Barlows for beginners!

[davidpretorius]

Ok, to help complete my telescopes and suggested equipment for beginners article, I want to include barlows (or powermates).

Given most guys are getting 8", 10" or 12" dobs and there seems to be always a 6mm or 9mm eyepiece floating around to start with, I want to advise what barlow / powermate to get for non tracking and then tracking. Also lets say you will get a few great nights per year and the noobie will want to have a really good view of the planets in as higher magnification as practical.

My conclusions from reading the excellent info from the threads


When you first get a dob, a 2x barlow (orion shorty plus) is the limit that is practical. Any higher magnification (aprox 300 x to 400x) is too hard to manage as the view will zip across the screen.
If you have tracking, then a 5X would be the best option.
Barlows seem to mean planets for most guys, so would i be right in saying that if you get a 5x then saturn, jupiter, mars will be very large in the field of view? Is there any need for a barlow at 2.5 or 2x if you have a 5.
Is the 5x only able to be used in very rare perfect seeing? What is the practical limit for magnification for a 10" 1250 fl dob?

I want to produce a summary shopping list, no list will be perfect for all, but by the repetive response we all give, then a common list would be very useful

[Starkler]

The seeing in Melbourne where I live is usually crappy, so most times planetary observing for me is at about 200x, or occasionally 260x in good conditions usually pre-dawn.
Times that I could use more are very rare indeed.

[davidpretorius]

thanks ie based on a 1250mm fl, the 6mm by itself or 9mm with a barlow (2x)

[janoskiss]

You can also barlow longer focal length eyepieces to have the advantage of longer eye relief and to have more choice in magnification.

For a cost-optimised setup the focal lengths of the eyepieces should be selected so that the barlowed magnifications are significantly different from those obtained with the unbarlowed eyepieces; e.g. 25, 15 & 10mm gives you 12.5, 7.5, & 5mm through a 2x Barlow, i.e.: 25, 15, 12.5, 10, 7.5, 5mm; a good range with good coverage. Then you might also add a 2" wide view 30mm to assist with the hunt, if budget allows.

Note that the Shorty Plus is not a cheap Barlow, and a beginner may not want to spend that much. A cheap but functional barlow is around $50, and from what others on these forums have said, it should do the job. So that should be under $200 for a carefully selected set of 3 good Plossls + 2x Barlow.

On the other hand, a premium 1.25" set might go like: 24mm Pan, 19mm Pan, 13mm Nag, + 2.5x Powermate, giving: 24, 19, 13, 9.6, 7.6, 5.2mm effectively. (Now you know what I'm buying next if I like the 19mm Panoptic + 13mm Nagler I already have. God bless the credit card! And may the same God help me pay it off! :prey: )

[iceman]

The 5x is only used by guys who do planetary imaging to get large image scale. I don't know anyone who uses a 5x powermate for visual use - it just gives too much magnification, there's no need for it and the seeing would never support it.

A 2x or 3x barlow is all you'll ever need for visual use, tracking or no tracking.

[Starkler]

Just for reference, my 24mm panoptic doesnt like my Ultima barlow :doh:

[janoskiss]

In what way, Geoff? Too much eye relief with touchy eye placement perhaps?? I presume the 2.5x Pmate would work well with the 24mm Pan. :shrug:

[acropolite]

In the LX (2000mm focal length) my 2x barlow gives 285x with a 14mm EP, slightly more if placed after the diagonal, useable only when the seeing is very good.:astron:

[Starkler]

Hard to describe but it was just yuk ! . I couldnt see the field stop either.
The powermate page on the televue site mentions combining a powermate with a 24 pan no problems.

[davidpretorius]

thanks guys great feedback. ice, that makes sense, so a 5x for imaging and a 2 or 2.5 for viewing.

[asimov]

Usually my maximum usable magnification is 200X barlow or no barlow, with or without tracking.

Mike, are you saying you can go way higher magnification photographically than visually?
What about seeing conditions? IE: If it's crap seeing & the highest mag you can get *visually* for the night is lets say 180X....then you can go way higher mag *photographically* & still get a successful image??

If that's the case, I've been doing it all wrong!

[iceman]

Yes, by a long way.

