miguel87

Just a heads up to all of the 200p users looking at making a DIY Bahtinov mask.

If you can get your hands on a celebrations plastic tub as pictured, it is almost exactly the same diameter as the 200p tube assembly and just slides nicely onto the end of the scope 😁

My problem is that I have always been a 'why' person. It's not good enough if it works, I want to know WHY and HOW it works 😬

2 minutes ago, bingevader said:

And this is still true if the magnification remains the same but the diameter of the EP increases (so no reduction in magnification to brighten the image)?

I will quite happily stand corrected, I've been living with a misconception for some years!

There was a vogue for larger and large EPs at one point and I remember the same conversation then.

Maybe I didn't remember it and that's the problem!

This is what I am wondering now because I was imagining the same magnification but a larger exit pupil. I guess I didnt think about the fact that you would have to use two telescopes to compare the effect.

👍

7 minutes ago, Don Pensack said:

 

Your supposition that a larger exit pupil is not wasted is correct.  The reduction in magnification brightens the image and the light loss exactly equals it, so for the field you see, it will be exactly as bright as the image when the exit pupil matches your pupil diameter.

Thankyou for a great explanation. I think I have trouble visualising how AFOV works.

Glad you cleared up the wasted light issue tho. It had been troubling me to be honest!

Telescopes are so simple and so complicated at the same time.

Mike

19 minutes ago, bingevader said:

If the exit pupil is too big, Jupiter will be dimmer. It's an overall effect, not just a centre and edges, or bright things and empty space effect.

This assumes that some of Jupiter's light exists outside the image of the planet, which I dont think makes sense.

Image a 7mm diameter, circular camera sensor with the exit pupil being projected onto it. If the outer ring of pixels were removed, the image of Jupiter in the middle would not get any dimmer.

6 minutes ago, bingevader said:

 

It's not that some bits are bright and others aren't, there's an overall reduction.

But some bits ARE brighter than others, or else there would not be a resolved picture to look at. Of course jupiter is brighter than an empty bit of sky.

I would agree if we were talking about the unresolved light cone. But the exit pupil is a resolved image and it's brightness by definition is not uniform.

Thanks for your reply, the topic interests me.

And I understand what you say exactly, if you cant see the whole exit pupil 2d image, then some light is hitting your iris and not going into the pupi. However, the exit pupil is an in-focus image, so the light not entering your pupil is just resolved stars at the edge of the FOV. I cant see how this would dim any object in the centre of the view? For example a planet, the light from the planet is hitting the mirror, then secondary then eyepiece lenses, being resolved into an image of the planet at the exit pupil plane and into the human pupil, all of it. Ok so the light from a mag 12 star at the edge of the frame is lost, but the planet will not be any dimmer.

Perhaps we are agreeing with each other but in different ways?!

I dont think you can say that 10% too large an exit pupil results in 10% reduced brightness, because it depends how the light is distributed across the image. If the outer 10% of the exit pupil image has no stars then surely 0% light is lost?

Mike

Probably getting out of my depth here but anyway...

I was  looking at exit pupil sizes and all that jazz and my largest is 6.5mm and with my eye snuggled up to the eyepiece I can only just see the entire FOV without moving my gaze. This fits with the idea that my pupil size is similar to the exit pupil image.

BUT, imagine that the exit pupil was 7.5mm and I had to 'look around' a little to see the edge of field. The exit pupil image is a resolved image, so not being able to see the edges would not dim any objects in the middle because my pupil is receiving all the light available for the object in the middle. This makes me think that a 7, 8 or 9mm exit pupil is not 'wasting light' like other people sometimes suggest. That's like suggesting that once an oil painting becomes so big that you cant see it all without moving your head, that it is somehow dimmer and the artist was wasting canvas.

Finally this got me thinking about the link between AFOV and exit pupil. If I have one of these expensive 100° eyepieces, the exit pupil must be bigger than 7mm, otherwise I could see the edges without moving my pupil around?

I know I must be wrong because AFOV is not a factor in calculating exit pupil size.

So can anybody explain where I am going wrong and how (if at all) exit pupil and AFOV are related?

Thanks!

18 minutes ago, JonCarleton said:

 

You have a 200P and mine is basically a 250P, so very similar scopes.  You should be able to get good results.  Are you using a Dobsonian or Equatorial mount?

 

 

I am using an EQ5 mount just with the cheap motor drive. But I spent time setting up for this image and was impressed that there was no visible change in the frame over 72 exposures of 30 seconds each.

Looking forward to trying some globulars and waiting for a nice bright nebula to come round!

19 minutes ago, JonCarleton said:

M3 is a good target this time of year.  Very bright.

And in GIMP, you do most of  your heavy lifting with Colors->Levels using the sliders as mentioned.  I use the "Move-center-slider-left, then move-left-slider right." approach.  The press OK and repeat as needed.  Don't slide past/into the histogram curve area, or you will be removing data.  The curve is recalculated based on your slider settings each time after you hit OK, so taking multiple "small bites" is better than trying to do everything in one shot.

