Increasing image scale ??

[beamish]

If one wanted to increase the scale of an object in the fov of a w/cam using a barlow, I've heard of people using an extension.

Q: Is this placed in front of the barlow+ w/cam or behind it , ie between the the barlow and w/cam ?

Ta, Karlo

[Ant]

Hi Karlo,

Basically the distance between the lens part of the barlow and the webcam chip if the important part.

If that distance is small then the increase in image scale is lower, if there is a lot of distance between the two then the image scale is increased.

So put the extension tube between the webcam and barlow.

Ant

[beamish]

Aw cheers Ant, think I'll try and try that tonight . God's permitting.

Karlo

[Ajohn]

Just incase some one fancies modifying a barlow / taking a hack saw to there scope etc.

The magnification a barlow gives is determined by how far it is inside the normal focus of the scope. The more inside the higher the magnification. The new focal plain tends to go in the opposite direction away from the normal focal plain. Extension tubes allow correct focus to be achieved with the barlow further inside the normal focus position. Just how far this can be pushed depends on the scopes focuser. Refractors often offer more leeway and the barlow can even be modified so that it will slide inside the focuser.

John

[beamish]

So John, if I understand you correctly. In my case with the cape, I normally need to use the supplied ext. tube to bring EP's to focus. If I were to place a barlow in front of this (ie between the crayford and the ext tube and EP/webcam) then the image scale would be greater.

It'd be easier with pics really wouldn't it.

CAPE 200mm F6 f/l 1200mm

Normal optical train using a w/cam:

Scope- crayford-ext.tube-barlow-w/cam....... results in disappointing image scale

Ant's suggestion:

scope-crayford-ext.tube-barlow-ext.tube-webcam

Your suggestion (?)

scope-crayford-barlow-ext.tube-(possibly another ext.tube)-webcam

I suppose I'm asking, cos when the skies are clear one doesn't wish to see/fritter it away faffing, however probably likely to give it a go anyway.

Karlo

[Ajohn]

It isn't possible to say without numbers so the best thing to do is try it terrestrially on something at least 40 to 50 times the focal length of the scope away. Further if you can manage it. 1 to 200yds will definitely be very close to the infinity focus.

Your biggest expansion will be scope - barlow - extensions - web cam. The question is whether the scope can focus like that - or if you have enough extensions - or if the barlow sticks into the tube which isn't much of a problem anyway.

If using the camera is a bit of a problem when trying it terrestrially it's possible to hold an eyepiece and position it so that the image is in focus. That can give you an idea where the focal plain is. On a plossl the focus of the eyepiece is very close to the outside of the inner lens. ie The one you can see if you look in the wrong end of it.

:oops: Your right about the prose - I re arranged it several times. Maybe I should have googled. This is a half decent explanation.

http://www.licha.de/astro_article_barlow_lens_projection.php

You might find the link at the bottom of the page of interest too - eyepiece project. Usually the best route to high levels of magnification.

John

[beamish]

Thanks John, I'll take a look at that.

As for EP projection ... too faffy for me but thanks anyway.

As an update : I tried just the 4x Imagemate + w/cam last night, it was the first time I'd seen Jupiter thru the Cape. On the K3 screen the image scale was actually rather good- which is more than can be said about the sky conds. It was bigger than I expected bearing in mind my experience of seeing Saturn with the same kit. Of course the problems then are tracking !! Because I've set it up at the top of the house -NO Polar alignment !!(it's the ONLY way I'll get to see the giant planet this year) So the bigger the image the less time I have available before it disappears off the the K3 screen unless I am prepared to guide manually !

Karlo