How long for objects to cross field of view?

[digitaldave]

Daft question, but how long does it take for an object to cross the field of view at:

1) 100x, 1 degree TFOV, 100 degree AFOV Ethos EP (this is what I get with my TV85 and a 6mm Ethos).

2) 200x, 0.5 degree TFOV, 100 degree AFOV Ethos EP (proposed 10" dob with 6mm Ethos, or the TV85 with 6mm Ethos and 2x barlow).

3) 300x, 0.33 degree TFOV, 100 degree AFOV Ethos EP (10" dob with 8mm Ethos and 2x barlow).

Thanks,

Dave.

[brianb]

The time it takes for an object to cross the centre of the field of view, when the object is on the celestial equator, is 4 minutes times the actual field of view. For an object not on the celestial equator, divide by the cosine of the declination.

e.g. actual field of view 0.5 degrees, declination of object 25 degrees (M45)

4 mins x 0.5 / 0.91 = 2.2 mins

actual field of view 0.3 degrees, declination of object 58 degrees (Double Cluster)

4 mins x 0.3 / 0.53 = 2.3 mins

But it's more usual to use the time to get the absolute field of view. Focal lengths of scopes and eyepieces are rarely exactly "as marked" and apparent fields of view are really little more than an approximation which can be seriously affected by distortion, especially with super-wide types.

[digitaldave]

Brian,

Thanks for that, it's very useful. The reason I'm asking is I don't currently have any high power EPs for my scope (max is 100x at the moment), and I want to get some idea of how often I will have to move my scope if I go up in power. Or should I just accept the fact that it'll be easier to get a tracking scope?

Dave.

[arad85]

Dobs are really easy to use with Ethos eyepieces. VERY natural once you get your head around which way your slight shove moves the image. I don't have any problems with my 13mm barlowed down to 6.5mm - you just nudge it along gently. Having said that, to be honest, the only time I use the Barlow is for planetary views and then you only have a small (and bright) object to keep in view.

[John]

Brian,

Thanks for that, it's very useful. The reason I'm asking is I don't currently have any high power EPs for my scope (max is 100x at the moment), and I want to get some idea of how often I will have to move my scope if I go up in power. Or should I just accept the fact that it'll be easier to get a tracking scope?

Dave.

I've mostly owned and used scopes on undriven alt-azimuth mounts so manual tracking is a must. It's one of the reasons that I use ultra-wide field eyepieces and that does help as you nudge a bit less often but, after a while, it becomes 2nd nature and you find yourself adjusting as the power increases.

I actually really enjoy finding and tracking objects manually - it's a really important part of the hobby for me and makes me feel more "in touch" with the universe as I view it.