11 replies to this topic

[sciguy125]

I'm looking into getting a solar telescope, mainly for imaging. I have a DSLR that I've been using for deep sky imaging as well as a QHY5L-II for guiding. I'd probably try both of them with the solar telescope to see which I like better.

 

The 60mm single stack Coronado and Lunt telescopes are in my price range but I need some help deciding between them. From what I've been reading, it looks like the general consensus is Lunt, but nobody is really clear about why. Based on my independent research, I've been leaning toward the Coronado mainly because it's a few hundred dollars cheaper.

 

Can anyone explain why Lunt is "better"?

[skyward_eyes]

Lunt has better light transmission on their coatings. These coatings provide better contrast across the image.

For imaging the pressure tuner from Lunt provides a more even tuning across the image as aposed to tilt tuning.

If you're going to get into solar imaging I recommend using a fast planetary style camera in monochrome. A DSLR isn't the best for h-alpha solar as you're missing at long if the data because of the color sensor.

[bandazar]

Lunt is slightly better, but beware that pressure tuning has its issues as well too.  Weight, possible air leakage issues, decontacted etalon, etc.  I sort of "fixed" my pressure tuner.  Whoever assembled pressure tuner put the larger ring toward end of the end of the cap, making it very difficult to put the pressure tuner back on after I screwed it on.  I just switched the O-rings (using the thinner black one near the end, and the larger orange O-ring near the back. 

Getting to the O-ring was in of itself difficult, as I had to use an allen wrench, a pickle jar opener with a set of pliers. 

To me, tilting is not "that" bad.  I don't mind panning around a little bit to see things.  I already have to do that in the lunt 50 anyway (probably because of the small internal etalon).  It's just not as bad as the pst.

[edintexas]

Most Lunt scopes have very long delivery times, if that's a factor for you.

[Holltim4103]

If you have a chance, try to take a look up close and personal at the Lunt scopes and the Coronado scopes. I did a lot of research and asked for the opinions of others. I received a lot of recommendations that leaned toward Lunt. For me, my mind was made up when I went to NEAF and was able to look at the many different solar-scopes and see for myself. For me, it was the Lunt pressure tuned scopes without a doubt. I really like the pressure tuner, as the detail would pop out at me and it would appear more even. I felt the tilt was kind of a pain to use as the sweet spot would move a bit. I like the quality of the Lunt scopes and they seem so much easier to use and give a more pleasant and appealing view. The Lunts appeared more solid and the focusers were smooth and of quality. The customer service at Lunt is outstanding. They always answer the phone or reply back to emails. I asked them many questions and they always made me feel like I wasn't wasting their time. This is why I would pay the extra couple hundred bucks for a Lunt scope.

[rigel123]

I throw this out every time someone asks about purchasing a Lunt.  I would not rule out the Tilt Tune Lunt either.  That's what I have and love it.  I use the B1200 blocking filter since I mostly image.  I set the tilt once when I got my scope 4 years ago and haven't touched it since.  I don't seem to have an issue with the sweet spot on full disk with this scope as some have reported.

 

You can see examples of images with the LS60T in my Astrobin site in my signature below.  As stated the QHY would work out better for you for imaging the sun, particularly if is the mono version.

 

One other note, I believe the Coronados only come with helical focusers.  If you want to image I'm not sure those are great for fine focus.  That alone might make them less expensive than the Lunts that come with either a Crayford or Feathertouch.

Edited by rigel123, 01 February 2017 - 04:26 PM.

[Bill Barlow]

I saw that Lunt once sold a 35mm solar scope much like the 40mm Coronado PST.  Why was the 35mm Lunt discontinued?  It was the one that I was most likely going to buy.

 

Bill

[Cometeer]

I saw that Lunt once sold a 35mm solar scope much like the 40mm Coronado PST.  Why was the 35mm Lunt discontinued?  It was the one that I was most likely going to buy.

 

Bill

 

Discontinued in favor of the 50mm- which is a much better value at $50 more than the $800 the LS35mm had retailed for.

[Traveler]

Lunt vs. Coronado is a valid question of course. But at this moment, is the question Lunt vs Coronado vs Daystar Quark not a litlle more valid question? (without wanting to hijack the OP's thread).

[BYoesle]

Not if you're looking at filter type, configuration, and cost.

 

Both Coronado and Lunt use air-spaced etlaons configured within a customized collimating lens system placed within a telescope specifically designed for the filter system. The Quark is a solid mica-spaced etalon used near the focus and employs a telecentric lens system which can be used with a wide variety of telescopes, albeit not usually optimized for the telecentric-filter system. Each system has advantages and disadvantages with regard to field angles and instrument angles through the filter which determines how well the filter will perform with regard to contrast uniformity and meeting a bandpass specification. Here the Quark's generally will not be operating in a optimized optical system, and additionally have not passed the requirements to meet DayStar's SE or PE filter uniformity standards. Therefore one should not expect that they will perform as well, and why they are not given a band-pass specification and marketed as "cheap, easy, and fun."

 

All things being equal (and they seldom are ;-) the Coronado or Lunt scopes would therefore generally be expected to out-perform the less expensive Quark used with the the same aperture telescope. So in this case the Quark's advantage is simply one of cost. The other advantage of the Quark is that it can be used with a larger aperture telescope, thereby gaining significantly increased resolution. However, in order to obtain better contrast uniformity and band-pass performance, you'd have to spend considerably more money for a SE or PE grade filter. Additionally you'd generally need to employ a custom telecentric lens system to get good contrast uniformity, and to truly obtain the specified band-pass specification with these higher quality filters, an f45 or greater focal ratio is usually required, which begins to limit practical use with large aperture systems - at least for visual use under average daytime seeing conditions.

