PST sm40 double stack and the miniERF

[marktownley]

Only thing I would add is that if you are planning to remove the ITF and replacing with KG5 I would use a nighttime Ha filter in the stack keep a bit of the visible heat off the blocker.

Ken is in the process of organising some plots for the ITF - that side of things is hos department!

[Merlin66]

David,
I obtained some spectroscopic transmission data (using the sun) for both the "min-erf" from my PST and the BF15 from my Coronado SM60.
I've done the testing three times now to ensure consistency of results.
The graphs I'm currently getting are different from the one published above.
Before uploading them, I'm trying to make 100% sure they are representative of the filters and not some weird artifacts from the processing.
They both show UV-IR cut-off and significant attenuation across the visible portion of the spectrum ie good candidates for an ERF.
I will upload the data I have for "further discussion"
Mark is arranging to get some filters spectroscopically tested in the UK for further comparison.
Stay tuned....

[DJD]

Mark,
if using BK5 have you a particular visual Ha filter in mind or would something like the Baader H-alpha 35nm CCD Filter
be inappropriate ?
Ken,
waiting patiently for your findings :-)
Best wishes,
David

[marktownley]

You should be fine with the 35nm Baader David

[Merlin66]

Dave,
Still not finished doing the full instrument response and corrections but the attached graphs show the basic data.
Filters taken from a PST (the "mini-erf") and an ITF from a BF15 blocking filter. Using the MG80 spectroscope and fully modded 1000D camera - solar spectrum. The "normal" solar spectrum exposure would be 1/800s; these needed 1/4s - some x200 more equivalent to >ND2 attenuation.
The first two are profiles taken from the RAW images (in ADU/ pixel)
The first, the PST "mini-erf" shows a peak transmission of 44k ADU and the second, the BF ITF 57k ADU (about 20% more)
The third graph shows the two transmission curves calibrated for wavelength and normalised (pink=PST, Blue=BF). The height scale is arbitrary but shows the relative intensities.
These still have to be instrument corrected to accurately establish the final filter transmission characteristics, but the fundamental bandwidth and relative transmissions won't change.
You can see that neither of these graphs match the one given earlier.
Comments welcome.....

[marktownley]

So, does this plot go from 378nm to 882nm Ken?

[Merlin66]

Yes.
The Canon, even with full mods only goes down to 370nm and the NIR, if extended would be contaminated by the 2nd order spectrum. You would need a separate filtered exposure to accurately record above 800nm or so.
If the "demand" is there I can certainly do that.....

[DJD]

Thank you Ken for that information.
I am hoping that those with more "savvy" than me can put it into context of replacing the "miniERF" with off the shelf
filters for the PST and alternatives for the SMxx type scopes if different, both visual and CCD based.
As to KG5 filters and empty filter holders , I have found a site in the UK that supplies KG5 filters mounted in 1.25"
screw in filters and also supplies a wealth of other type of filters of interest ( and blank filter holders ) if anyone is interested. It is Apex Optical Services and my dealings with them have been first class ( absolutely no affiliation )
http://www.apexservices.co.uk/astro_optical_filters.htm
Best wishes,
David

[Merlin66]

The "executive summary".....
Basically a UV-IR filter with ND2 attenuation across the visible spectrum.....

[marktownley]

The "executive summary".....
Basically a UV-IR filter with ND2 attenuation across the visible spectrum.....

Lol! That's a good summary there Ken!

If it is do-able would be interesting to see the extended response if this is something that is easy for you to do...

Mark

[Merlin66]

Can be done.
Just need to check the best camera to push into the IR...
I have:
Modded Cannon 1000D
DMK21
DMK41
QHY5
Lodestar
Atik16ic
Atik16icS
Atik314L+
I'll check the curves see which one goes furthest into the NIR

[DJD]

The "executive summary".....
Basically a UV-IR filter with ND2 attenuation across the visible spectrum.....
Just to show off my ignorance, does this mean that a KG5 3mm filter + a variable polarising filter is
sufficient to satisfy that definition ? Or does it need some more ND attenuation ?
Best wishes,
David

[DJD]

Might be getting confused with ND 2 = 2.0 ie 1% transmission
with ND 2 = 2 stops ie 25% transmission

[marktownley]

[quote]The "executive summary".....
Basically a UV-IR filter with ND2 attenuation across the visible spectrum.....
Just to show off my ignorance, does this mean that a KG5 3mm filter + a variable polarising filter is
sufficient to satisfy that definition ? Or does it need some more ND attenuation ?
Best wishes,
David

Yes. But need a Ha filter too.

