Collimating the LS152T

[Davidm]

Just wondering if anyone with a LS152T had ever had a go at collimating the objective? The sweet spot on this scope has always been a bit off-centre, which hasn't proved much of an issue for high-res imaging, but I feel it might have drifted a little further off and I would like to have a go at sorting this out. I imagine the off-centre sweet spot is likely to have contributed to my lack of success with the DSII as well. I have tried tilting the etalon unit a little to see if that changed things at all, but it doesn't seem to do much, and that seems like a bodgy fix in any case. Lunt have suggested collimating during the daytime, but I am not quite sure how to do that on an extended object such as the sun. I was wondering if it might be best to set the scope up in white light mode and do a 'standard' collimation on a start at night? Anyway, if anyone has experience with collimating this scope, or perhaps other Lunt refractors, I would be keen to hear about it.

Regards,
David

[gabrieli]

Hi, David

I have the same situation as you regarding the sweet spot on my LS152. I have a Cheshire collimator to check or adjust the collimation on all my non dedicated refractors. The LS152 passes too little light for the collimator to work. There's the option of buying a calcium module which has a white light backing plate but that's far too expensive just to allow collimation. I haven't heard of any other useable method unless there's a cheaper way of holding
the collimator at the eyepiece end aligned with the optical path.

Lou

[pedro]

Hi David

I also have a sweet spot on my LUNT152. I remember reading (LUNT blog) that this scope was mainly designed for high resolution work and not for Full Disk imaging. The sweet spot I have is not uniform. I see it with all the BFs I have (LUNT BF3400, BF1200, Coronado B30...). I never tried to collimate the F/6 lens.

[eroel]

David:
Here is a link:
http://philjay2000.tripod.com/usefulstu ... ntures.pdf
Hope it helps,
Eric.

[Davidm]

Thanks for the comments guys. I don't have a problem with having the sweet spot as part of the setup, but because it is significantly off-centre it is getting close to impinging on even my high-res imaging. I do have the CaK module and white light backing plate so can try collimating at night using the link Eric sent (thanks for that). When Lunt sends more suggestions about how to collimate I will pass the info on.

[gabrieli]

Hi again, David

The best way to collimate is a star test as you intend to do. However, a cheshire eyepiece will allow collimation indoors with an artificial light source and they are not expensive. They work by pushing and pulling your lens with the adjustment screws until the 4 reflections (for a doublet lens) from the air glass boundaries become superimposed. You could always confirm the collimation this way with a star test.

Lou

[Davidm]

Thanks Lou, I have a Cheshire somewhere, so will give it a go. Lunt got back to me and confirmed that the collimation is indeed best done as a star test using the white light backing plate. They also said to double-check that the origin of the off-centre sweet spot was not further down the train by rotating the etalon and blocking filter to see if the position of the sweet spot changes. I will do this first to see if there are any issues there before playing with the collimation, in case that is not the cause. Just need a chance to experiment now.

[eroel]

David:
Good luck with your experiments.
BTW, rotating the etalons and blocking filters make a difference, I don´t have a Lunt, but with the Coronados and even the Daystars, rotating the etalons and even the ERF gives a difference, specially with the hotspots, there are also polarization light issues through the different filters.
Regards,
Eric.

[pedro]

I agree with Eric, rotating the etalon+ERF make a big difference (LUNT & Coronado). I usually spend some time finding the best position as far as my Coronado SMs are concerned.

[zorgdotnl]

Same as Pedro ans Eric, twiking BF vs étalon orientation . you can check collimation in Ha with a pin hole on an aluminium foil un front of a broadband light source as hallogen far, by night.
If collim is OK, something on the etalon cell might be too much tilted. Lunt might help you then (they changed my first LS100 for that)
BTW, if you have an Atmosphere Dispersion Corrector (the one with 2 prisms), you can use it to centre the sweetspot

Chers from cold sunny Paris

[george9]

I would think that collimation the objective lens is unlikely to affect the sweet spot much. If the center is high contrast with all the H-alpha features visible, but it is very slightly blurry, and if off-center it is sharper, then that could be collimation. But if the center is off-band, then that's not the objective lens at fault.

My LS80 DSII's sweet spot is about the size of the sun but very slightly low, and I am very happy with it. Then recently the sweet spot went very high in the eyepiece inexplicably. Everything was tight, nothing seemed to move, etc. About a week later I disassembled everything I could (focuser, adapters, etc.), reassembled, and now it is back to normal. I may never know what happened, but something must have shifted.

George

[Davidm]

It does indeed seem that the collimation is not be the issue. I had a go at a star test, which was a little hard to do since the seeing was awful and the CA is quite strong, but as far as I could tell things were reasonably good. I struggled a bit with the Cheshire (the reflections I think I am looking for in the eyepiece seemed to wander around depending on how I moved the light source, and I think I need to work a bit more on how to use this).

I set up for Ha today and experimented with various rotations of the etalon and the sweet spot seems to rotate along with the etalon. So, it would seem that it is not something that I could alter myself and I would just have to cope with this as is. Not the end of the world, but it would have been good to have a way to centre the sweet spot.

Thanks for the comments and suggestions.

[george9]

Have you given Lunt a call? If needed, can you ship the scope to them? I expect if the sweet spot is that far off, they will do something to fix it.

For example, I had an odd Newtons' rings effect after the first year with my scope, and they just swapped the BF1800 for me to fix it.

George

[Bob Yoesle]

The RG630 ERF ahead of the etalon will have about a 2 degree tilt, and this will also shift the optical axis off the mechanical axis to some extent. Then collimating optics may also magnify this shift a bit as well.