H-alpha comparison

[A McEwan]

I'm interested in any comparisons in the difference in view between a Lunt 60mm PT scope and a PST-modified refractor, of say 70mm, 80mm or 90mm etc.

My limited understanding of the etalon suggests that the tuning in the Lunt with its Pressure Tune system would give a better overall "sweet spot"-free view, but the increased aperture with the PST mod scope would allow more detail to be seen. Would the PST Sweet-Spot issue detract substantially from the improvement given by the larger aperture?

With thanks,

Ant

[A McEwan]

And, while I'm at it, how important is the quality of the refractor's objective lenses?

Thanks! Ant

[keithatrochdale]

You would be welcome to come up and try some solar scopes, that would then give you a side by side comparison - would let us try a Lunt too! ;-)

[Merlin66]

The objective could be a simple lens - monochromatic light makes things easy.

I think you just have to look at some of the results being obtained with the modded PST to answer your questions....some of them are truly amazing!

( I show some representative samples in my PST mod write-up - available on request)

[MLyons]

..... I show some representative samples in my PST mod write-up - available on request......

Consider a copy "requested"

Seriously though, I would appreciate any info you care to share on the Ha system and any enhancements/mods that can improve things.

My email - mlyons at cybersmart dot co dot za

Thanks

Martin

[Merlin66]

Copy sent!

[A McEwan]

Got my copy thanks - very interesting read!

Ant

[A McEwan]

It's taking a while to acquire the bits for this project, which is fine as it's giving me lots of time to think. I can save a fair bit of money by having a sub-aperture ERF inserted into the OTA. From what I've read so far a 75mm ERF in 70mm clear cell could be positioned in a suitable part of the light cone of a 100mm f10 OTA and still cover more than enough of the light cone diameter to be considered safe. Unless anyone has further information on this that might dissuade me?

My thinking is that a 100mm OTA at f10 is a perfect match for the etalon without having to sacrifice aperture, either in the objective or ERF.

Any comments gratefully received...

Ant

[Merlin66]

In theory it will work 100% - the only negative feedback I've heard is that the heat build up between the rear of the objective and the ERF causes loss of clarity and resolution....

I think Brian may have better first hand experience than I.

[A McEwan]

Hmm... ventilation holes/channels then maybe?

[PeterW]

No worse that putting an nd3 at the back of a wedge, warm, but not failed yet! Setting up some fans would be useful.... Have one to help keep the observer cool too.... Man is it hot today.

Peter

[Peter Drew]

I've had success with experiments with large aperture (125mm and 220mm) refractors with sub diameter ERF's. Provided that they are placed no further down the lightpath tthan halfway, heat does not seem to be a problem. It's not neccessary to stare through the scope for hours on end so it can be placed off-Sun periodically to "cool down"

[MLyons]

Merlin, thanks so much for the info - very interesting read.

If I were to put two 77mm diameter ERFs off axis on opposite sides of the secondary on a 10" SCT, would it permit the same resolution capability as one 10" ERF?

What about putting a smaller ERF at the entrance to the baffle tube in a SCT? Maybe a good 90% ND filter then the ERF?

Martin

[Merlin66]

No.

You'd get the equivalent to the combined areas...approx 108mm diameter.

Using an SCT - which etalon would you add?

[A McEwan]

Well I've more or less decided to go full-aperture ERF using a 90mm D-ERF in cell on a Skywatcher Evostar 102. I know that will cut down the available aperture, but budget limitations etc... I can keep an eye open for a 110mm ERF later on.

In the meantime, curiosity got the better of me and I unscrewed the tube off my Lunt. Once the tube is removed, the etalon chamber with threaded front sits exposed, and I wondered.... if a step up adapter was made to go between the threaded etalon chamber front and the "donor" OTA, and if the focal ratio was kept as near to 8.3 or above (8.3 is the native f-ratio in the Lunt 60 models) then in theory should it be possible to simply screw on a larger front tube and aperture?

Surely someone somewhere has tried this, no?

Ant

[Merlin66]

As far as I know, there have been no Lunt mods cariied out so far....don't know why??

There must be a barlow/ collimating lens in the assembly somewhere to give the correct parallel beam to the etalon...then you need to determine the necessary "off set" to get the alignment....

[A McEwan]

Perhaps I need to look more carefully... and remove the focuser assembly too to see the chamber from both sides...

Ant