have stated 6 arc min field for 2 min doing a rough polar alignment. Is that considered good or good enough?

Err... I think that's depend on you... unless you are going for rough polar alignment competition... ha ha ha Anyway, it's definitely good enough for me.

Have a nice day.

[vhuang168]

I'm confused. I've never really looked at doing a tracking platform for my dob so never really thought about it.

Does the tracking platform turn your azi axis into the RA axis? If so, doesn't it mean your mirror box should be almost turned on its side since the center rotational axis of the azi needs to be pointed at the celestial pole?

I'm equating the tracking platform to wedges used by fork mounted SCTs for AP.

Vincent

[rcj]

Now I am confused too, for the same reasons as Vincent has mentioned. Looking at Brian Reed's website FAQ:
http://www.roundtableplatforms.com/wst_page3.html
however, it does not mention much about the usable latitude but it does say a plus-minus 5 degree latitude range of adjustment.

I cannot access Johnsonian website.

Nevertheless, it would be really great to see this work here! (and open up more purchasing devils )

[weixing]

Hi,

however, it does not mention much about the usable latitude but it does say a plus-minus 5 degree latitude range of adjustment.

I think it mean that the platform can be used +/-5 degrees of it set latitude... so if you order the platform to be used in Singapore which is around 1 degree North, you can still use it in latitude from 6 degree North to 4 degree South.

So I think the platform can be use on any latitude, but the maximum tracking duration is limited.

Have a nice day.