All about SCT

Post by **weixing** » Sun Jan 28, 2007 1:23 am

Hi,
Very nice ISS photo

My telescope field is nearly 2 degrees

Below is OT:
How you get nearly 2 degree true FoV using a C5?? I can only get around 1 degree when using a 2" GSO 42mm 65 degree AFoV eyepiece (theoretically should be 2.1 degree of true FoV) ... I remember that I put the full moon in the centre of the FoV and only half a full moon of free space surround the full moon. I later measure the baffle of the C5 which is around 25mm and I use the forumla True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3 and the result is around 1.1 degree of true FoV, so I thought C5 can only get around 1.1 degree of true FoV or did I do anything wrong?? Or did my eye got stamp and measure the C5 baffle diameter wrongly... Hmm??

Have a nice day.

Canopus Lim · Post by **Canopus Lim** » Mon Jan 29, 2007 9:15 am

I use my 35mm Panoptic 68 degree AFOV so theoretically it should be about 1.9 degree. It is definitely not 1.1 degree as I have looked at the moon with it. It could fit quite many moons (0.5 degree TFOV) across its field. Using my 17mm Nagler (1.1 degree TFOV about same field stop as a 32mm plossl), the 35mm Pan is much noticeably wider in true field (about twice as wide). Also I did not observe vignetting with the 35mm Panoptic. Stars look bright across the field. Also, with the 35mm Pan, I could put the entire Pleaides (about 2 degree size) comfortably.

rlow · Post by **rlow** » Mon Jan 29, 2007 7:33 pm

I use my 35mm Panoptic 68 degree AFOV so theoretically it should be about 1.9 degree. It is definitely not 1.1 degree as I have looked at the moon with it. It could fit quite many moons (0.5 degree TFOV) across its field. Using my 17mm Nagler (1.1 degree TFOV about same field stop as a 32mm plossl), the 35mm Pan is much noticeably wider in true field (about twice as wide). Also I did not observe vignetting with the 35mm Panoptic. Stars look bright across the field. Also, with the 35mm Pan, I could put the entire Pleaides (about 2 degree size) comfortably.

I concurr with that. I had used a 40mm MK-70 with 70 deg. AFOV previously on my C5+. It doesn't matter what the maths or theory says, its the 'practical' that counts for me - enjoy the view!

Post by **weixing** » Tue Jan 30, 2007 3:47 pm

Hi,
Sorry OT again:

I concurr with that. I had used a 40mm MK-70 with 70 deg. AFOV previously on my C5+. It doesn't matter what the maths or theory says, its the 'practical' that counts for me - enjoy the view!

I think most of us also want to enjoy the view, but in order to do that we need to get the correct equipment/accessories. Unfortunately, most of us don't have unlimited budget for trial and error and depend on those theory and maths to help us get the most suitable equipment/accessories that fit our need at first attempt. If those theory and maths was not reliable or apply wrongly, I think most of us will have big problem in decide what is the correct equipment/accessories to get... (hmm... now I know why Astromart is so popular and why HST doesn't work at first time... ha ha ha

) So I think it's importance to know when and how to use those theory and maths correctly.

Have a nice day.

PS: Hope the secondary mirror I order base on those theory and maths (hope I apply it correctly) will work as it suppose to...

ariefm71 · Post by **ariefm71** » Tue Jan 30, 2007 4:53 pm

C5 at F10: 25/1250*57.3 = 1.15 degree
C5 at F6.3: 25/787.5*57.3 = 1.8 degree

Canopus Lim · Post by **Canopus Lim** » Tue Jan 30, 2007 5:59 pm

Actually, the formula used will calculate the maximum unvignetted field and that is 1.1 degrees (100 percent illumination). After that 1.1 degree, that will be drop off in illumination. The baffle tube will cut light more than 1.1 degrees and will cause vignetting. However, how severe the vignetting depends on how the scope was design, the secondary size, the baffle tube design (length, angle, size etc). Also, the C5 is a slow scope, so I guess vignetting is not too serious since the rays are not steep when passing through the baffle tube; hence less rays get cut off and less drop in illumination.

In the case of the C5 for visual having more than 1.1 degree field, there should be vignetting but it is NOT OBVIOUS to my eyes. The vignetting is definitely not more than 50 percent as it would have been obvious. As in a Newtonian, the secondary size will also limit the illumination on the focal plane; you don't get fully illuminated Newtonians across the field unless the secondary is huge. For refractors, it is the tube and hole opening at the rear end that will limit the illumination. For visual use, the C5 is capable of going to 2 degrees and provides view that is nice. You don't see stars at field greater than 1.1 degree look as though they lost a magnitude of brightness.

rlow · Post by **rlow** » Tue Jan 30, 2007 11:59 pm

depend on those theory and maths to help us get the most suitable equipment/accessories that fit our need at first attempt. If those theory and maths was not reliable or apply wrongly, I think most of us will have big problem in decide what is the correct equipment/accessories to get

I agree with that statement. However, sometimes practice has proven that theory may not be reliable or maths may be wrongly applied.

An example that theory may not be reliable: Some people tend to take it for granted that a eyepiece's AFOV is correct. A 15mm 'P' series eyepiece with 68 deg AFOV should show a wider true field than a 14mm 'X' eyepiece with 65 deg AFOV in the same scope, but it was proven in the field that the 14mm has a bigger TFOV than the 15mm. Moo saw that with me at Mersing.

I later measure the baffle of the C5 which is around 25mm and I use the forumla True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3 and the result is around 1.1 degree of true FoV

The other example: Maths shows 1.1 deg on paper, I saw 2 degs thru the 40mm MK-70, so which is correct? In this case, I would go with what I see, rather than what I think. It so happens that I have my C5 and 40XW with me in the office this evening so I did another check by measuring the TFOV seen of Pleiades and the Orion Nebula region and measure it against the same field in my Sky Atlas 2000.0 and I still get 2 degs. So from 'practice', I would say that the max. TFOV is limited by the effective field stop of the eyepiece and not the rear aperture of the C5. Hence the calculation is probably 46/1270 x 57.3 = 2.07 degs which corresponds well with what I saw. Note that the effective FL of the C5 for visual use is not 1250mm.

So allow me to rephrase my earlier statement: in this case, 'practice' counts for me more than the 'theory' or 'maths' as I have shown in the above examples. As Wxg knows, I still do rely on maths and theory in other cases, such as the theory & maths regarding the secondary mirrors of our respective Newtonians which Wxg and I discussed over dinner the other evening.

ariefm71 · Post by **ariefm71** » Wed Jan 31, 2007 12:08 am

I would say that the effective field stop is limited by the eyepiece and not the rear aperture of the C5.

Unless the baffle is transparent (

), I have a very hard time visualizing that

. There must be a logical explanation for this "phenomenon".

rlow · Post by **rlow** » Wed Jan 31, 2007 12:10 am

Arief, that was an editing error, but you are faster than my correction!

jermng · Post by **jermng** » Wed Jan 31, 2007 1:18 am

Hmmm .. then in that case, in practice, waht's the largest TFOV you guys have gotten in a C8?