what lense to use

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what lense to use

Postby Premacy10 » Tue Apr 12, 2016 3:50 pm

Hi all

I am still new to astronomy and wondered what lense sizes you would use to look at the moon, planets etc. I have tried messing around but some lenses appear never to go into focus as if they are too powerful. I also use a 3x and 2x barlow.

Any help gratefully received.

Regards tony
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Re: what lense to use

Postby Aratus » Tue Apr 12, 2016 11:00 pm

You need to work out the magnification of each eyepiece with your telescope and the effect of using the barlows.
Take the focal length of your telescope which happens to be 900mm, and divide it by the quoted focal length of the eyepiece. So with your 10mm eyepiece the magification is 900/10 or x90. If you use a barlow with it as well then multiply by the barlow strength. So with a 2x barlow 90x2 = x180.
I'll leave you to work out all the figures for all your eyepieces and barlows!
For my telescope I've typed them all out and I have it handy when observing.

Now your telescope is a 5". For general use you shouldn't use more that 40x or 50x the number of inches. So you stand a good chance of getting a bright image if you keep below those figures. So 40x5 = x200. Use an eyepiece/barlow combination which is below that figure unless the conditions are really good. So to use the example at the top, your 10mm eyepiece with a 2x barlow will give you 180x magnification which should be fine. If you use the 3x barlow you will get x270 which might be too high for average conditions.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: what lense to use

Postby Supercooper » Wed Apr 13, 2016 9:59 pm

HI,

I had someone write to me complaining that the telescope I sold them was rubbish because it wouldn't focus at all using the Barlow lens!

I pointed out that when you use the Barlow lens you also have to use an eyepiece in the Barlow lens or it won't focus.

This did the trick and aparrently the telescope was rather good after all :o)

Barry
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Re: what lense to use

Postby The Man with the Corrugated Iron Roof » Mon Apr 18, 2016 7:48 pm

You can exceed these limits with an APO or Maksutov.
How can I be one with the universe when we don't know what 96% of it is.

My website: http://www.philippughastronomer.com/

My blog: http://sungazer127mak.blogspot.com/

Photos: https://www.flickr.com/photos/philippughastronomer/
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Re: what lense to use

Postby Aratus » Mon Apr 18, 2016 11:12 pm

In the end we need to know what combinations Premacy10 is using to get these 'out of focus' conditions.

I'm only guessing that the reason is too much magnification based on the OP.

In the end it is up to the individual to find out what their telescope can and can't do. I would never tell people not to go over the 40 or 50x per inch of aperture, but if they do and they get a dim indistinct image, then that is likely to be the problem.

Phil, if you want to start a new thread on the topic of Maksutovs and how they yield better images at higher magnification than other kinds of telescopes, I would be very interested to know the reasons behind it. It is not widely known.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: what lens to use

Postby Supercooper » Mon Apr 25, 2016 11:58 pm

The Maksutov gives similar results to a refractor - Which yields great results at up to 100x per inch. Always has - It's physics.

the reason newtonians are poor is because of:
1 Open tube
2 Diffraction due to spider and secondary.

Maksutovs and Refractors have no tube currents and less diffraction (Although the Mak has some it's considerably less than the Newt).

Aratus: An SCT should be the same - Don't tell me you've been restricting yourself to 400x? :O/
Last edited by Supercooper on Tue Apr 26, 2016 12:06 am, edited 3 times in total.
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Re: what lens to use

Postby Supercooper » Mon Apr 25, 2016 11:58 pm

Patrick Moore always said that it was upto 50x FOR A NEWTONIAN and SCTs - Everyone seems to have forgotten this little caveat - It's upto 100x per inch for refractors (and Maksutovs)!

I'm getting wonderfully detailed views of Jupiter with my 120mm refractor at 366x and more than I get at your restrictive 235x.

Patrick published a book in which there were observations he had made with his Cooke 5" refractor. They showed magnifications of 500x on Neptune and Uranus. I trust Patrick.

I give the reasoning in a new post lower down.
Last edited by Supercooper on Fri May 06, 2016 12:39 pm, edited 1 time in total.
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Re: what lense to use

Postby Aratus » Tue Apr 26, 2016 2:33 pm

I'll leave the choice of what magnification to employ entirely to the user. They will quickly find out for themselves what works and what doesn't. There is no harm in trying. Similarly I am always happy to push the magnification of my own equipment to get the best performance. Any restrictions will be dictated by the conditions not theoretical mathematics, but the mathematics gives a good starting point. I have successfully observed Jupiter with my 8" SCT magnifications higher than 340x on several occasions, but rarely. Even when I have done so, I have not felt it necessarily gave more detail or a better image for being bigger.

As far as advising people who ask questions on this forum, I personally will not encourage people to go out and buy equipment combinations that experience of general conditions suggests won't work well. I value money too much to do that to anyone! :) I’m happy to acknowledge that there are people who believe differently, and of course you are welcome to add your opinion to any thread I’ve contributed to. (and vice versa)

As for arguing the point, I’m still uncertain as to why you think the general rules of thumb regarding aperture and magnification apply significantly differently to different kinds of telescope. As I have said before, if this is true there must be reasons for it. Unless you mention the reasons we cannot discuss it.

Without that information all that would happen is that you and I would end up quoting different opinions from different sources. A bit pointless really, and not really moving the discussion on.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: what lense to use

Postby dave.b » Thu May 05, 2016 1:23 pm

The 50x per inch of objective is guidance for the practical limit for visual observing with the Mrk1 eye ball. (40x per inch of objective is a better figure to work with.) It effectively translates into the smallest usable exit pupil. I've written about this on several occasions on this forum. When using a camera, such as a small ccd with small pixels, higher effective magnifications are possible. However, limiting factors to this are the clarity of the sky (transparency and seeing) and the sensitively of the sensor (camera or eye ball) to detecting dim objects.
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Re: what lens to use

Postby Supercooper » Fri May 06, 2016 12:12 pm

Refractors have a clear apperture, closed tube without any noticeable currents or interruption to the lightpath. The light therefore can be almost perfect when entering the eyepiece. The atmospheric seeing contributes but, because the apperture is generally smaller for the same light transmission (refractor / reflector) the atmosphere makes less contribution to image distortion. Telescopes of above about 120mm in diameter are affected more than smaller ones because 120mm is the approximate size of the air pockets that cause bad seeing. Telescopes over this size see more overlapping cells of air and have bigger distortions - They have to wait for nights of good seeing where the air cells are larger than their apperture. A refractor of 120mm therefore has less chance of being affected by poor seeing, whilst at the same time having a better light source to form the image.

Simply put - Refractors have steadier, more detailed images (size for size) than reflectors, and have better detail available at any given magnification.

Newtonian telescopes have an open tube full of currents and the spider and secondary to cause interference and scatter light in the image.

SCT's have a secondary mirror central obstruction that is a sizeable portion of the apperture which causes diffracftion problems in all types regardless of the tiny differences of the size of obstruction. The 'corrector plate' has a flat front surface that introduces its own problems.

Maksutovs aproach the image quality of the refractor because of the smaller obstruction and curved meniscus lens and closed tube.

In my 40+ years experience:
Approximately equal image detail can be had from the following:

80mm Refractor
90mm Maksutov
127mm SCT
130mm Newtonian

120mm Refractor
140mm Maksutov
200mm SCT
250mm Newtonian

But the smaller instruments will have more nights of 'good seeing' and generally be able to employ 50% higher maximum magnifications than those over about 120mm.


Cheers, Barry
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Using fab Helios f8 150mm Achromatic Refractor on SkyWatcher EQ5 - enjoing the views!
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