First light of LS60FHa. Tuning problem?

[solarahn]

Recently my new LS60FHa arrived, front mounted filter with B1800. Ever since arrriving the weather has been poor, until today and I could make a brief test in between the clouds. It is mounted on a Takahashi FS-60CB and an ASI178mm was used for a quick test. Having looked through a Coronado PST before, the view was a bit disappointing. Visually prominences were visible, but surface detail was not great. For some reason, it seems I couldn't get it to tune properly. See the attached full disk image. It also seems that when the sun is on the edge of the camera field, see other attached image, the tuning seems better. The sky was not super transparent and also the sun was low in the sky. I didn't focus yet on the sampling or the camera artefacts, the focus now was on the tuning and to see if the filter system works correctly and as expected.

What are your thoughts?

[robert]

How is the filter attached to the scope? Could there be some sagging? Definitely not tuned.
Robert

[solarahn]

Thanks for your opinion Robert. The front filter is attached using a custom made filter adapter, see photo below. Do you mean sagging at the front or the back, or both? The screws that are used to tighten the filter adapter at the front were tightened as evenly as I could.

[Carbon60]

Assuming the thumb wheel isn’t giving you the range, you could try ‘clocking’ the whole front mounted unit to see if there’s a position that brings the etalon on band. Additionally, any ‘slop’ in the connection between scope and etalon unit could give rise to the etalon being tilted by gravity (droop, as has been mentioned) and this can also take it out of range in the worst case. Gently push the front mounted unit up and down/side to side to see if the image changes for better or worse.

Stu.

[solarahn]

Thanks Stu for the suggestions, and indeed the thumb wheel isn't giving the expected range. Definitely a good idea to try 'clocking' the front unit. I could also try to move the front unit a little bit up (it was down and as close to the telescope front objective as possible), but it could be only minimal because there is limited 'length' in the filter holder. What do you mean exactly by slop? Do you mean that there is some kind of tolerance in the filter holder dimensions so that by gravity it could be pulled down a little bit? Below I have attached a picture of the front filter holder mounted on the telescope, but without the front unit attached. The filter holder seems to fit perfectly on the telescope.



Tomorrow there may be sunny weather again, so I could possibly try out your suggestion.

[robert]

Thanks for your opinion Robert. The front filter is attached using a custom made filter adapter, see photo below. Do you mean sagging at the front or the back, or both? The screws that are used to tighten the filter adapter at the front were tightened as evenly as I could.

Yes I suspect the holder is not keeping the filter exactly parallel to the objective, sag or droop. They are heavy and it may be best to hold it as close to the objective as it can go.
Nice looking setup (when it works)
Robert

[solarahn]

Brilliant Robert and you gave me the idea to remove the dew shield, now the etalon is much closer to the front objective! See below the result. Hopefully can test it tomorrow again.

[rigel123]

You may have already done this, but just to be sure, did you first turn the tuning wheel all the way clockwise (while at the eyepiece) until it stopped, and then start turning the wheel counterclockwise until you start to see some surface features. You may encounter some resistance but it should still move until you see the filaments darken. You should not need to force it but it may start out fairly loose. You should be able to turn it enough to see it darken and then lighten again as it goes from being on band to off band, so you would bring the wheel back to the right (clockwise) to bring it back on band.

[marktownley]

I agree with Robert. Looks like sag to me on the dewshield.

[Carbon60]

What do you mean exactly by slop?

Hi,
Sloppiness in any of the connections between the etalon and scope. The dew shield is a good example of where there could be a poor connection leading to movement resulting in etalon droop (effectively tilting the etalon off band). The other end of the scope is also important. The face of the BF must be mounted square (perpendicular) to the optical axis. Any excessive tilt here can also affect the ability to get on band.

Hope this helps.

Stu.

[solarahn]

Thanks everyone so far for the help, as soon as the weather permits, I will test again and report back on the result.

You may have already done this, but just to be sure, did you first turn the tuning wheel all the way clockwise (while at the eyepiece) until it stopped, and then start turning the wheel counterclockwise until you start to see some surface features. You may encounter some resistance but it should still move until you see the filaments darken. You should not need to force it but it may start out fairly loose. You should be able to turn it enough to see it darken and then lighten again as it goes from being on band to off band, so you would bring the wheel back to the right (clockwise) to bring it back on band.

Thanks for the suggestion! Yes, I followed that process but from my perspective I couldn't reach an satisfying view. At some points, as also seen a little bit in the image, you could barely see a filament, but then it would disappear again.

[rigel123]

One last thought, not knowing where you are at, did it happen to be cold out? Often below 40F you can lose a lot of contrast due to the BF getting cold.

[solarahn]

I’m testing right now and it’s bad news… the view is even worse than the previous session as this time I cannot get a different view than a continuum view. At some points I can see a tiny bit of promincences, but not a single Ha surface detail. I have also tried to “clock” the front filter but it seems this makes no difference. The tuning wheel also seems loose in a way that you can push it a little bit in and out, and it seems not to “catch” until you reach the ends, when no details appear. Throughout the tuning process I can see sharp sunspots on screen so focus should be sufficient for the tuning

Right now its around 48F / 9C outside.