Not sure why you want to use this polarising filter though? :dry:

[marktownley]

Can be done.
Just need to check the best camera to push into the IR...
I have:
Modded Cannon 1000D
DMK21
DMK41
QHY5
Lodestar
Atik16ic
Atik16icS
Atik314L+
I'll check the curves see which one goes furthest into the NIR

Good stuff Ken. I'm most curious as to what is happening >1500nm.

[marktownley]

Thank you Ken for that information.
I am hoping that those with more "savvy" than me can put it into context of replacing the "miniERF" with off the shelf
filters for the PST and alternatives for the SMxx type scopes if different, both visual and CCD based.
As to KG5 filters and empty filter holders , I have found a site in the UK that supplies KG5 filters mounted in 1.25"
screw in filters and also supplies a wealth of other type of filters of interest ( and blank filter holders ) if anyone is interested. It is Apex Optical Services and my dealings with them have been first class ( absolutely no affiliation )
http://www.apexservices.co.uk/astro_optical_filters.htm
Best wishes,
David

If there is no need to replace it - ie it is not rusted, then I would not replace it. You can always remount it elsewhere in the optical train by sandwiching it between two 25mm washers, and then mounting these in a blank filter holder.

[Merlin66]

David,
With astro filters and most applications the ND# is the 1/10^power ie ND1= 1/10, ND2=1/10*10, ND3= 1/10*10*10 etc.
IMHO All filters used within the converging beam in a solar application, must be interference film type filters (ITF)...not the "dyed in glass" type. The dyed filters work by absorbing the energy,(ie heating up and re-radiating etc) whereas the ITF reflect the energy back towards the objective.....

[swisswalter]

Hi Ken

thank you for that excellent explanation. Now I see the point in using the ITF

[Merlin66]

Hmmmm
Just checked through the curves I have for all the cameras...
The response >1000nm starts around 2.5% and just falls off...I can't find any definative QE curves beyond about 1100nm.
Does anyone have such data?
I will, however, give it a go....

[marktownley]

David,
IMHO All filters used within the converging beam in a solar application, must be interference film type filters (ITF)...not the "dyed in glass" type. The dyed filters work by absorbing the energy,(ie heating up and re-radiating etc) whereas the ITF reflect the energy back towards the objective.....

A very very good point there Ken

[DJD]

Ken, Mark,
Thank you both for both for this information and effort.

I am still a little confused, so bear with me and see if I have totally grasped what I am being taught on this thread.
I will restrict it to the PST and I will adopt the nomenclature that I am most comfortable with ( but may be wrong
in pedantic terms , sorry Mark ).
In the later PSTs we have the etalon that passes an Halpha signal + extraneous harmonics that are to be filtered out.
In the eyepiece stack we have two components, the so-called "mini-ERF" and the so-called "blocking filter(BF)".
Ken has pointed out that the mini-ERF attenuates the visual signal and acts as a UV and IR cut signal. It also
cuts all the extra long ( for want of a better term ) IR signal . The BF then restricts the Halpha range to cut
out all the harmonics.

Now if that is correct, can I to return to what I want to do ie remove the miniERF alone from the stack and replace it
( in the same order of components ) with off-the shelf screw-in components. I had planned on basically using a KG5
filter (which I understood removed all the extra long IR signal) and a visual ND 2.0 filter together to protect the BF
and had the pre-requisite characteristics of the miniEF. I think I may be wrong in that assumption?

If I am correct so far, I am not sure what an extra nightime Halpha filter is removing.

I have now also gathered that it is better to use reflective coated filters so that internal heating in the filters does not re-radiate towards the eyepiece.

Ok, where have I gone wrong :-)
Best wishes,
David

[Merlin66]

David,
Your logic is OK.
BUT I think the problem is finding an ITF ND2 type filter to "balance" the dyed in glass KG5 filter.
If I had a $$$$ effective solution I'd offer it up.
A collection of ITF type filters:
A ITF UV-IR cut
A visual ITF ND2 filter
would do the job.....

