miguel87

12 minutes ago, John said:

Does this shed any more light on this topic (see what I did there 😞

https://www.telescope-optics.net/eyepiece1.htm

 

An interesting read, thanks.

"The image formed by the telescope objective is real, and can be observed directly"

So at the point of the field stop of an eyepiece, the eyepiece 'selects' a portion of the image (a real observable image with non-uniform brightness) to work with. It passes through various optics and produces an exit pupil.

The converging cone is made parallel (collimated into pencils), therefore at infinity (the paths would never converge).

If the image that the eyepiece is working with is 'real' and 'directly observable' AND a camera sensor can be placed in the light path to sample the photons and create an image. I dont understand how brightness can be uniform?

There surely must be a real image at the exit pupil.

19 minutes ago, andrew s said:

 

The exit pupil is where the entrance pupil is imaged not the object e.g the star.

 

In a telescopic system the entrance pupil is just the aperture of the instrument where light enters. Therefore not relevant to the properties of the exit pupil in this case.

I know it is very complicated an I'm sure you have more experience than myself but...

Unless I can understand how a cross section of the light cone taken by a DSLR chip shows the full detail of the image, without the image apparently being present there?

In this instance, what is focussing the image?

10 minutes ago, andrew s said:

Quit right, but the focal point is not the exit pupil. That what you are  misunderstanding. 

Right, but we use the exit pupil to view the image. Which comes just after the focal point. Once past the focal point, the light is not uniformly distributed, it is the image of whatever we are looking at. Otherwise eyepiece projection would result in a blank, evenly lit image.

And I know you dont like my DSLR sensor analogy but it works because it literally samples a 'slice' taken through the optical cone of light after the focal point. But it shows that the light at this cross-section is not evenly distributed but forms an image. 

20 minutes ago, andrew s said:

Light is not focused at the exit pupil @miguel87 that's were you analogy fails. With a telescope/eyepiece the rays from a star are parallel through the exit pupil.

You need to look at some ray diagrams to really understand what is going on.

Regards Andrew 

It is focussed or else how would a DSLR sensor with no lens and no focus ability be able to take a sharp image straight from the exit pupil plane?

The whole purpose of the telescope is to bring light to focus at a point.

That point is the exit pupil.

Attach the camera and telescope with no other filters, lenses, barlows etc. Then get the moon in the centre of the image and roll the focusser all the way in and all the way out.

If you find focus(I believe you will) then job done.

If you dont find focus then note which 'end' of the focus range is closer to in-focus.

If you need to go further out, then you can get an extension tube.

If you need to go further in, you must use a barlow lens or alter your focusser arrangement in some way.

It is hard to advise when the only images you have shared so far appear to be taken through an eyepiece, which is not how to get your best photos.

31 minutes ago, andrew s said:

Simple bit first. If the entrance pupil of the eye is smaller that the exit pupil of the telescope/eyepiece it will limit the effective aperture and hence light grasp.

I would use the analogy I used before of a 7mm diameter circular camera sensor with the exit pupil placed perfectly on it.

Let's use the full moon as an example, taking up the centre of the resolved exit pupil image on the sensor.

If you remove the outer circumference of pixels on the sensor so that it is only 5mm in diameter (equivalent of the human pupil being smaller than the telescopes exit pupil) then the resolved image of the moon remains entirely in the centre of the sensor.

The image would not be any dimmer because the light has been focused into an image with brightness distributed non-uniformly across it (I.e. the moon is brighter than the sky around it.

So as was explained so well to me earlier, there is NO brightness loss in the part of the image that enters the eye as the exit pupil gets larger.

You have to remember the exit pupil as a little floating 2d image in the air, it is focussed already. It is not evenly spread light.

Many  many people use a newtonian and DSLR combo without a barlow, without a nose piece, just a T-ring adaptor, this is how I get all my photos.

You dont need a moon filter either, this will just dim your image and you may lose detail. All cameras can focus a full bright moon without glare.

You are fiddling with too many things between the camera and the telescope which is more likely to cause glares, reflections etc etc.

Just DSLR-Tring-telescope:

+1 for black diamond

Quite something isnt it. It's real and it's right there! 

My first attempt at DSO. My fourth astrophoto ever! I need some flats desperately and not to spend too long messing with colours. I like some of the detail in M82 tho. Considering the cloud came in early last night and this is only 15 minutes of exposure.

6 hours ago, GiggaKubicca said:

Just putting it out there for everyone, I'm still looking for barlow help.

What specific question do you need answered?

Wow, amazing some of the scopes that are out tonight. Here's my 200p getting some Bode's nebula snaps.

Love it, nice smooth texture.

I am just waiting for enough dark to start exposures on M81/82. Not tried a galaxy before 🤞

Start without the barlow of you can, as has already been said. Use your telescope as the only lens. Barlow of you want a more zoomed in view but a poor barlow will degrade your image.

Aiming a camera down an eyepiece can be very difficult, at least it was for me!

11 minutes ago, Aramcheck said:

Was it "Dr. Maggie's Grand Tour of the Solar System" by Maggie Aderin-Pocock (Buster Books £12.99), from the Dec 2019 edition?

Dont think so but will google it to check. The book was described for young kids like toddler sort of age with a lift up flap on each page and a big picture of each animal constellation. I think Haha!

Thanks everybody yes I will send them an email and hopefully they will reply.

✌

Thankyou very much but that it not the one I remember, I must be confusing my months, sorry 🙁

Anybody still got their March sky at night.

I think there was a childrens book recommended in the book section near the back and I meant to write it down but no I cant remember what it was.

Something about the constellations for younger children.

Thanks.

I just found this post as I am choosing between the same 2 pairs of binoculars! 

Anybody any experience?

Thanks

I have been stargazing regularly for a good 5 years now but I have never looked at a comet, I sometime try to persuade myself to look online track one down but the current options just dont seem worth the effort of locating.

Tonight's skies should be great where I am but the moon is obviously an issue so I am trying to decide what to observe.

Think I will continue my quest to sketch the messier catalogue and hang around in ursa major opposite the moon.

Nice picture thanks!

Do you know if any of the tail can be seen visually?

Mike

There are free print out sheets online with the correct sizes for almost any telescope at http://www.deepskywatch.com/Articles/make-bahtinov-mask.html

I then cut out with scissors on the printed paper, stuck that to the base of the sweet tub, spray paint to mark the holes. Removes paper and slice with stanley knife and metal ruler.

It's a nice thickness to work with as it cuts quite easy but is rigid enough to hold when finished

19 minutes ago, MarkAR said:

And for the 250P-DS ???

Obviously this will not fit. I'm sure some random product out there will fit!

Nothing?

Just now, John said:

Fair enough. I think quite a few on the forum have a similar approach, and why not ?

 

I wish I didnt ask so many questions! Life would be simpler haha.