Quality Barlow?

[Aenima]

The focal ratio in planetary photography must be matched to the pixel size of the CCD to capture all possible detail (technically, you want to sample the image at the Nyquist frequency). Planets have high surface brightness, so being slow is not really an issue.

Many thanks, Michael. This kind of information really helps a noob like me to understand a problem rather than just following advice without knowing why. I have been advised before that the atmosphere is causing a lens rainbow effect and now you've reenforced the idea, I can approach the issues a little better, thank you.

CCD pixel size and 'Nyquist' is a little beyond my range at the moment - any basics you think may help, I'd be grateful for the info? - but I really liked the look of AE imagemate 4x but wanted assured quality rather than strangth and therefore went with x2 but now maybe not.

Thanks again,

Regards

Jay

[michael.h.f.wilkinson]

Many thanks, Michael. This kind of information really helps a noob like me to understand a problem rather than just following advice without knowing why. I have been advised before that the atmosphere is causing a lens rainbow effect and now you've reenforced the idea, I can approach the issues a little better, thank you.

CCD pixel size and 'Nyquist' is a little beyond my range at the moment - any basics you think may help, I'd be grateful for the info? - but I really liked the look of AE imagemate 4x but wanted assured quality rather than strangth and therefore went with x2 but now maybe not.

Thanks again,

Regards

Jay

The Nyquist frequency is actually a simple concept, though it seems complex at first. Think of an image of a simple grating of a certain spacing between two consecutive dark (or light) lines, e.g. 0.5mm. The spatial frequency is simply the inverse of that number (i.e. two per mm, or 2 mm^-1). Suppose this image is projected 1:1 onto your CCD. If I want to resolve a grating in the image, I will need at least one pixel for the dark lines, and one for the light, i.e. in our example we need 4 pixels per mm. If I have 200 lines per mm, I will need 400 pixels per mm. The Nyquist frequency is simply twice the frequency of the most closely spaced features you can still resolve.

So why is this related to the focal ratio?

First of all, when talking about the resolution of scopes, we are talking angles, not mm. Assume a scope can split two stars at an angle a = 1" apart. the distance d between the two point images in the image plane depends on the focal length f of the scope:

d = f tan a

This can be approximated to

d = f a

if we express a in radians rather than degrees (360 degrees = 2 pi radians).

Pixels will have to be spaced by half the distance d due to the reasons given above.

The diffraction limit is proportional to the aperture of your scope: ideally, for an 8" scope the minimum angle a is half that of a 4" scope. Thus, for a fixed focal length, the distance d is halved. However, if I double the focal length, I get the same value for d. Thus, for a fixed focal ratio, d stays the same.

This means that if you know the size of pixels on a chip, you can compute the required focal ratio to ensure you have pixels spaced in such a way in the image plane, that the finest detail is captured.

Hope this helps.

[Aenima]

The Nyquist frequency is actually a simple concept, though it seems complex at first.

Wow, thats a lot of info.

I appreciate your intention to help, but I dont really follow math very well. Unfortunately not much of the above made sense to me. Duh.

I wonder if you could put it into laymans terms?

My grasp of algebra and such is really quite small, sorry, else I would have learned much from your post.

Pretend I'm a ten year old

Regards

Aenima

[michael.h.f.wilkinson]

I will post a better explanation with graphs tomorrow.

[Aenima]

I will post a better explanation with graphs tomorrow.

That would be great, thank you!

Although I must stress that I was probably underestimating my lack of math skills at best, I genuinely didn't understand the formulas and algebra, which for me is a shame as I would love to understand planetary imaging better.

I appreciate your time spent putting it down in a post for me, though.

Thanks again,

Jay

[JamesF]

I wrote up some ideas on planetary imaging that might complement what Michael has said here:

http://www.tanstaafl.co.uk/2012/03/focal-ratio-capture-length-planetary-imaging/

James

[Aenima]

Cheers James.

I will check the link out in a mo whilst having a well earned coffee.

Ta.

Jay

[Merlin]

I have the Tal X2 and X3 Barlows. They are very good, but small. Eyepieces with focal lengths greater than about 15mm will be vignetted.

[Aenima]

Thanks,

A lot of people like the Tal's and I'm guessing they have good reason. Problem is, when I first asked about a quality barlow I was fairly sure it was completely necassary for CA correction. Now, after a few knowledgable members posted about atmospheric diffraction causing a lot of my CA, I'm hoping my planetary imaging can benefit in a slightly different choice of barlow - specifically a AE Imagemate x4 - as my set-up could use a bigger barlow and wont be gaining much relief from CA that probably isnt barlow related to begin with.

Anyway, I'm really grateful for the many helpful posts and comments, I also learned stuff I wouldn't have otherwise so if anyone has an Imagemate 4x, let me know how they are.

Regards

Jay

[JamesF]

If you're using the ImageMate in the 200P then you may well be fine, but in the 130P you may well find there are few targets that are bright enough for a good image. Over time I've collected a range of barlows of different powers and tend to swap them around to find the best combination on the night for the target I'm interested in.

You may also find that it's quite a struggle to get the image on the camera sensor at 4x focal length. Probably the easiest way is to get the image in a 10mm eyepiece and centre it, then put in the 2x barlow and re-centre. Then the ImageMate and re-centre once more. Then swap to your 4mm ep and centre once more before swapping to the camera. Don't worry about focus with the eyepieces as long as you can tell where the target is. It's actually quite tricky to tell when a target is centred within the margin of error required to get it on the camera and if all else fails an illuminated reticle eyepiece works wonders.

James

[Aenima]

Yes, those ones that allow you to view whilst cam is still attached? I have looked more than once at those, the idea is very tempting.

Thank you for your advice,

Jay.

