bingevader

10 hours ago, bomberbaz said:

Regarding the ES O-III and HB they appear to have some bandwidth drift which in the case of the O-III is quite significant on the face of the lab tests. It would be very interesting to run a comparison on relevant bandwidth appropriate DSO's. 

I'd be interested in the results. Will be looking to get an O-lll at some point in the future. Would be the ES or the Optolong, but noticed the shift in the ES too!

And before anyone starts! No, it's not like looking through a 4mm plossl!

Nice 'scope though.

51 minutes ago, FLO said:

Dobsonian telescopes are like television screens, wherever you start (4", 5", 6" or higher) it is only a matter of time before you want the next size up 🙂 

Not true sat Steve. We only bought the 28" TV because they'd stopped making anything smaller!  

3 hours ago, heliumstar said:

Give me that box If that is shallow, so be it....

If it is a shallow box, it has probably got a filter in it.

11 hours ago, bomberbaz said:

IMHO it out performs the BST's too, sharper across more of the fov.

https://www.firstlightoptics.com/celestron-eyepieces/celestron-x-cel-lx-eyepiece.html

That's interesting, only used the 8mm BST and thought it was very good, but never compared it to my X-Cel LX.

However, having said that, for the 82°, if the Nirvana 16mm is as good a the 4mm and 7mm, I'd go with that.

Why not indeed!

You can probably find it elsewhere.

3 hours ago, Gina said:

On way back from obsy to house, slipped on the mud in the winter, fell over and rolled into the pond!!  No injuries other than to my pride.

Fully clothed I hope Gina!

23 minutes ago, bomberbaz said:

Yes cat's out of the bag Gerry, it is currently being built.

With all the whistles and bells?

I quite like the sound of the DSC.

I have an 8mm plossl and that's plenty low enough for me!

If you want a replacement for the 6mm that is a reasonable price, then I can highly recommend the WO SPL 6mm.

1 hour ago, Ricochet said:

What was the problem with the BSTs? If we know the problem we can try to avoid it with any suggestions, although the BSTs are probably the best all rounders at the £50 price point.

Definitely.

The BSTs would be the goto recommendation atm.

It'll be difficult to give any suggestions otherwise.

I like all the EPs in my sig with the 8" dobs.

1 hour ago, Merlin66 said:

I’ve got to say, when I started out back in the 60’s I was told even the bottom of a coke bottle would work well at f10.........

If you want to see the bottom of a coke bottle, maybe! 🤔

Nice one John!

What are you using to mount the phone?

I've just received a Celestron NexYZ, but have not yet had the chance to give it a go.

The "Lectures" bit was slightly misleading!

I gave up after realising it was an interview.

I flew straight by!

Nice view of the ISS!

2 hours ago, miguel87 said:

If I replaced it with an eyepiece that matched my dilated pupil size perfectly, what would I be gaining? A bit more mag? I have that in spades in other pieces.

Then why not use them?

Yes indeed.

I have a 1.25" to 2" adaptor from Revelation Astro.

Not as cheap as ebay, but threaded.

I bought mine to use my (1.25") 14mm ep whilst using my 2" eps.

Means I don't have to swap everything out when swapping between.

On 09/05/2020 at 17:07, Don Pensack said:

Remember, every point on the focal plane of both eyepieces is illuminated by the entire primary (* see below)

I've always understood this.

On 09/05/2020 at 17:07, Don Pensack said:

A larger apparent field spreads the light farther into your peripheral vision, but does not brighten the image.

But why doesn't this then result in "wasted light" if the exit pupil is too big? That's where I need clarification.

5 minutes ago, John said:

This is Tele Vue's take on some of these issues. Myth #2 is relevant:

http://www.televue.com/engine/TV3b_page.asp?id=86

 

Sums it up for me very nicely, thanks John.

At these magnifications the loss of light makes little difference.

13 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.

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!

The AFOV is determined by the field stop (gives the edges of the FOV) and the focal length of the EP.

6 minutes ago, miguel87 said:

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.

Yes, absolutely, but the exit pupil (amongst other things) determines how much of the light from Jupiter hits the retina.

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.

You'll have to wait for someone who can explain the physics for a better answer, I just know that it's the way it is!

18 minutes ago, miguel87 said:

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?

I didn't. 

I made no correlation between exit pupil and brightness.

I said, that if you lost 10% (only as an example) of the light, then it is lost, regardless of where you are looking.

1 minute ago, miguel87 said:

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.

Nearly, but not quite.

The pictures are too complicated for me!

You aren't receiving all of the light from the planet, there is still an overall loss and so a dimmer planet.

Imagine a 5p coin as the ep and a 20p coin (okay, not round) as your pupil. The 5p fits nicely in the 20p, all the light is received.

If you now swap the 5p for a 2p, it's now bigger than the 20p. Not all of the light can enter the pupil, there is an overall reduction in the percentage of the light reaching the retina.

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

Hi,

I don't think you are quite right with your middle paragraph. Using an EP with too large an exit pupil does result in 'wasted light' because you can't get all that light into your eye. You moving your eye around doesn't mean you are picking up any extra light, you are still only receiving the same amount of light on your retina. If the loss was, for example, 10%, it would still be a 10% loss regardless of where you looked. You can't physically get all that light into you eye so there is always 'wasted light'. With the 100° EPs (as long as they fall within the limits of your pupil size) you are not losing any light. Your retina is receiving 100% of the light leaving the EP regardless of the angle at which you observe through the EP.

I'm sure there must be some diagrams to explain, I'll have a look.

The 'wasted light' bit has never bothered me though, because if it is a pleasurable view, what difference does it make?