TAL/PST stage 2 mod disappointing first DSLR image - help needed

[DJD]

Dear All ,
finally managed to get my Tal 100/pst stage 2 mod up and running, but have had very odd first light results.
I would welcome comments on the image and possible explanations.
I am at approximate correct position for the objective and focus with respect to the etalon.
I am only using a 5mm BF from my original PST and have dispensed with the miniERF to get the spacings correct for
imaging.
The sun was positioned centrally to the DSLR chip for the image. The result was not what I expected.



Best wishes,
David

[DJD]

Sorry, I should have added that there is a 50mm sub-aperature ERF placed internally at the correct position
for the objective.

[marktownley]

Hi Dave,

Ok, first off. You don't have the etalon tuned 'ideally' - you can see the annulus of detail that you have running around the edge of the suns disk - this is the 'sweet spot' a slight tweak and you will find this annulus of detail turns into a central spot of detail. Turn the etalon the other way and this annulus of detail will expand towards the periphery of the field of view. This is perfectly normal for a PST mod, it's all down to field angles - basically the ratio of the size of the objective to the size of the etalon - there's nothing you can do to alter this, but there are ways in which you can make it work better than it is currently.

The main reason you are getting the result you are is that you are using a DSLR. This has a monster sized chip / field of view - i'm not sure exactly how many pixels it is but it's big! The problem is the DSLR encompasses the central area (when tuned properly) that is on band, but also the rest of the image which is off band - so resultant image does not look as we would like!

With my 100mm Tal PST mod I found I had to run it at f16 with a DMK31 (1024x768 chip size) to encompass ONLY the on band sweetspot in the field of view. Running it at native f10 focal ratio, even with the relatively small 1024x768 chip size of the DMK31, I found the image was going rapidly off band towards the edges of the field of view - this introduces all sorts of complications not only with the lack of detail due to the wider bandpass but there are also gradients introduded from lack of even field illumination.

The issue we're dealing with is the size of the CCD chip - with the same scope setup and the larger chipped DMK41, IMHO, you would need to be running the scope at f20, ideally f25 to be imaging only in the onband sweetspot. If the much smaller chipped DMK21 was used you could get away with imaging at the native f10 f ratio of the TAL. I think, to use a DSLR and only image in the onband sweetspot you would be running the scope at somewhere in excess of f50(!) - not really possible or practical!

The other main issue is that despite the huge chip of the DSLR you are only using a quarter of it's effective resolution - in Ha we are only using the red of the bayer matrix that covers the DSLR sensor - even if you record in mono


bayer_matrix by Mark Townley, on Flickr

You won't have found this to be an issue with the same DSLR and the unmodded PST due to more favourable field angles in this optical configuration. Realistically a PST mod is only ideal as a high res / closeup scope. If I was you I would buy a CCD to image with.

I know this post doesn't help your predicament, but hopefully helps to understand what you're seeing and why it's going on!

Mark

[colinsk]

Nice mount!

[swisswalter]

Hi David

there is nothing to add what Mark did not mention. Still congratulations on the mod

[DJD]

Thanks all.
Mark, thank you for the detailed breakdown. I shall have to digest it and come up with a plan of action.
I know I am flogging a dead horse again with the DSLR, but have not got access to a laptop I can use
at present to give webcams et al a run out.To be honest I was trying to avoid the need to do stacking and mosaics
for a full disk - I am inordinately lazy. The crop mode of the 550D does a reasonable impression of a wbcam for avi
work, especially in my seeing.
As to tuning the etalon, I have found I am at one end of the travel, unlike when it was mounted in the PST.
Don't understand why, perhaps I reassembled in the wrong place after having trouble removing it from the PST.
I have re-set that but will have to wait patiently for another glimpse of the sun.
I assume that you mean to use barlows to increase the f-ratio - that I can experiment with.
Colin, as to the mount in the avatar, it was made by our very own Peter Drew. I have a bigger version with a
Slevogt on it also made by Peter when he was making telescopes professionally.
Thanks again,
Best wishes,
David

[marktownley]

Thanks all.
Mark, thank you for the detailed breakdown. I shall have to digest it and come up with a plan of action.
I know I am flogging a dead horse again with the DSLR, but have not got access to a laptop I can use
at present to give webcams et al a run out.To be honest I was trying to avoid the need to do stacking and mosaics
for a full disk - I am inordinately lazy. The crop mode of the 550D does a reasonable impression of a wbcam for avi
work, especially in my seeing.
As to tuning the etalon, I have found I am at one end of the travel, unlike when it was mounted in the PST.
Don't understand why, perhaps I reassembled in the wrong place after having trouble removing it from the PST.
I have re-set that but will have to wait patiently for another glimpse of the sun.
I assume that you mean to use barlows to increase the f-ratio - that I can experiment with.
Colin, as to the mount in the avatar, it was made by our very own Peter Drew. I have a bigger version with a
Slevogt on it also made by Peter when he was making telescopes professionally.
Thanks again,
Best wishes,
David

Ahhh, if you don't have access to a lappy then that doesn't help things I guess with a webcam / ccd then David. However, let's see what we can do to maximise what you can do with the setup you've got. You say in the Tal setup you are at the end of the tuners travel compared to the PST - my money is that there is some 'sag' in the optical train somewhere in the setup, I would hazard a guess it is on the focusser on the Tal - what happens is this accentuates the angle the face of the etalon makes with the optical axis and you don't have sufficient twist to tune this away. Ok, how do we sort this? Well next time you get some sun try rotating the etalon assembly in the focuser and view this on the DSLR screen, you will see there is an optimum position where the most contrast is visible, when you find this index mark it so that everytime you use this setup you can return to the optimum position. If you're still finding you're at the end of the tuning range and that could probably do with a bit more twist then we can look at resetting the tuning ring on the etalon - it's really easy to do, Alexandra did it on her PST and was pleased with the results, however, one step at a time, let's try rotating first.

Mark

[DJD]

Problems with the Tal focuser is a distinct possibility and one that I will be looking into ( sic ).
Thank you for all the suggestions Mark. I am starting to find similar threads that have discussed this problem before, I guess I should have looked harder, I hope you don't mind going over old ground again.
Best wishes,
David

[marktownley]

No problem at all David, glad to help.

Forgot to say in my previous reply btw, barlows will definitely improve things with your setup