Dedicated solar scope or a filter?

[mytola]

Not much darkness left now, so I've started playing with the thought of getting some solar gear. Only problem is I don't know much about it.

Is it possible to get good H-alpha results with a filter on my existing rig? If I've understood correctly I would then need a H-alpha filter and a blocking filter in front of the tube? Or is it easier or better to just go for a dedicated scope like the Lunt LS35THa?

Any input would be greatly appreciated!

[kirkster501]

Been wondering exactly the same thing....

[DrRobin]

Hi

You have a few options if you want a Ha for your scope, either a front mounted Etalon and a rear mounted blocking filter or something like a Daystar Ha filter with a front mounted Energy Reduction Filter (ERF). These options could be used with your refractor, but wouldn't be suitable for your C8. For Ha both options are expensive and it is nearly as cheap to buy a dedicated solar telescope.

There is a third option, a mid mounted Etalon, but this nearly always means making your own scope, so doesn't really qualify.

Google Lunt, Coronado, Solarscope for front mounted Ha and blocking filters.

Google Daystar Ha for their range of rear mounted filters, but make sure you are sitting down when looking at the price.

Robin

[michael.h.f.wilkinson]

You can put H-alpha filters in front of an existing scope, combined with a blocking filter behind it. Typically, a 60mm front etalon and blocking filter will set you back more than a dedicated 60mm solar scope. Alternatively, you can first buy a PST, and then mod that into a bigger H-alpha scope.

[Pete Presland]

the coronado PST is a lovely scope to use during the "long balmy summer days" i bought mine 2nd hand in January for a little less than £300, unfortunately they are little more this time of year.

watch out if you are buying 2nd hand for any older models that may have the dreaded rust problem, it tends to be the older versions with gold colour objective lens, there is thread below with some known PST's with problems

http://www.cloudynig.../5/o/all/fpart/

[Peter Drew]

In my experience a good quality refractor fitted with a stand alone front etalon and rear mounted blocking filter always seems to give a better result than a dedicated unit of the same gerneral spec.

[ollypenrice]

The Etalon is the expensive bit and dedicated scopes reduce its size by putting it near the back where the light cone has a smaller diameter. Also monochromatic light only needs a singlet front lens since no colour correction is needed. This, I guess, is why dedicated scopes are cheaper. I have only tried one front mounted filter (A lunt 60 on a ZS66) and I think it was pertty comparable with my own Lunt LS60. It would be interesting to try more.

Olly

[Steve Ward]

Front mounted is best but more expensive .

From what I understand the etalon requires a parallel beam of light to work properly , being further back requires more glass to correct beams etc and is always going to compromise the view.

The very large Lunt 152 Ha has only a 3" etalon mounted at the back , bit of a swizz really as everyone always assumes it's a 152mm Ha scope ( not to mention the 230)

Best option , a 100mm unit from Ken Huggett on the Isle of Man , different class.

Don't go jumping the queue though , I'm saving hard for one myself.

[michael.h.f.wilkinson]

A mid-to-rear mounted etalon can produce excellent results, so using a 3" etalon in a 6" scope is not really a problem. The reason is that the field angle is just twice that of the front-mounted ones: 0.5 deg field angle vs 0.25 for the front-mounted ones). Yes there is a slight deterioration or sweet-spotting effect, but a field angle below 1 deg is considered to be no problem.

All PST mods are rear-mounted, and they can produce some stunning results, even though the field angle can exceed 1 deg in some mods with larger scopes.

[mytola]

Thanks for all the replies people! I could be wrong, but I wouldn't be comfortable with mounting a blocking filter at the rear of my scope since it has an integrated flattener. I guess a front mounted ERF is the way to go if I want something for the scope I have. But I must admit it's not easy to navigate in the jungle of products...does anyone sell a package with a combined ERF/Ha filter?

Those Daystar Ha filters are way out of my budget...

