Let's talk Lunts.

[maw lod qan]

I might be dreaming, but I do fancy the idea ofa Lunt.

I know there are many members who have them so I really would appreciate your knowledge and advice.

What I'm considering is the Lunt 50mm, but as far as what all the bells and whistles do, I'm lost.

No doubt the feather touch focuser is a must, but what is the difference in  a 4mm or 6mm blocking filter in the diagonal?

Also, what do you gain with the double stack? That's a big jump at $895.00 price add on.

I would do more visual, but of course dabble in imaging. 

Any and all advice will be of great help, and thanks in advance.

[Peter Drew]

Some owners don't get along with the stock focuser, specially imagers, I manage visually with my Lunt 35.  A 4mm blocking filter is just adequate to contain a full solar disc, a 6mm blocker will give you more space round it so higher magnifications will still fit the view.  Double stacking, the using of two etalons in tandem lowers the Ha passband width, this markedly enhances the contrast of Ha features at the expense of image brightness.  And your wallet!     🙂

[maw lod qan]

Thanks! @Peter Drew, that's the kind of information and knowledge I was hoping for.

Very understanding!

Now to sell the toy I've put up on line to start gathering the funds!

[Ricochet]

18 hours ago, maw lod qan said:

No doubt the feather touch focuser is a must

I think so. There are single speed and double speed options. I've heard other people say that they get on fine with the single speed but I've found that l prefer a double speed on all my telescopes. With the Lunt I think l only use the fine focus knob.

18 hours ago, maw lod qan said:

what is the difference in  a 4mm or 6mm blocking filter in the diagonal?

If you look down into the top of the diagonal there is a small red window. I assume that the size is the diameter of this window. Now look at the size of the field lens on the underside of any eyepiece that you right expect to use with the telescope. It is probably larger than either 4 or 6mm so the blocking filter must vignette the image. With a Lunt there is a definite sweet spot where much more detail is visible in the centre of the field of view. I assume that the drop in detail off axis is at least partly determined by the vignetting and effective loss of aperture resulting in less resolution. Therefore the wider the blocking filter, the wider the high resolution section of the image should be. This sweet spot is another reason to upgrade the focuser. With the feathertouch it will be in the centre, with the stock focuser it will be somewhere else.

 

18 hours ago, maw lod qan said:

what do you gain with the double stack?

The double stack lowers the band pass so that the telescope is more finely tuned to h-alpha radiation from the sun and finer details become visible. If you've ever used a budget UHC or OIII filter and then upgraded to a premium version it’s a similar thing. You can get by with the budget option, but the premium is just better and once you've used it you won’t go back. The downside of the double stack is that the two filters create a blurry reflection of the sun. There is a tilt tuning adjustment that you use to move the reflection off axis and out of the field of view. This and the aforementioned sweet spot mean that you are best off using eyepieces with a relatively small field of view. I use 50° Vixen SLVs.
 

Unfortunately, none of this advice is good for the wallet. It con be summarised as:

• Buy the scope with the largest, most expensive blocking filter you can
• definitely upgrade the focuser and go for the most expensive double speed option 
• make the telescope even more expensive by buying the double stack
• You might want some new eyepieces
• You're not finished yet. It doesn't come with a finder so there's another £30 you need to spend on the TeleVue sol searcher which fits the holes on the clamshell. 

[Peter Drew]

I think the blocking filter diameter has been calculated to present an unvignetted image of the Sun in the stock offering, larger filters provide more space round the image which is useful for photography as it reduces the need for mosaics, they also don't increase the size of the sweet spot.  I also don't think that focusers have any impact on the sweet spot position.  My reason for this addition to my original post is purely to ensure that wouuldbe purchasers are reassured that the entry level specifications are still adequate for enjoying Ha solar viewing and that the desirable upgrades can be considered later based on experience.    🙂

[Rusted]

The danger is always that the H-alpha observer wants to share the fascinating solar details with their fellow astronomers.
The larger aperture blocking filters and etalons are then an advantage despite the extra expense.

Having to later "upgrade" to the larger aperture elements means you are having to buy them twice but the second set at a mark-up.
Leaving you with unused kit which is only suitable for observing. Not really for optimum imaging.

