Lunt LS80 - reflections and ghosts

[Rossco72]

Hi all,

I have had a couple of chances to look through my new Lunt LS80 now and there is definately some relections and/or ghost images showing up.

The sun is very bright and clear in the middle of the eyepiece but there are what definately looks like fainter suns in other places.

I was trying to image the other night and during my focussing attempts there were smudges around the image. As I never got the image finely focussed I didn't record any of these to show you!

I am going to try again over the weekend if the sun is out. In the meantime, I asked Ian King and he reckons that the pressure tuning system shouldn't suffer from reflections or ghosts the way older tilt systems did. He thinks there might be an issue with my blocking filter.

Does anyone else have a pressure tuned Lunt and can they comment on any reflections or ghost issues they might have?

Cheers

[Merlin66]

Hmmm, not sure I can help.... when we do the PST mod the front ERF has to be tilted to prevent reflections. I have a double stacked SM60 and if it's not set properly I'll get a couple of secondary images - these disappear off the edge of the field when the tilt is correct. In the etalon itself there should be no ghosts etc - I assume the blocking filter is the same basic design as the Coronado and has a "mini-ERF" up front and the actual blocking filter near the eyepiece. You could try and tilt it slightly in the focuser see if it makes any difference.....

Hopefully someone with a Lunt can explain the issue....

[anthony]

New to my Lunt and new to solar but here is my experience. It is a lunt ls35t - their most basic. It is BTW superb for visual and has produced a fine first image.

I have on occasion (3 times I think) had a full and bright solar disc that couldn't be focussed. I reckoned on a reflection and slewed around until a disc reappeared that could be focused.

Don't know the explanation. I am pretty sure that the Lunts are ERF and etalon at the front end. At the eyepeice end (on mine anyway) the blocking filter is housed in the base of the helical focuser that screws into the eyepiece. The blocking filter is securely housed and the fitting glued. My diagonal has a small pane of subtley tinted glass at the front. It is obviously there for a reason but I wondered if it might have a role in the reflections.

Might it be that it is perfectly normal? Is it not the etalon that reflects the light back and forth within itself depending on wavelength - in the end only passing frequencies that are multiple integers of the spacing. So you will get a number of images sent to the blocking filter. They might all be on axis. Could it just be that the blocking filter bandpass is broad enough for several closely positioned frequencies of Ha region light to still pass through. I am no good at explaining this but this article puts it into words better.

http://www.designerinlight.com/solar/EtalonArticlePt1.pdf

Just guessing and hoping it is normal!

[Merlin66]

I helped Colin write that article.

The finesse ie the bandwidth between the resonance images is greater (around 10A) than the pass bandwidth of the blocking filter (around 6-8A) so none of them can be visible when the etalon is tuned close to Ha.

If you see ghosts then they must be in and around the Ha wavelength ie 6563 +/- 2A

[Rossco72]

Anyone in central Scotland have a Lunt with a B1200 or B1800 blocking filter?

[ollypenrice]

I've had no such issues with my tilt tuned LS60/B1200. Hope you get it sorted, Stuart.

Olly

[Peter Drew]

Stuart,FWIW, I'm experiencing similar problems with my modded PST scope. It's definitely not caused by the ERF as no matter what the tilt or the orientation, the reflections remain the same. I have 3 PST based Ha "eyepieces" that can be swapped around other donor scopes, each one shows reflections to different degrees on the same scope, convincing me that the reflections are internal for some reason. The PST's were 2nd hand so have been round the block a few times so in my case it's possible that the etalons/blocking filters are past their best, however your Lunt components are new so should be at there best. This is very frustrating as the images in the unaffected parts of the field are fantastic. I used a friends standard tuning Lunt LS60 recently which had a lovely black field around the Sun with no annoying reflections, I have since been seriously considering looking out for a s/h Lunt as an alternative mod. Hope you soon get to the bottom of your issue. .

[A McEwan]

I don't know if this will be much help, Stuart, but in my Lunt 60 Pressure Tuned I have had no ghost images to date. Until I double stacked it. Then I do get a ghost image in the view along with the real Sun, but when I adjust the etalon tuner on the second filter that edges towards the edge of field of view and disappears off the edge - thankfully while the tuning is improving the view on the "real" Sun.

Ant

[Rossco72]

the area around the sun is definately not black! the sky is a reddish hue and the focussed sun in the middle of the field is red/orange. I was actually surprised by that as i was only expecting red...

If i push the sun to the edge of the field and look at an angle the sky darkens to near black and the contrast seems enhanced.

Also at the moment I am just snatching quick looks after work, what I really want is a nice sunny day at the weekend to really get to grips with it.

[A McEwan]

The background sky will not be black unless the sky is really clear of haze and cloud. I quite often observe with a reddish glow surroundin the disc, but it shouldn't affect the view of the disc itself too drastically. Time to get to grips with it is probably what's needed too, as they are a bit different from any other type of scope.

Ant

[Rossco72]

Thanks Ant,

The view of the disc is pretty stunning, I just wasn't expecting so much background colour. In fact I had thought the disc itself would only be red but it is definately more orange and that surprised me a bit as well...

There is definately an element of getting to grips with not only a new scope but a whole new way of observing, never mind imaging!!

Cheers

[Rossco72]

Here's a couple of shots to try and shot what i mean.

The smaller shot is using a 2x powermate the larger one is with a 4x and i rotated the camera 180 degrees, i haven't rotated the images at all.

Anyone have any insight into this kind of thing?

Cheers

[Merlin66]

Hmmm

Something definately not right there....

I've never seen anything similar on previous solar images......

The tuning is a bit off - did it change any with tuning???

[ollypenrice]

I may be wrong but surely that is just blooming? I get that on the sun and moon with a CCD camera. I can't get the exposure down enough to use CCD with our Lunt but on the moon cutting down exposure time kills it.

I think you just have too much light. To confirm, you could make a simple card apterture mask to stop down the objective.

In our LS60 the sky is black and the proms contrast against it strikingly. We do have a good sky though.

Olly

[Merlin66]

Good question!

I'd assumed Stuart was using the DMK or DFK camera....

[Rossco72]

well the shots above were with the 11002, but at f28 i had hoped it would be ok with exposures of 4-8 thousandths of a second.

I will hopefully get another chance tomorrow and will try stopping down the objective.

[ollypenrice]

well the shots above were with the 11002, but at f28 i had hoped it would be ok with exposures of 4-8 thousandths of a second.

I will hopefully get another chance tomorrow and will try stopping down the objective.

Those are big, sensitive pixels... I think any visual issues you have are likely to be independent of the blooming in the images you posted.

You'll get there.

Olly

[Merlin66]

Try using the DFK. The results, I'm sure will be better. I use the DMK21/ 41 and need about 1/120 sec for the surface and 1/15 sec for the proms.

[Rossco72]

A 20mm aperture mask and some playing around produced this:

http://stargazerslounge.com/imaging-solar/147728-first-solar-image-03-07-2011-a.html

So hopefully not too bad going forward, although makes me wonder why i bothered with an 80mm scope!!

[ollypenrice]

Hot dog!! Diggetty dang!!! Brilliant result. Oh yessss.....

Olly