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Barlow for asi224 at f/6


sorrimen

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Hi there

Just got an asi224 for my 8” manual dob (far from the norm but have got decent results as below). So far I’ve been using a 2x barlow, but with my limited experience and knowledge I think I should be imaging at around f/18 instead. A 3x barlow would be perfect for this. Is all of this correct? I’m aware that tracking would be harder, but with 2x I’m only dropping ~10% of frames from lack of object in frame (with a fairly small ROI) so I think it would be manageable.

If I am correct, are there any budgety 3x barlows that would be acceptable? Not looking to spend much more than £50. 

TIA

Ross

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6 minutes ago, sorrimen said:

If I am correct, are there any budgety 3x barlows that would be acceptable? Not looking to spend much more than £50. 

Ideal F/ratio for ASI224 is about F/15 - this means x2.5 barlow.

You don't need to spend any money - your x2 barlow can operate as x2.5 or anything in between (and possibly in range of x1.5 - up to x3 or more).

Magnification that barlow lens provides depends on where it is placed with respect to focal plane - which in this case translates into sensor - barlow distance.

Larger this distance - larger magnification factor.

All you need to do is to tune this distance to get exact magnification you want.

This is easy if you have few extension tubes - or best, variable extension, but you can also play with how deep you insert camera nose piece into barlow body.

To determine optimum distance - procedure is quite easy and can be done during daytime. Aim telescope at a distant object - like church tower or bridge or tall building. Something that you can measure in pixels.

Take image of this object without barlow and measure some length on it (height or bridge span or whatever). Now, insert barlow and camera into it and adjust distance between barlow element and sensor and measure same length on object - adjust distance between barlow element and sensor until you get x2.5 larger size of object compared to reference (shot without barlow).

 

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1 hour ago, vlaiv said:

Ideal F/ratio for ASI224 is about F/15 - this means x2.5 barlow.

You don't need to spend any money - your x2 barlow can operate as x2.5 or anything in between (and possibly in range of x1.5 - up to x3 or more).

Magnification that barlow lens provides depends on where it is placed with respect to focal plane - which in this case translates into sensor - barlow distance.

Larger this distance - larger magnification factor.

All you need to do is to tune this distance to get exact magnification you want.

This is easy if you have few extension tubes - or best, variable extension, but you can also play with how deep you insert camera nose piece into barlow body.

To determine optimum distance - procedure is quite easy and can be done during daytime. Aim telescope at a distant object - like church tower or bridge or tall building. Something that you can measure in pixels.

Take image of this object without barlow and measure some length on it (height or bridge span or whatever). Now, insert barlow and camera into it and adjust distance between barlow element and sensor and measure same length on object - adjust distance between barlow element and sensor until you get x2.5 larger size of object compared to reference (shot without barlow).

 

Oh wow, I never realise any of this. This has been a great help and I’ll try it all out. Thanks so much!

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Nice results, really nice considering you used a manual dob.

If you know the focal length of the Barlow (sometimes published by the manufacturer) the barlow power can be calculated by:

Power = (barlow FL + distance)/ barlow FL

The distance is from the lens of the barlow to the camera sensor. If you don’t know the barlow focal length you can calculate it by measuring the distance from the lens to the top surface of the barlow and rearranging the formula above using the native power factor of the barlow.

Knowing this you can then calculate how much extension you need to get to whatever power you want.

Edited by CraigT82
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