My PST stage one / two mod

[Moonshane]

well I think it is safe to say I can justify the purchase of a BF10. I tried Ian's today albeit briefly and the view is transformed. Full disc detail at 100mm aperture resolution is very very nice!

the jump in aperture did not make as big a difference as going from 40mm to 80mm of course but conditions were really quite bad with a lot of haze and cloud about. The sun is of course probably a little lower than last time out too.

the view is more effortless with the BF10 as with the BF5 at 4x less field approx is like using a smaller exit pupil at high power in a normal scope.

I cannot wait for a nice sunny day when the sun is higher - yummy!

all I need to do now is save up for a new one or buy a used one sooner. I think my Vixen 80M f11 might be on the chopping block although I may be able to save enough by spring/summer with any luck. 

[Moonshane]

one thing I have noted is that the cell which the DERF came with was rather clever in that it has a built in tilt which is required to ensure I understand the reduce internal reflections. However, looking down the scope (obviously not when looking at the sun!) from the objective end it seems to  send the light back on a path which hits the tube wall rather than going straight out of the objective - in other words the cell tilts the DERF too much. i can adjust this to send it out of the objective but is it worth it? The view does not seem to suffer.

[A McEwan]

Hi Shane.

The tilt of the ERF is more to do with reducing reflection in the eyepiece view. If the ERF is straight-on it tends to introduce "shadow images" that are very annoying. Tilting the ERF at an angle to the incoming light reduces or removes completely that artefact. I would leave it as is personally, unless anyone with more experience can chime in.

Ant

[Moonshane]

I note that I am getting some internal reflection/ghosting with my 'home made' bf5 but not Ian's borrowed bf10. Other than the size of bf the main difference is that the bf5 eyepiece holder (which has been reused compkete from the pst black box and screwed into a mirror diagonal. Therefore the mini erf is on the eyepiece side of the train. With the bf10 there is a mini erf in the nose of the diagonal so on the objective side of the diagonal.

I am pretty sure the ghosting is due to the position of the mini erf in the bf5.

My understanding of the reason for this smaller erf is that it is needed in blue objective psts which don't have any other erf in the chain (unlike the older gold coated objective versions.

My question is therefore can I safely remove the mini erf from the eyepiece holder given I am using a derf of 80mm effective aperture set inside the ota?

I am not keen to take any risks of course but the secondary erf seems to serve no purpose and in fact seems to be creating minor problems.

[Merlin66]

The blocking filter assembly is made up of the two elements - the front ITF and the rear "sorting" filter.

In theory, a good D-ERF does the same job as the built-in ITF.

It doesn't do any harm to leave the ITF in place. If you experience additional ghosting with the BF5 assembly I can only assume that the tilt of the ITF in the eyepiece "cell" may not be enough for the set-up being used.

I have used a CP filter on the eyepiece to suppress some reflections - doesn't work in all cases, but can help.

[Moonshane]

Cheers Ken

it is not a massive issue. I may have a look at the eyepiece first filter as I had not realised that there is a tilt on this. Maybe I can adjust it.

[Uplooker]

Ha ha Shane,

That's quite funny/ interesting, as I Do get reflections with the BF10 that you are using (mine).

Ian

[Moonshane]

Lol. Maybe we need to swap ;-)

As I say it is not a big issue but if I can sort it....

I am still debating whether to buy a bf10. It is great to get full disk but I often increase mag anyhow.

[Uplooker]

I note that I am getting some internal reflection/ghosting with my 'home made' bf5 but not Ian's borrowed bf10. Other than the size of bf the main difference is that the bf5 eyepiece holder (which has been reused compkete from the pst black box and screwed into a mirror diagonal. Therefore the mini erf is on the eyepiece side of the train. With the bf10 there is a mini erf in the nose of the diagonal so on the objective side of the diagonal.

I am pretty sure the ghosting is due to the position of the mini erf in the bf5.

My understanding of the reason for this smaller erf is that it is needed in blue objective psts which don't have any other erf in the chain (unlike the older gold coated objective versions.

My question is therefore can I safely remove the mini erf from the eyepiece holder given I am using a derf of 80mm effective aperture set inside the ota?

I am not keen to take any risks of course but the secondary erf seems to serve no purpose and in fact seems to be creating minor problems.

The blocking filter assembly is made up of the two elements - the front ITF and the rear "sorting" filter.

In theory, a good D-ERF does the same job as the built-in ITF.

It doesn't do any harm to leave the ITF in place. If you experience additional ghosting with the BF5 assembly I can only assume that the tilt of the ITF in the eyepiece "cell" may not be enough for the set-up being used.

I have used a CP filter on the eyepiece to suppress some reflections - doesn't work in all cases, but can help.

Ken,

If Shane was to remove the ITF filter would it then be sensible / logical for him to replace it with a 1.25" UV/IR blocking filter?

Ian

[Merlin66]

Ian,

If it was just a "red" ERF being used , then the UV/IR option would be good value.

The Baader D-ERF blocks everything other than the Ha.

(I use the Baader 35nm Ha CCD filter as an ITF replacement on a BF15 diagonal)

[Moonshane]

Cheers Ken

I have the baader derf (blue coatings but red if you look up the drawtube). May try and adjust the tilt on the mini derf