[Merlin66]

The best input I can get from the "experts" - say that silicon based CCD chips are transparent to light above 1100nm - the wavelengths are too long to be captured in the matrix. Special sensors are required to accurately record in the IR.....
So, basically above 1100nm the back of the hand is a better sensor than a CCD chip...heat!
I've finished the BF filter calibration and correction for the response curve of the modded Canon 1000D.
The attached graph shows the final bandwith profile.
Looks very much like a typical ERF to me (the prominent dip shown at the peak is the Ha absorption)
Any questions let me know.
I'll continue to finalise the PST filter for comparison.

[marktownley]

Thanks for this Ken. Can you just clarify the Y axis scale for me - the bigger the value the more or less transmission?

[Merlin66]

mark,
It's an arbitrary scale based on the background continuum. The higher the number, the higher (slightly) the transmission.

To get a good measure of % transmission I tried measuring the ADU response with the same exposure with/ without the filters - the attenuation is so great (>ND2) that it's very difficult to get a %age.
The best I can offer at the moment is that a 1/4 sec exposure with the filter gives similar ADU counts of a 1/800s without the filter....
If any has a good technique for measuring such a difference - let me know...

[swisswalter]

Hi Ken, Hi Mark

thank you for all that valuable information

[marktownley]

The higher the number, the higher (slightly) the transmission.

Are you saying it is a non linear scale?

[Merlin66]

Mark,
No, it's linear.
But in terms of the total transmission the peak is still low....

[marktownley]

Gotcha

[DJD]

I don't want to clutter up the other thread on miniERF alternatives, so I will ask here.
Just to recap I am trying to replace the PST miniERF unit with nosepiece screw-in filters in my mod to
gain me a bit of focus room.
I am also trying to use what I have.
In that quest, I am using a 3mm KG5 filter and a Baader 35nm Ha filter screwed into the nosepiece of a diagonal and the PST BF5 unit screwd into the eyepiece side of the diagonal. This is working in getting me the wiggle room that I need, but visually I seem to have a problem. I am using the PST gold tube and etalon with the mod back-end just for experimentation.
Using the PST miniERF in my mod I have a decent image.
Using the above configuration I get a brighter but less contrasty image. Is there something I could add to increase
contrast, preferably something easily obtainable, or am I flogging that same dead horse as before ?
I have seen the KG3/interference filter thread but don't want to dump what I have and buy more kit just yet.
Thanks,
David

[Merlin66]

Hmm
They should work.
Can you clarify what you see in the "less contrasty image"?
Could be reflections between the elements (slight tilt required?)
What happens if you add a cp filter to the eyepiece?
Only with the DS set-up or with the base PST etalon....

[BelOptik_Oliver]

Hi David,

Is the image in base PST simply to bright, you can reduce with slightly grey filter or an very slightly green filter.
Are reflexes a problem, tilt slightly the Baader H-alpha filter. Reflexes or "ghost images" arising are often, when optical elements stay nearly in curvature radiuses of spheric optical elements.

Oliver

[DJD]

Ken,Oliver,
thank you for the replies.
At the moment I am just using the base PST configuration.
Ghost images do not seem to be a problem ( but I will check when I can ).
On a practical note - how do you tilt a screw-in filter if it is in a stack ?
I can try a green filter ( I have one of those :-) ).
In terms of contrast, I mean filaments are not standing out as much as in the normal/base component
PST configuration. It is as if it was not tuned, but re-tuning does not make it better only worse.
There is a possibility that the KG5 filter I have is not as optically perfect as it could be,
but in other experiments I have viewed through double glazing with the original PST and got better results.
I am assuming that the filter is optically flatter that two panes of window glass.
If only we had some sun I could try out these suggestions, but sadly will have to wait until the
current 100% cloud cover ( and occasional snow flurries ) passes over.
Thanks,
David

[marktownley]

When the sun finally returns a process of elimination is what is required. I think my first change would be to put the ITF in the objective end of the diagonal nosepeice (obviously removing the KG & 35nm) - I wonder if you have an echo of reflections occuring within the diagonal from the back side of thebaader, off the diagonal mirror and the back of the blocking filter? Lots of mirrored surfaces here...