[marktownley]

Did you remove and reseat all etalons and adapters when you retested?

[Bob Yoesle]

The best way to definitively evaluate the etalon is to use a H alpha spectrum tube.

Are you near a school that has a science department with a Hydrogen spectrum tube? If you can get access to one, you can place a frosted plastic diffuser over it and check the etalon fringes. For an air-spaced etalon the central etalon fringe ring should be no larger than about 1/4th the diameter of the next fringe ring. Anything larger means the CWL is too far off-band high, and the etalon likely will not come on-band, and even if it does manage, it will have a non-uniform "sweet band."

Ideal v high CWL.jpg (630.93 KiB) Viewed 2283 times

TOP A very good pre-Meade Coronado SM90 0.7 A air-spaced etalon ($3950 in 2003 = $6472 today) needing very little tilt to come on-band. BOTTOM A Meade Coronado "rich view" SM90II 0.7 A air-spaced etalon ($2639) with a CWL too far-off band to be able to come on band with any tilting.

In general, you would want a tilt-tuned etalon to have a CWL no higher that 0.5 A off-band high = 6563.3 A:

Ha lamp tuning range clr labeled.jpg (40.55 KiB) Viewed 2283 times

Peter Zetner

Some purposely made double stacking filters likke the SM90II shown above are made with a high CWL so that they are tilted more to move the retro reflections out of the field of view. This is a compromise, and these may work OK for double stacking, but are poor primary etalons, where the less tilt needed at your location (barometric pressure) - the better.


Bob

[solarahn]

Thanks a lot Bob for the detailed explanation, every day we learn something new! I would be very curious to know the CWL of this etalon, although I'm not sure if I will find this kind of testing equipment near.

With the front etalon being closer to the front objective of the telescope, I was expecting an image that was equally as good as the first session, but as I mentioned I couldn't get any proper tuning this time. As an example, below is a single frame from a recording I took while tuning. In the centre it shows a darker band, this is caused by the etalon. I could tune in both directions, but the surface details wouldn't change from a white light view. Only at some point was I able to spot a little prominence. As I mentioned the tuning wheel seemed much ''looser'' today and would only catch on either end, but no surface detail would appear.



And here is another one after ''clocking'' the etalon (turning the position):

Did you remove and reseat all etalons and adapters when you retested?

What do you mean exactly with reseating the etalon? Between the two sessions, the etalon was removed from the custom adapter for storage in its case.

[Bob Yoesle]

Looks like case 3 or 4 in Christian's "sweet spot banding" article - the CWL of the etalon appears too high to give uniform and on-band contrast. I would consider return for exchange / replacement by a new one, or a refund. Lunt is usually very good about these situations.

[solarahn]

Thanks Bob for your opinion, I’ve contacted the shop where I got the filter from and will let you know once I receive any feedback.

[robert]

"As I mentioned the tuning wheel seemed much ''looser'' today and would only catch on either end,"

Could the adjusting mechanism be beyond its limit and have become detached in some way?

Robert

[marktownley]

What do you mean exactly with reseating the etalon? Between the two sessions, the etalon was removed from the custom adapter for storage in its case.

Taking off and putting back on the alumiuim etalon cell holder. How tight a fit is it to the tube of the scope?

[solarahn]

"As I mentioned the tuning wheel seemed much ''looser'' today and would only catch on either end,"

Could the adjusting mechanism be beyond its limit and have become detached in some way?

Robert

That would certainly be a possibility, although at each end (left or right) it seems to tighten again, so not sure if that is possible with a detached tuning wheel.

Taking off and putting back on the alumiuim etalon cell holder. How tight a fit is it to the tube of the scope?

Yes, between the sessions I would take off the aluminium holder (also shown on the image below). The cell holder seems to fit just right around the telescope, in the image below you can see it attached to the telescope front with dew cap removed.

[solarahn]

Update: after discussing the issue with the shop and having test images and a video looked at by their engineer, the filter is now in the return process and a replacement unit will be shipped as soon as the current unit arrives. They will then test the filter system to investigate on the issue. Not knowing what is exactly wrong with the filter, I can only speak of a top service from the shop that they ship a different new unit.

Once there are any updates, I will report back (it be on the replacement unit and/or the unit under investigation).

[marktownley]

Glad to hear it is being sorted and about good customer service from the retailer.

[solarahn]

Thanks again to everyone, for assisting in this thread and for your patience. Recently I was able to give the new etalon its first light, below is the result. Any feedback or comments are welcome! Unfortunately no news on the previous etalon.

[marktownley]

Looks tidy!

[Bob Yoesle]

Can't see the original pics you posted, but as Mark states, looks very uniform - good to see the customer service has been good.