[DrRobin]

Andreas,

You have to use a rear mounted blocking filter. If you use a front mounted Etalon, it normally has an ERF included, but you do need all three. The reason is that the Etalon is a 'comb' filter, so it passes light in very narrow bands, but is repeated all the way across the spectrum. You then need the narrow band blocking filter to select just the Ha band.

Daystar filters are out of most people's budget so I wouldn't worry.

It is looking as though your best and probably most cost effective solution is to go the dedicated route.

Robin

[Merlin]

Does this mean that my front-aperture LUNT (LS50F) has a built-in ERF, besides the one at the blocker (B600) end?

[DrRobin]

Yes it does.

Interesting because if you double stack the DS Etalon unit also contains an ERF, so you have two, but only one is required. This is one of the reasons that the front mounted Lunt DS reduces the brightness.

Robin

[michael.h.f.wilkinson]

This reduction in brightness is particularly evident in older ERFs partly based on dyes. The transmission in the centre of the passband is often well below 90% in those cases. The D-ERF has a transmission of up to 93-94% if I have read the graph correctly.

That means that stacking two loses only about 6-7% more light when two are stacked (i.e. two stacked would transmit about 87%). If your transmission is in the order of 70%, stacking two would result in 50% transmission; a very noticeable change

[DrRobin]

Michael,

Yes transmission curves and analysis is correct. Lunt used to make a front mounted Etalon for double stacking and a also one to go on an ordinary scope, but now they only make one and of course it has the ERF on the front. I don't know if the ones designed for double stacking had the ERF, probably did. With a double stack there is no need for two ERF's just one on the front would be okay, but with a mid-mounted Etalon a SS scope will always have an ERF on the objective, so it is difficult to avoid the two filters.

Robin

[mytola]

Hmm, solar filtering is obviously more complex than I thought. I would like to understand the details...Is there any articles available online that describes the different approaches and why they are needed? I don't mind technical stuff.

[Steve Ward]

These are a great read.

http://www.cloudynights.com/item.php?item_id=1791

http://www.cloudynights.com/item.php?item_id=2015

[mytola]

Thanks for that! I'll take a look at those

[mytola]

Those were good reads. Would it be correct to say that if we had a perfect Ha bandpass filter, the comb filter would be redundant? In other words, is the etalon there to make the local Ha spike so it can be passed by the less than perfect Ha trim filter?

I see that in one of those articles, he mentions the TV127is. Isn't that a petzval design? I'm thinking if that one can be used with a front-mounted ERF+etalon, than pherhaps my scope can also be used.

[DrRobin]

If you could make the perfect Ha bandpass filter it would only have one passband, or if you could make the distance between the passbands so large that they were outside the visible spectrum then you could get rid of the blocking filter. However, the Etalon is two plates with a resonant gap between them, that gap resonates at several wavelengths and this is what gives rise to the multiple bandpass (combs).

I doubt it is possible to get rid of them or even move them outside visible light and if it is the two glass plates would have to be so close to one and another and so flat, that the cost would make a Daystar filter seem cheap.

Robin

[mytola]

Just trying to understand why the Etalon has to be there at all. I.e. for imaging there are 3nm Ha bandpass filters. Why not use something like that together with an ERF?

[DrRobin]

A basic solar Ha Etalon is 1A (Angstrom) or 0.1nm, which is 30 times narrow than the astronomy 3nm filter. A 3nm filter wouldn't show any of the surface detail, other than sunspots which are visible with a low cost white light filter.

You have to tune an Etalon (either by tilting or by changing the distance between the plates) to get it exactly on the Ha band, otherwise the bright outer layer of the sun wipes out all of the surface. At anything more than 1A, the surface detail would be lost.

Robin

[Steve Ward]

If it were that easy I wouldn't be saving hard for one of Ken Huggetts masterpieces . . .

[mytola]

Thanks for spoonfeeding me guys, now I get it

[Steve Ward]

As a rough idea , if you stretch the visual Solar spectrum from Red to Violet so that it's about 1.5m across , the Ha band you're after is about the thickness of a sheet of paper edge on.

Give or take a few microns . . .