Cannot get in prime focus with barlow

[JCdeBoer]

Hi there,

Lately, I ve been able to take decent planetary shots using a webcam on my newtonian telescope in prime focus. Focussing went well as I could just get close enough to the tube. Because the images were quite small, I used a 3x barlow last night, but somehow I could not get any image on the screen. Not even an out of focus blob. Could my barlow possibly moved the focal point further inwards the tube or might it have been placed tens of cm's outward?

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[Cornelius Varley]

One possibility is that you didn't get the target onto the sensor. With a 3x barlow the FOV becomes very small and you could miss the target by a fraction and not know it. One thing you could try is to make a medium/high magnification eyepiece parfocal with the camera. Get the target into the centre FOV of the eyepiece and then swap the eyepiece for the camera. 

[Mr TamiyaCowboy]

the first time, the light cone coming into the focuser was just about

perfect, this light cone sits just right for the camera to see it.

the cone is also big enough to cover the whole sensor ( few mm x few mm big)

we pop a barlow in. now what we have to remember is the light cone is now 3x smaller

than it first was and it has now moved.

so pop the barlow in the focuser.

pop a 25mm eyepiece into the barlow, now center target.

quickly swap over to a 10mm eyepiece, and recenter again.

slip out the eyepiece and now replace it with your camera. 

you should notice on the screen the blackness, moving scope a little you will notice a brightish hue.

the brighter the hue gets the closer you are to the image/object.

keep moving scope till your object appears. now focus and there we go

barlowed at 3x with the camera

[Langy]

Another thing that I also find is that the light levels reduce. So if your exposure is spot on in Sharp Cap (if you are using that) without a barlow you are best to move it up to the max to find the object again.

Initially use the bright object for a rough focus as you can tell by the size, then reduce the exposure.

As Peter said the most probably cause is that it is not on the sensor, 3x Barlow is quite a big difference where I suspect the sensor pickup area then becomes three times smaller.

[Steve Ward]

And ramp up the settings in SharpCap , Gain and Exposure , the image at 3x is a lot dimmer and you could well miss it at the usual settings.

Whoops , too slow again  ... 

[Mr TamiyaCowboy]

its also handy to have a try at this in daylight. 

get used to swapping out eyepieces and centering the object. 

your correct on the light cone moving UP the focuser to, but this should not matter much.

[Steve Ward]

Sound advice there , a little mark on the focus tube at the 3x focus point will help get you in the ballpark next time out.

[JCdeBoer]

Thanks all.

I did indeed first use a 26mm lens on the barlow, then a 9mm and then popped in the camera. Normally, whenever my scope is out of focus but the target is centered, I do indeed see the bright blur and the secondary mirror which then disappears as soon as I get my focus right.

But with the barlow I got no light at all on my screen (yes, I thought about turning exposure and gain up and contrast down to compensate for less light). I even aimed the telescope at a big view filling steady object (illuminated crane a couple of km's away) but couldn't get anything on my screen. Maybe the ccd is just slightly off center which wasn't noticable in the wide cone without the barlow. Nevertheles I would think that the cone would become wider as I move further away from focus (though upside down).

Arg... It's probably something stupid . Will definately try again at daylight. Thanks again for the advice!

[JCdeBoer]

Tried the barlow and webcam at daylight and was able to get an image. The problem was a combination of a much dimmer image and very small range where the camera was (nearly) in focus. I now know the right distance on the focusser and should be able to get an image when I get time and the sky is clear. Again thanks for the advice!

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[Mr TamiyaCowboy]

Your welcome and we all look forward to seeing the images you 

and you webcam/scope produces.

maybe be worth waiting for the next moon to come round, nice and big so will give you a good marker in the sky

and should not be so dull. 

have fun and clear skys

[JCdeBoer]

This is the best I could get out of my xbox camera with 3x barlow and light pollution filter. I used this to protect my ccd from dust as I have no clear or ir filter. The only other filter i have is a moon filter which takes out way too much light for use with a barlow.

Seeing was quite poor and focus hard to get spot on. Had to process the image extensively to get any detail, but for given the conditions I guess its not bad. I figure the blue/green-ish ring is due to my achro barlow lens?

Sorry for the late reply but I ve been quite busy with my study and the weather has been very - lets say disappointing . Anyway, thanks again!

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