"dobserving" and manual tracking

[kniclander]

when people talk about manual tracking with a dob are they saying that they move the scope very slowly so as to counterract the rotation of the earth and thereby keep the object in the middle of the FOV or are they saying that they let the object drift across the FOv and then move the scope back to get it in the FOV again (which is what I do)?

I cannot imagine doing the former as my Dob does not move smoothly enough and any movement of it causes little vibrations that impair the image (at high powers). Is it really possible to do it like that if you have a smooth enough mount and enough skill?

Also, if you have a driven mount is it smooth enough to actually track an object without any loss of visual image quality?

thanks

dan

[John]

I do as you do Dan - that's why I'm such a proponant of wide field eyepieces - you get a longer drift time to study the object

When I have had scopes that were driven I did enjoy being able to study an object at high power while the drive motors kept it in the field of view. The old fork mounted Celestron C8 that I had a couple of years ago was particularly good at this - I recall fabulous views of Saturn at around 300x with the planet rock steady in the centre of the FoV.

I've not personally noticed that drives introduce vibrations however I have read that some notice the "steps" of a stepper motor when observing at very high power.

John

[WillEyedOney]

I am in the process of fettling the horrible azimuth axis of my Skyliner 200P.

I have cut some washers from 4l plastic milk cartons and intend to do something similar to this to remedy the jumpiness in the movement.

If that doesn't work then I have my eye on a lazy susan bearing that might smooth things out a bit.

[John]

I am in the process of fettling the horrible azimuth axis of my Skyliner 200P.

I have cut some washers from 4l plastic milk cartons and intend to do something similar to this to remedy the jumpiness in the movement.

If that doesn't work then I have my eye on a lazy susan bearing that might smooth things out a bit.

I used to have a Skyliner 8" that was OK in movement right out of the box however a 10" version that I owned previously suffered from this and I found the milk carton washer trick did make a big difference - the trick is to find the right balance between the number of washers needed and tension on the azimuth axis bolt which you can only do by experimentation.

But it really does work

John

[arad85]

Also, if you have a driven mount is it smooth enough to actually track an object without any loss of visual image quality?

thanks

dan

I certainly don't notice any loss of visual quality due to the mount with the C9.25 at high mag (around 427x) on a CG-5.... Loss of quality due to seeing and transparency is another matter at that mag tho...

[Doc]

It all depends on what magnifiction you are using.

But I'll do it the same way as you and if I'm using a really high mag I'll try and keep it centered.

[Casus belli]

The 10" lightbridge is great for manual tracking at low to medium magnification so thats what I do. At higher magnification I use the second method

[Beulah]

With high mags I prefer an EP with a wide FOV....gives me an extra few seconds...

[Phil Fargaze]

I once noted in my log book that when I first managed to find /observe Neptune with my dob I tried using a 5mm orthoscopic eyepiece (240x) and it took 20 seconds for the planet to cross the fov. I would use the drift then nudge technique for that. I would nudge the object to where it just enters the fov, let it drift, which would give me steady observing time then nudge again to let it drift across the fov. I found that wide field eyepieces really do make a big difference though by increasing the drift time.

[Mutley]

I think the wide angle route is the way to go for dob owners. I got hold of my 1st Nagler a few weeks ago - a 16mm T5. I found it really makes a difference. I've hardly taken a plossl out of the case since.

[haitch]

What about coma in the widefields on fast dobs?

My only wide angle EP is a cheap 30mm 2" UWA 80 degree which does show significant coma in the last 1/4 of the fov on my F4.5 14" Dob (better than I expected when I bought it actually).

Do the premium EPs deal with this to some degree? Or is it less noticeable in say a 15mm UWA over a 30mm UWA? Or would I need a coma corrector to get reasonable views?

[PeterW]

What Naglers were made for, the reaon I got my 5mm one. Save on having to keep nudging the scope. Not sure about Coma, but it will depend on the focal ratio of the scope and can be counteracted by using a Paracorr unit.

Cheers

PEter

[Astronut]

What about coma in the widefields on fast dobs?

My only wide angle EP is a cheap 30mm 2" UWA 80 degree which does show significant coma in the last 1/4 of the fov on my F4.5 14" Dob (better than I expected when I bought it actually).

Do the premium EPs deal with this to some degree? Or is it less noticeable in say a 15mm UWA over a 30mm UWA? Or would I need a coma corrector to get reasonable views?

What you're probably seeing is not coma at all. I read somewhere (awhile back) that coma is often masked completely by aberrations from the eyepiece which can be 4x greater than coma.

AFAIK no eyepiece corrects for coma.

For instance, Televue eyepieces are tested with F/4 optics. If coma is the only aberration left then it's a 'pass'. You might be seeing spherical aberration which AFAIK means that while the central field area is focussed the outer edges are defocussed and vice versa.

Any fast reflector (classic newtonian design) will show coma in any eyepiece (even premiums) without a coma corrector eg. Paracorr.

[Doc]

Coma is introduced by the fast parabolic mirror not by the eyepiece. An eyepiece normally does not correct coma, but as far as I'm aware a few items do. You have a coma corrector of course and I think the Klee barlow also corrects. Then you have the 20mm Brandon Eyepiece and an eyepiece I've never seen but have read about the Pretoria eyepiece.

[Mutley]

I've also got one of the 30mm cheap UWA's that Haitch mentions. It is awful in my Dob (F4) - terrible coma and a pin-cushion effect that makes me feel sick. But the same eyepiece more than acceptable in my travel scope (F9 refractor). The Nagler give me a loveley flat field and barely discernable coma. Shame about the price tag though.