The reason is the way that we create the images, taking video and getting rid of the bad frames and keeping the best. At such high magnification visually, all you see is an out of focus blob.
But when imaging, you can capture 2000 individual frames, and out of that 2000 there might be a hundred or so that were captured in a brief moment of steady air. Then it's the stacking and processing procedures that can turn the rough raw frames into the work of art that we post ;)

[asimov]

Wow..You learn something new on here everyday! Thanks Mike. I've just had a sudden renewal of interest in astrophotography by you telling me that!

[anthony2302749]

Hi all

When I want to work out the maximum magnification for any given telecope, I use the following basic forumla. It works like this, the maximum magnification per inch of objective is around 50x. Therfore a 3" telescope has a maximum magnification of 150x, a 10" would have a maximum magnification of 500x. This formula will work with any given apature.

Of course due to factors, such as weather condition, these maximum magnifcation may not be obtainable.

A beginner would start out with say an 8" reflector with an f/ratio of about f6. The maximum magnification would be around 400X. So we need to look at an eyepiece/barlow combination that would give use the best range of magnifications. Usually these scope will be supplied with a 25mm and 10mm eyepiece, giving a magnification of 48x and 120x. If a good quality 2x barlow is include we the get the following magnification 48x, 96x,120x and 240x.

As you see by just including a 2x Barlow we have gained a good range of lower, medium and high magnifications. The maximum magnification is 240x well short of the 400x but remember due to factors, such as weather condition, it may not be obtainable.

When it comes to imaging the magnification of the telescope with the camera mounted directly to it (prime focus) can be determined by dividing the focal length of the telescope by 50. For example, a 1,000mm focal length telescope would yield 20 x magnification compared to a normal 50mm camera lens.

When imaging the planets most people use negative projection e.g. teleacope+barlow+camera. With the setup I used to image Mars the magifiction would be around 250x e.g. 10" f10 has a focal lenght of 2500mm mulitply that by 5 (5x Powermate), we end up with a focal lenght of 12500mm. Divide this by 50 and we end up with a magnification of 250x. In terms of image scale we end up with about 0.12 arcsec per pixel.


Anthony

[janoskiss]

Any eyepiece + barlow/no-barlow combinations that produce magnifications beyond 300x are pretty much useless for most Australians, and very rarely useful for a few. If useable, it will be much less than once in a blue moon.

[asimov]

I think I've squeezed out 300X visually maybe 5 times in 6 years from memory.

[davidpretorius]

thanks anthony, great explanation,

can you do me a favour and post your mars pic as it exactly seen in size. I have a 10" at 1250mm fl, no barlow The toucam at prime focus gives me around 200 - 250 x mag.

I would be keen to see how big your scope actually shows mars. I am only assuming you have made it bigger in photoshop.

The amount of detail you have is great.

With 200 x already, if i get a 5x televue. i will get 1000 X with the toucam at prime focus???

This is mars as i see it thru the toucam at prime focus

[asimov]

:confused: :D ..

[anthony2302749]

Here is my last image of mars. The image size is 640x480. I used a Meade 10" LX200 f10, 5x Powermate. The distance from the top of the Powermate to surface of the CCD is 20mm which gives a multiplying factor of 5.5x (see table from prevoius Posts). There for the effective focal length is 13750mm. Divide this by 50 and we end up with a magnification of 275x. In terms of image scale we end up with about 0.11 arcsec per pixel. Mars diameter in this image is 17.65 arcsec.

Anthony

P.S. the wife got the nappy job.

[davidpretorius]

Thanks,

don't forget the nappy!!

here is asimov's 400x, i reckon you are getting more than 250 x at time of imaging or i am missing something basic.

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=4868

[anthony2302749]

David

If the toucam is at prime focus your magnification should be 25x not the 200-250x that you mentioned (Magnification at Prime Focus = Focal Lenght/50). Looking at the size of Mars in your image I would say this correct.

When imaging planets, image scale is an important factor, since you will be taking pictures of small targets (Mars etc) you will need to magnify these object in order to capture sufficient detail. For imaging, magnification is a function of focal lenght and the focal lenght will determine image scale.

I you were to use a 5x Powermate with your setup you will end up with a focal lenght of 6250mm. Divide this by 50 and we end up with a magnification of 125x. In terms of image scale we end up with about 0.18 arcsec per pixel.

This is a respectable image scale for your setup and you will find that you will record more detail.

Anthony

[bonox]

does this generally mean that all barlows should be placed directly at the focuser or visual back? I have always done it purely because i can't fit the barlow into my diagonals without them contacting the prism surface.