Televue Powermate on a 11" EdgeHD?

[AstroEd]

Since a Televue Power-mate is a corrected Barlow for aberrations I was wondering if there were issues using one on a EdgeHD's optics since they also are corrected optics? (Or am I mistaken). If there are issues what is the best matched Barlow and what magnification? mainly to be used to image Jupiter, Saturn and Mars.

[michael.h.f.wilkinson]

The PowerMates are corrected in the sense the the aberrations present in many Barlows are absent. They do not correct for coma or other aberrations of the scope. They should be fine on your Edge HD

[jambouk]

Here's an article by damian peachy on imaging jupiter. He says f/30 is ideal focal ratio. I guess you are at f/10, so a 2x powermate will give you f/20 plus a bit more. Other suggest f/30 is too much for most optics/cameras/UK skies.

http://www.damianpeach.com/images/articles/jup2011an/Jupiter%202011%20AN.pdf

James

[AstroEd]

Thank you both for your replies.

[michael.h.f.wilkinson]

The optimal focal ratio depends on the pixel size on the sensor. For DMK21/ASI130/SPC900 type sensors, F/25-F/30 is optimal (I prefer to use my 2.5x PowerMate on my F/10 SCT to my 3x Meade TeleXtender on many objects). The ASI120 has much smaller pixels, so has an optimal focal ratio closer to F/15-F/20.

[jambouk]

Michael,

Can you share the maths behind that please?

Thanks

James

[michael.h.f.wilkinson]

Michael,

Can you share the maths behind that please?

Thanks

James

This has to do with the Nyquist sampling theorem. Assuming diffraction-limited optics, a given focal ratio means that the diffraction pattern of  star has a given size in the image plane. For example, doubling the aperture while keeping the focal length identical halves the diffraction disc, but doubling the focal length magnifies it such that the diffraction pattern gets the same size as before. Thus, the size of the smallest details in the image plane depends on the focal ratio. This means that the highest spatial frequency in the image plane (in the Fourier transform of the image) is fixed by the focal ratio as well. To reconstruct the image accurately (without loss of detail) you need to sample at twice this frequency (Nyquist criterion) or just above. Put differently, the selected focal ratio should scale linearly with CCD or CMOS pixel size.

[jambouk]

Oh! Wish i'd never asked

Thanks.

Is there a graph somewhere to show the ideal relationship between the two parameters?

James

[michael.h.f.wilkinson]

Oh! Wish i'd never asked

Thanks.

Is there a graph somewhere to show the ideal relationship between the two parameters?

James

The graph is a simple straight line. The DMK21 sensor has 5.6 micron pixels and is optimal at F/25, the ASI 120 has 3.75 micron pixels so it is optimal at F/ (25 x 3.75/5.6) = F/16.75.

[jambouk]

Ah i see. Yeah, that makes sense. Plotting my canon 6d it would seem it would be optimal about f/30, so might have to give that a go.

Where does the formula come from for pixel size x 4.5 = optimum f/ ratio?

Regards

James

[michael.h.f.wilkinson]

Do not forget this is optimal for planetary imaging not for DSOs, where you want fast optics to grasp enough light per pixel to get sufficient signal to noise. On planets this is not such an issue. I personally would not use a DSLR for planets.

[AstroShomy]

I know this is an old topic, but could someone tell me if EdgeHD 11 + Televue 4x Powermate + ASI120MC-S is a good combination for planetary imaging? I am trying to decide which planetary camera to buy. If this is not an ideal combination, which would you recommend for this combination?

Thanks!

[jambouk]

I suspect a 4x powermate on a c11 with most cameras will be tough, unless you've got absolutely exceptional seeing which is very uncommon in the uk. But see what michael says if he re-reads this thread. Maybe drop him a message and ask him to contribute.

James

[Cornelius Varley]

I know this is an old topic, but could someone tell me if EdgeHD 11 + Televue 4x Powermate + ASI120MC-S is a good combination for planetary imaging? I am trying to decide which planetary camera to buy. If this is not an ideal combination, which would you recommend for this combination?

Thanks!

The optimal focal length for imaging with that camera / telescope requires a barlow / powermate of 2x - 2.5x power. At f40 (4x power) the image will become darker and diffused and you won't get any more detail in the image than at f25.

[jambouk]

I also find that with my crayford focuser, flip mirror etc that the focal length is longer than the native focal length of the scope, so even with a 2x powermate, i am much more than f/20.

James

[Grotemobile]

The most I used to use on my C11 was

a 2x barlow. 2800 x 4 gives a very small

fov.Also gain on the camera will need to

be right up to get a image.Just my findings.

I found that you were better off not using

a barlow with the C11.

Steve.

[AstroShomy]

Thank you all for your input. How would different pixel sizes affect this combination? Would I do better with a larger pixel sensor?

[jambouk]

At a focal length of 12000mm, the image of say jupiter is spread over a very large area. If you have massive pixels, the image of jupiter may land on say 50x50 pixels (i'm making the numbers up); if your sensor has very small pixels, with the same focal length the image of jupiter may be spread over 200x200 pixels... But then you can see that influences resolution, but also there are fewer phons per pixel with the small pixels... This is my understanding anyway.

James

[AstroShomy]

At a focal length of 12000mm, the image of say jupiter is spread over a very large area. If you have massive pixels, the image of jupiter may land on say 50x50 pixels (i'm making the numbers up); if your sensor has very small pixels, with the same focal length the image of jupiter may be spread over 200x200 pixels... But then you can see that influences resolution, but also there are fewer phons per pixel with the small pixels... This is my understanding anyway.

James

...meaning, in this case, the bigger the pixels, the better...