PS I usually get the object centred in at least one plane viewed through finderscope, then swap around the bigger to smaller ep's until just before the webcam is in, then check that the object aligns along at least one axis and slowly sweep that slo-mo controller to locate it on the camera. There are probably better ways......

PPS yes this is for webcam in 200p

[michael.h.f.wilkinson]

It is a struggle to get an image on the sensor of the webcam. I got a Vixen flip mirror, which allows me to centre the planet in the EP, and simply flip the mirror and refocus slightly. A flip mirror is fine for refractors, Maks and SCTs, but will not work for most Newtonians, because the focal point is too close to the tube.

[Aenima]

Hi

I took another look at the Nyquist thing and honestly find it a bit too mathematical for my understanding, which is embarassing really, but true so i'm hoping that if I wrote the details down regarding my webcam and scope then maybe I could just get some advice as to which barlow to get and how best to use my webcam........?

Hoping so.

Webcam- The philips spc880nc flashed to spc900nc.

Scope- SW F/5 200mm Explorer

Targets- Saturn, Jupiter, Mars, Venus, Moon and our Sun.

Software to capture is Sharpcap - not sure exactly which settings though, if anyone knows anything of these details, drop a post in.

Much appreciate any info..

Regards

Aenima

[JamesF]

Have a go with the kit you have now and see how you get on. The practice is always valuable. See how it goes with Jupiter or try on the moon. Do some runs capturing without the barlow, some with and so on. If you get some colour alignment issues see if they can be sorted out during processing. Times for imaging runs on the planets I've put on the same web page I referenced in my previous posting. If you look at the planetary images in the site then I've also posted the settings I used in SharpCap with the SPC. Those are all with a 127 Mak though. With the 200P you may well be able to back off on the gain and/or exposure because of the much increased aperture.

And if you're struggling with processing, post what you've got and people will help.

James

[Aenima]

Nice1 James, thank you.

Will check back on your link for sharcap settings.

Not really much to practice on in this weather but soon as its clear I will be out.

Cheers

Jay

[Solar_observer]

I believe The tals are The most popular but if I can't get hold of one, which would be a good alternative x? Budget around £40

tony

[John]

I recently had a Revelation 2.5x barlow for a while and thought that was a nice barlow for it's price. Telescope House sell them:

http://www.telescopehouse.com/acatalog/Revelation_Astro_2_5x_Barlow_Lens.html

I have owned the TAL and other excellent barlows in the past and thought the Revelation did pretty well.

[Solar_observer]

I was looking at this one but with my workings out, this would give me an 8mm option which I already have. I was looking at more The x2 Barlow. Scope n skies do a revelation x2 & x1.5, anyone have any experience with this Barlow?

tony

[KaStern_Former_Member]

Hello Anaenima,

I'm currently finding issues with my ep's and barlows regarding a colour aberration - namely white light showing coloured edges, redish one side, kind of through a spectrum, to greeny blue the other side (of a star, or Venus etc) of the usual whiteish object.

I have not read the whole thread. But what you tell might be atmospheric refraction.

This appears to be most prominent on objects low above the horizon, but can be seen even on objects up to 50° or so.

Blue is stronger refracted, red lesser, and the other colours are in between of them.

You can cure this by using a wedged glass in front of your objective.

Cheers, Karsten .

[Aenima]

Actually yes, karsten, quite a lot of what I was seeing was the atmosphere, I just hadn't noticed until - like you say - the object was low in the sky, but it became worse through my cheap barlow and I assumed that was my problem.

Truthfully, though, I do need a decent barlow, but this atmospheric refraction can be drastically reduced in registax rgb align and so then I just got hold of a bog standard barlow and use software to fix the colour issue.

but when I do buy a new one - tal x2 seems the winner right now.

Thank you for your explanation,

Regards

Aenima

[michael.h.f.wilkinson]

The essence of the Nyquist theorem is that you want to match the physical size of your pixels on the CCD to the size of the Airy disk (or seeing disk for big scopes) of your scope. The size of the Airy disk in arcseconds depends on the aperture of the scope. The size in microns depends on the size in arcseconds and the focal length of the scope. In practice, this means the size of the seeing disk on the CCD depends on the focal ratio of the scope.

Simple rule of thumb for SPC900 cameras, or any with the same pixel size, means somewhere between F/20 to F/30 is best, depending on conditions, and the surface brightness of the object. More does not give more detail, and lowers the intensity of the image (increasing photon noise). Less means that you lose detail. I use F/30 on Mars, and F/20 on Jupiter and Saturn. I find F/20 perhaps a little short on Jupiuter, which is why I recently got a 2.5x PowerMate, so I can try F/25 for Jupiter.

[Aenima]

Hmm. I kinda get the idea, but some of the maths I lose the thread. Its easier when I think of the F/ numbers - ie mine is a F/5 so a 4x gives the needed F/20 ?

But the photons and working out what pixel size by exact amounts, math's not my best subject.

Also, I'm not entirely sure if I see the airy disc much either, only when I defocus....? Is that not right?

Aenima

[Rob]

You just cant beat the Orion shorty Plus. To be frank its the best barlow I've ever used and owned!.

Try and get one second hand...

[fredster]

If you are lucky enough to find a Celestron Ultima or Orion Shorty Plus for a good price it would be worth it, they are superb. As good as it gets in the sub-Powermate league. Otherwise i agree with John, the Revelation 2.5x barlow is surprisingly good, especially considering its price. I was less than pleased with my Tal 2x, it gave a yellow hue to all my views but i could have had a lemon.

[JamesF]

I've already voted once in this thread for the Revelation 2.5x as well. It's one of my regular pieces of kit for planetary imaging. The Ultima (which I also have) is indeed very good, but be aware that it is far closer to 2.3x or 2.4x than 2x.

James