 

As with most things, there is no free lunch, and you get what you pay for.

Edited by BYoesle, 04 February 2017 - 09:09 AM.

[BYoesle]

To best understand how narrow-band etalon filters work, you must first understand that everything hinges on the incident ray angles that pass through the filter. Each filter is designed to work at an optimum center wave length (CWL), and when light diverges away from “normal” (perpendicular) to the filter, the CWL will shift to shorter wavelengths. Once the angle exceeds the “acceptance angle” of the filter, the CWL has shifted far enough for the filter to be considered “off-band.”

 

The ideal narrow-band solar telescope would have a filter which is relatively small to keep costs low, and present all light rays going into the filter normal to the filter so that all parts of the filter remain on the CWL:

 

 

Unfortunately, this ideal telescope-filter system is impossible in reality. The sun is a relatively large object subtending 0.5 degree, and when placed on the optical axis, the edge of the sun’s disc will therefore present the objective with a “field angle” of 0.25 degree. There is no way to reduce the field angle to be smaller than this. Therefore placing the etalon filter ahead of the objective will give the best possible filter performance. The field angles are as small as they can be, and there are no instrument angles to contend with. These filters are designed with a slightly “high” CWL so that they may be tilt-tuned with little ill effect in order to be exactly on the H alpha wavelength, which is necessary due to changes in atmospheric pressure and temperature which can change the filter's CWL. Unfortunately, etalon filter costs go up dramatically with increased size (e.g. surface area). The traditional maximum typically found is about 150 mm, and 100 mm etalon filters are not cheap.

 

In order to make solar telescopes more affordable, a smaller etalon filter can be placed in the optical system after the objective. However, this introduces more complexity in the light rays presented to the etalon in the form of “instrument angles.” Therefore additional optical elements are employed to better control the angle of the rays presented to the etalon.

 

The diagram below will be useful in understanding the discussion that follows (and forgive me if some details are incorrect - I'm not a optical engineer, and didn't stay at Holiday Inn last night ;-)

 

 

In the collimator system, a negative lens is placed before focus, or a positive lens is placed after focus.  This lens renders a “collimated” or parallel bundle of instrument rays to the filter, and after that a positive lens is used to re-focus the image where it can be viewed or imaged. The focal length of the re-focus lens is usually adjusted in order to render the effective focal length back to that of the objective alone.

 

While it can be seen that the collimator produces no incident angle to the etalon on the optical axis, as one moves farther off-axis, field angle magnification increases, and at some point across the etalon the field angles will exceed the acceptance angle of the filter. This increase in field angle magnification also makes it more difficult to use tilt tuning, and pressure tuning is generally considered a better method to adjust the CWL. It is also generally considered that for good full-disc on-band contrast performance, the focal length of the collimator lens should be no less than half the focal length of the objective, which results in no more than a 2x magnification of the field angles (just as an eyepiece does – same thing in fact.)

 

It can also be seen that the size of the etalon is also inversely proportional to the magnification, and therefore ideally is no less than ½ the objective diameter. Any shorter a focal length for the collimator (and the resultant smaller etalon) results in less than ideal contrast uniformity across the full-disc as the filter shifts more quickly off-band with greater field angle magnifications. This results in the well-known “sweet spot” phenomena observed where portions of the disc shift to a continuum view of the photosphere verses the chromosphere. This can be acceptable for those interested only in narrow close up views of portions of the solar disc, and is often the case with DIY solar telescope projects which use smaller collimators and etalons.

 

An even smaller etalon filter can be used, but now the optical system usually changes to the “telecentric” system in order to achieve acceptable etalon performance. In the telecentric system, a positive or negative lens is again used to essentially collimate the light rays similar to the collimator lens system. However, after this a relatively weak positive lens is used to focus the image using a sufficiently narrow converging light cone to keep the angles under the acceptance angle of the smaller downstream etalon near the focus. At the same time the telecentric system ensures that the off-axis light cones are also brought to a focus in the same manner, with the cone axis axis normal to the filter. This results in a uniform spectral performance across the etalon. An f 30 or even longer focal ratio light-cone is required for these systems. Remember again that the narrower the filter band-pass, the narrower (e.g. the longer an f ratio) this light cone must have in order to achieve the bandpass specification of the etalon.

 

The caveat with these filter systems is that generic “telecentric” barlow lenses, or a telecentric lens system used with the wrong focal length objective – e.g. “non-optimized” – may not result in true telecentric performance. Non-optimized telecentric systems therefore may not present the filter with an appropriately narrow light cone to achieve the specified filter band-pass (f 30 or greater). They may also increase angles from tilted peripheral field angle cones, and therefore exhibit a “sweet spot” in addition to being significantly band-pass widened on axis.

 

Not understanding the principles of how these filters work can result in solar enthusiasts spending a lot of money on a really narrow bandpass filter, only to realize the performance of a much cheaper and wider bandpass filter due to lack of telecentric system optimization.

 

See also:

 

http://www.cloudynig...-etalon-basics/

Edited by BYoesle, 04 February 2017 - 10:49 AM.

[sciguy125]

Just an update:

 

I got a Lunt LS50THa. I did a quick imaging session to try it out and was happy with the results.

 

However, the mounting holes on the clamshell weren't drilled properly, so I couldn't attach the dovetail. Lunt is supposed to send me a replacement clamshell. In the meantime, I put it on a camera tripod so that I could play with it. Obviously not ideal, but it let me play with my new toy a little.