Pentax and Dioptrx

[anat]

There is a discussion on using Dioptrx with Pentax eyepieces. It may be useful for a lot of Pentax'ers here
http://www.cloudynights.com/ubbthreads/ ... ost1612895

Dioptrx is no longer a reason to choose TV over Pentax

Anat

[Canopus Lim]

That's interesting. Now there is no reason to choose a Panoptic to a XW; at least for me. I anytime prefer the XW to a Panoptic especially since some Panoptics have way too much distortion and some of their eye relief is too short for glasses even though at long focal lengths.

Well there are reasons for choosing TV over Pentax.
1. AFOV
2. some people prefer warmer eyepieces on certain objects (eg Jupiter)
3. correction (off axis)
4. size and weight
5. much more available focal length choices and some may fit better to their setups

[VinSnr]

That's interesting. Now there is no reason to choose a Panoptic to a XW; at least for me. I anytime prefer the XW to a Panoptic especially since some Panoptics have way too much distortion and some of their eye relief is too short for glasses even though at long focal lengths.

Well there are reasons for choosing TV over Pentax.
1. AFOV
2. some people prefer warmer eyepieces on certain objects (eg Jupiter)
3. correction (off axis)
4. size and weight
5. much more available focal length choices and some may fit better to their setups

Another reason is simply because TV is American and these people just wanna support American companies.

[Canopus Lim]

Yeah some people are very pro TV (especially some Americans) but so do other people who are pro the brand that they like too.

TV eyepieces have their advantages and disadvantages and so do other brands too.

[Tachyon]

Yeah... I remember some time back some Americans were shocked to find out that some TV EPs are produced in Taiwan and Japan!

[VinSnr]

Yeah... I remember some time back some Americans were shocked to find out that some TV EPs are produced in Taiwan and Japan!

If you follow the forum in Astromart, you'll find that many stick with AP or willing to queue for AP simply only for one reason : AP is American.

What the heck....

[rlow]

3. correction (off axis)

I would have thought that Pentax XWs have better correction for off-axis aberrations than Panoptics. :roll:

[Canopus Lim]

"Dioptrx is no longer a reason to choose TV over Pentax "

"I would have thought that Pentax XWs have better correction for off-axis aberrations than Panoptics. "

I was saying there are other reasons to choose TV over Pentax as stated by Anat. Not necessary refering to Panoptics. There are so many TV eyepieces types and focal length available. Some of them better than others in different areas. Yup and I would anytime prefer the XW over the Panoptic.

From what I gather:
Main reasons why people like about XWs are the 20mm eye relief (which basically is one of the biggest reason), sharp (especially on-axis), transmission, less distortion and colour (so called neutral). Those that really like XWs are those that observe very faint galaxies or very faint details and there will be differences between XW and TV since the Pentax has the SMC coatings. There are people who just simply ignore the fc in some of the XWs for the slight improvement in transmission. But these people have tracking.

For information, max transmission of Pentax is 96 percent at 550nm (yellow green). Max transmission of T6 Naglers is also 96 percent but at 600nm (red). Due to higher transmission in red, the Naglers are warmer (white becomes cream). For blue and green, Pentax XW is about 2 percent higher in transmission. So the difference is actually very slight but probably noticeable for objects near the limit of visibility. It also depends on objects as different objects have different colours (even though we cannot actually see colour since they are too dim). For example the galaxy cores tend to have older stars and are red biased while the spiral warms tend to be younger stars and blue. Hence Pentax XW do have a very slight edge (2 percent) in detecting spiral arms but Naglers have an edge in detecting fainter galaxies (core since it is more red biased). However this also got to do with the person's 'spectral response' to colour too. Nobody see colours exactly the same and hence that is why there are always so varied reports on transmission. For Jupiter, to see the details in GRS, the warmer eyepieces does better all the time as the details in GRS are red/brown. Cool (blue biased) eyepieces will just show it as white and becomes difficult to see the details in GRS. There are pros and cons in colour reproduction. It is not so simple as saying one is better than the other. Having both types of eyepieces will get the most detail out of a planet/galaxy or other objects (that is why I have warm and cool eyepieces). Warm eyepieces btw are not coffee filters as claimed by some people. Filters are meant to remove. I would say warm eyepieces are red biased which does not mean that transmission in blue or green is bad just that it is less than in red. I found the transmission data of quite many eyepieces and if I am not wrong done by Markus Ludes.

For older Panoptics the max transmission is less than 90 percent. That is why the XWs have a clear transmission advantage not so with the modern Naglers though as the transmission is rather close. I feel that only experienced people will notice a transmission difference and that depends on the colour of the object.

Reasons why people like TV:
1. Well corrected off-axis and generally sharp
2. Generally flat field
3. For Naglers, larger AFOV (noticeable to some all the time)
4. Size especially T6 and smaller T5
5. Good transmission (for T6 and T5 Naglers, Plossls). As good as UO ortho and even slightly better.
6. Many available models and focal lengths to choose
7. Some americans just want american products

[acc]

wow that's good information.

3. For Naglers, larger AFOV (noticeable to some all the time)

I am one of those afflicted with this 'condition'... The main reason I favour my Meade and TV 80deg FOV eps over the more expensive Pentax XWs is that looking through the latter feels a bit like staring down a straw (when compared to the wide vistas in the former).

[rlow]

As I have discussed the other day on the way to Mersing, I find it amazing that many people here still have the misconception that Pentax XWs are more expensive, when XWs from Japan can be generally cheaper than TV Nagler Type 6s from USA.