low power....

[Andy78]

Hi all, could you SGLers offer advice on a low power EP for my 150/750 f5 scope? My lowest as yet is the 25mm BST which is awesome, but i was wondering about a lower EP. But my BST as you all know has an AFOV of 60dgs but is there a 1:25" EP lower than 25mm with a wider FOV?

Im wanting to start hunting down DSO's... i know that my scope aint the best for this but seen as i was forced to sell my 250px dob! Im trying to get the best out of my smaller scope until i can afford to upgrade again.

Thanx in advance... Andy

[John]

Are you able to use 2" eyepieces Andy ?

In the 1.25" format you can go a little wider than 25mm BST but only just a little bit. That would need a 32mm plossl. It's only going to show you around 10% more sky than the 25mm BST though.

The 2" format would enable you to go wider.

Most DSO's will fit in the 2 degree true field that the BST 25mm gives you though.

[Andy78]

Hi john. No i cant use a 2" EP with my scope unless i get the adapter to do so. I did look at the SW 32mm Panaview as it gets good reviews on here. But i reckon i will stick with the BST, at least untill i upgrade again.

Thanks for the input though.

[John]

Well the 2" adapters don't cost much but a 2 degree field is quite decently wide for all but the largest deep sky objects.

[faulksy]

Hi john. No i cant use a 2" EP with my scope unless i get the adapter to do so. I did look at the SW 32mm Panaview as it gets good reviews on here. But i reckon i will stick with the BST, at least untill i upgrade again.

Thanks for the input though.

Hi john. No i cant use a 2" EP with my scope unless i get the adapter to do so. I did look at the SW 32mm Panaview as it gets good reviews on here. But i reckon i will stick with the BST, at least untill i upgrade again.

Thanks for the input though.

i think the pan is 2" format

[Piero]

Your BST 25mm is a very low power already. It gives you 5mm exit pupil, and I am afraid to say that if you go lower than that you will start seeing the shadow of your secondary mirror, plus a greyish sky.. An eyepiece of 32mm focal length gives you more than 6mm exit pupil. Unless you regularly have superb dark skies, I doubt you will really use it. 

[Piero]

Regarding field of view, you can get a 24mm 68degrees afov, but I am not sure you will have a much larger fov w.r.t. your bsd 25mm 60degrees. 

This staying with 1.25" eps.

[ronin]

As best I know the widest 1.25" eyepiece at 25mm you will get is the Antares W70 (70 degree) from Rother Valley they are £60.

How good they are not sure, I have the shorter ones but cannot recall the last time I used them. Got too many eyepieces. In general Antares produce reasonable items just they never became main stream. The Antares Speers Waller's are reckoned to be very good.

So if you want wider and do not mind the outlay that is an option.

Think however that that is it in 1.25" format.

[Stu]

To accurately know the true field of view at these focal lengths in 1.25", you really need to know the field stop size. Just using the afov/magnification gets quite inaccurate as the field stop approaches the limiting size of the barrel.

My understanding was that 32mm TV Plossl and 24mm panoptics offer the largest fov in the 1.25" mm size??!'

[John]

If you know the diameter of the field stop of an eyepiece the following formula can be used to work out the true field of view, that is the width of the sky that can be seen:

EYEPIECE FIELD STOP DIAMETER / TELESCOPE FOCAL LENGTH x 57.3 = TRUE FOV IN DEGREES

The ultimate constraint is going to be the internal diameter of the eyepiece barrel becuase that will limit the diameter of the field stop. With 1.25" eyepieces the maximum field stop diameter is about 28mm and with a 2" eyepiece the limit is around 46mm.

The challenge is that field stop diameters are not regularly available for all eyepieces on the market but you can see from the above why a 2" eyepiece can have a much wider field of view, around 60% wider in fact, than the same focal length in the 1.25" format.

[Dom1961]

Just out of curiosity, how do 1.25"-2" adaptors work? Because surely if you put the 2" into a 1.25" focuser then only 1.25" worth of light will get through? If you get what I mean?

[John]

Just out of curiosity, how do 1.25"-2" adaptors work? Because surely if you put the 2" into a 1.25" focuser then only 1.25" worth of light will get through? If you get what I mean?

From your mirror the light path is cone shaped narrowing as it travels up the scope from the mirror, bounces off the secondary mirror and travels sideways into the focuser tube. By the time it gets to the eyepiece adapter it's only a few mm across.

There are no optics in the adapter, it's just a tube to hold the eyepiece in place.

[Andy78]

Dom 1961... i thouhgt of that aswell! So thanks to john again for answering that question.

Ronin.... i will have a look at that Antares from RVO because that where i plan to buy my upgrade from. Its only about 1.5 hrs away from me.

Once again thanx to all.

[Stu]

From your mirror the light path is cone shaped narrowing as it travels up the scope from the mirror, bounces off the secondary mirror and travels sideways into the focuser tube. By the time it gets to the eyepiece adapter it's only a few mm across.

There are no optics in the adapter, it's just a tube to hold the eyepiece in place.

Hi John,

I've always assumed a 2" eyepiece will vignette in a 1.25" to 2" adaptor? Will the 1.25" not act as a smaller field stop and thus reduce the fov in the 2" eyepiece??

Never tried it so I don't know!

Stu

[ronin]

I only know of them as I have the 4.3mm, 6.0mm and 9.7mm.

To say I have not used them much is an understatement. :eek:

Pretty sure one has never been in a scope and reasonably sure the other 2 have only once - I seem to recall thinking they are a bit long for the scope in question - long in the physical sense not the focal lengths. So they never looked right. So I guess that 2 of them went in a scope once, the 3rd one I bought from Green Witch, used to have a shop close to me so easy to call in. Difficult to explain why I bought that one - I intended to get 3 but the place only had 2. So they let me have a Speers Waller at the same cost as the W70 of about the same focal length. But I still wanted the 3 W70's so I bought it at GW. The others came from Vancouver on a visit there.

Then I ended up with the BST's and tended to grab them, so the W70's are in a case not used a great deal.

[John]

Hi John,

I've always assumed a 2" eyepiece will vignette in a 1.25" to 2" adaptor? Will the 1.25" not act as a smaller field stop and thus reduce the fov in the 2" eyepiece??

Never tried it so I don't know!

Stu

You are right Stu. Assuming that the 2" eyepiece has a field stop larger than the internal diameter of the 1.25" adapter there would be some vignetting (loss of light) from the edges of the field of view. The diameter of the light cone produced by the mirror / objective has not changed though as far as I'm aware. I have to admit that my optical system understanding gets hazy at this point as to how the interaction between the focal plane of the eyepiece and that of the scope optics actually works in detail in 1.25" and 2" eyepieces. Perhaps some kind soul will enlighten us !

[Stu]

You are right Stu. Assuming that the 2" eyepiece has a field stop larger than the internal diameter of the 1.25" adapter there would be some vignetting (loss of light) from the edges of the field of view. The diameter of the light cone produced by the mirror / objective has not changed though as far as I'm aware. I have to admit that my optical system understanding gets hazy at this point as to how the interaction between the focal plane of the eyepiece and that of the scope optics actually works in detail in 1.25" and 2" eyepieces. Perhaps some kind soul will enlighten us !

Plenty of clever chaps out there in SGL world who will put us straight :-)