If, like some enthusiasts, you decide to follow the modification route to larger H-alpha telescopes then the larger elements win here too.

I specified the R&P two speed focuser on my 60MT and can highly recommend it.
I own two other larger, FT focusers and recognize the same qualities in the Lunt R&P focuser.

The Lunt MT series allows you to easily dismantle the instrument. It is intended to be separated into modules.
You can use the H-alpha components for H-alpha viewing and imaging.
I opted to use a straight though blocking filter instead of the Lunt elbow.
This was a personal choice because I already owned the 12mm straight through.
Those buying a complete Lunt scope should probably choose the elbow blocking filter for far more comfortable viewing.

With the MT you can remove the H-alpha components and use the instrument for normal astro viewing.
Or even as a spotting scope.  You have a small, heavy, high quality APO refractor.

You can then add a front, solar filter and use it for white light solar viewing and imaging.
Or add a Herschel wedge and enjoy white light viewing and imaging. 

The total expense is truly terrifying but will reward you endlessly.
By allowing you to enjoy your unique hobby in daylight and the [usually] greater the warmth of the daytime.
You can even take your compact, H-alpha scope on holiday. Where the sun is rumoured to shine all the time.
Do some outreach and become the life and soul of the beach party. 

 

[Ricochet]

23 hours ago, Peter Drew said:

I think the blocking filter diameter has been calculated to present an unvignetted image of the Sun in the stock offering

Correct me if l make any mistakes in the following. 
 

Consider this diagram from https://www.handprint.com/ASTRO/ae2.html

Which gives us the equation for image height:

h’ = tan(a) x fo

Where a is the field angle and fo is the objective focal length. If the angular size of the Sun is 0.5° then a= 0.25° and the focal length of the 50mm Lunt is 350mm so h'= 1.5mm. This means that at the focal plane the Sun forms an image 3mm in diameter so on first impression it looks like both 4 and 6mm blocking filters are sufficient. However, in order to focus an eyepiece we must place the field stop of the eyepiece at the focal plane, not the blocking filter. As most eyepieces place the focal plane approximately at the shoulder, the blocking filter is moved towards the objective by the height of the eyepiece holder. As we move from the focal plane back towards the objective the light cone must approximately increase in size by

Light cone diameter increase = 1 / f# x distance towards objective

The telescope focal ratio is 1/f7 and if we assume that the height of the eyepiece holder is 28mm then at the point where the light cone exits the blocking filter it is 28/7 = 4mm larger than it is at the focal plane. This means that with the 4mm blocking filter only the central point is fully illuminated and if we wish to have an unvignetted view of the whole Sun we require a blocking filter of at least 3+4 = 7mm, which is larger than either filter available with the telescope. For those of us observing with a manual mount where the Sun may drift across the field of view an even larger filter would be preferable. 

However, the sweet spot may also be influenced by other components, namely the etalon so increasing the blocking filter size may not have any advantage. I guess that the only way to find out is to try it. Does anyone have a spare b1200 that I can borrow find to out? 

 

[Peter Drew]

You may well be right but my practical experience over many years with my range of 4mm, 5mm, 10mm, 15mm and 30mm blocking filters has not given me a visual impression of vignetting.  Ha components have  fairly strict positional parameters and don't vary much from their optimum settings.  There is of course the vignetting of the image due to magnification as there is with observing the Moon.  I was disappointed to find that after spending a considerable amount that larger blocking filters only increased the field of view and not the sweet spot.  I am a visual only observer, interested mainly in high magnification high resolution Ha images, my self built 150mm F10 solar telescope utilises a 20mm PST etalon and a 5mm PST blocking filter, followed by a 2x Barlow lens screwed to the nosepiece of a binoviewer.  In this configuration, 40mm eyepieces give a magnification of around 150x, the final image is vignetted to about 1/4 of the solar image but not that of the prime objective, the field appears uniformly illuminated and correctly on band.  Interestingly, the Barlow seems to magnify the small blocking filter to appear that a larger one is in use, also my 5mm blocking filter, selected from several PST's gives a better result than any of my larger ones.      🙂 

Edited February 19, 2023 by Peter Drew