I also spend a lot of time in Colors->Hue-Saturation after I get image density that I can live with.  I prefer it to Colors>-Curves, though it accomplishes the same thing in the end.

After that, you can do things with color selection masks and things, but if you did a good job stacking with darks, flats and bios integration, you reallly don't need to do so much of that.  It is easy to get caught up in over-processing.

Thanks Jon, that's really helpful. Just need practice on multiple images. Hard to know how well I have done with the particular area of sky I targeted as there is not alot going on.

Mike

I'm saving my cash for the astronomik UHC. Cant decide whether to get 1.25" or 2".

I only have one 2" eyepiece, but it is my wide field deep sky viewer.

Decisions decisions 😂

What allows you to see 'further'is gathering more light (aperture) not magnification.

I have a microscope that can acheive 500x, much higher than my telescope. But my microscope is useless for deep sky observing 😂

1 minute ago, Aled B said:

great advice i appreciate all of this. with the barlow x3 lense if ive understood this will change the magnification by a 1/3 so with the 25mm it will act as a 8mm? this will help me to look further to the deep sky right?

No, not further into the sky. In fact, if you are viewing something really distant and faint I would stay under 100x magnification.

Magnification just tries to make the image bigger but dimmer.

So your barlow will make your image bigger but dimmer. You can only afford to go high mag on bright objects like the moon, planets etc.

I never use more than about 150x on deep sky. Whereas Jupiter might take 333x on a good night.

Yep I would say around 30-50x for finding. Remember that these things are not small really, just dim.

I use 31x mag for scanning for galaxies then maybe anywhere between 50x and 111x for viewing, depending on the object.

Enjoy the skies!

4 hours ago, NGC 2419 said:

I just came across this GIMP astrophotography tutorial today. It should give you some ideas.

 

Clear skies!

Brilliant thanks I will give it a watch

3 minutes to cross the sky, that's an odd amount if time. Not in keeping with something falling to earth. More like the time it would take an aircraft to pass. Did it make a sound?

What do you remember about Halley's comet passing by in terms of movement?

I doubt you will ever positively identify it now without a photograph or similar.

Good luck 👍

Bortle two is amazing. No filters at all! I would have to drive a lot of hours to beat Bortle 2.

Thanks I will google the term. And I guess fiddling around is the best way to get better. Interested to try a bright target next, perhaps m13 or Bode's.

✌

7 minutes ago, lenscap said:

Hi Miguel,

The Stellarium graphic shows a fov of 80 arcmin X 50 arcmin, which is an aspect ratio of 1.6 to 1.

Thanks 😂

Think my brain is a bit slower after tearing down my setup at 2am.

I had no idea what the TFOV would be like when using my DSLR at prime focus on my 200P. After my first pictures last night I tried to replicate my view on stellarium.

Problem is, it is suggesting 1.333° x 0.5° and that ratio seems all wrong. Surely it is closer to 6:9?

I want to know so that I can plan what I am framing better.

Thanks

Ok so I tried again with the original exposures. The only thing I did different was not ticking an option about making a black value 0 or something.

Anyway, considerably better results and I am very happy for my first ever stack and first process.

Please tell me where I could do better tho. The colour levels confused me because the sliders were not doing what I wanted, I set them where I though was least terrible then went onto use the black dropper thing.

After doing that I went back to the sliders and they were having a much better effect??

Anyway, my first ever stack, an odd target of 56 Ursae Majoris (a friend had this star 'named' after them as a present) and picked up NGC3675 in the top corner.

200P, EQ5, RA motor, unguided, pentax K-r

Need some basic advice.

Took 75x30s exposures at iso 1600 last night. Stacked them in DSS, all going well so far, opened the image in GIMP and it looked OK just very dim (I had been warned that this would be the case).

Followed some online advice about processing in GIMP because I havent got a clue!

It suggested I go to Colour, levels, all channels, auto input levels.

This made the image totally white! Bright white!

So my image out of DSS is just ok (the individual 30s exposures are better) but they have stacked together neatly.

Any tips appreciated.

It may just be the limitations of my cheap setup. If that's the case I dont mind and will stick with visual. BUT the individual photos look pretty good so I thought stacking them would improve things, not make it worse!

The sat-nav of the skies!

43 minutes ago, Jiggy 67 said:

It depends on the level of light pollution in your area. Bortle 6 and above makes star hopping and finding objects very difficult, there is nothing wrong with GoTo if it helps you see the wonders out there

Never made any sense to me, if you can view the object e.g. galaxy or nebula or whatever. Then you can definitely see the stars to hop there.

If the LP is so bad you cant star hop, you have no chance of viewing the object 

There is nothing wrong with GoTo. Just my own personal preference.

40 minutes ago, Xsubmariner said:

The site shows my location as Bortle 4, so not too bad. Unfortunately imaging does suffer from Wye valley mist on some of the scarce clear nights.

I used to observe at garway and skenfrith not far from there. Great part of the world!