Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Largest eyepiece f/l for f/10 Mak.

This topic has been archived. This means that you cannot reply to this topic.
16 replies to this topic

#1 CHRISTOS

CHRISTOS

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 07 May 2004

Posted 23 November 2005 - 06:29 AM

I want to complement my eyepiece collection I use with my Intes Micro 5" f/10 mak. I was looking for a 42mm Superview which will give ~1,8 deg. TFoV, but I got the opinion that it will severely vignette and will be like looking thru a straw, because of the small (true) AFoV of only 55 degrees. Although I am not completely excluding this eyepiece, I am thinking for a Meade 36mm QX as an alternative.
Can anybody, preferably user of a similar telescope, give an advice on this ?
Thank you very much.

#2 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5,746
  • Joined: 30 Sep 2003

Posted 23 November 2005 - 07:43 AM

55 degrees is hardly a straw. Only perhaps to people with Naglers, but I think the wide AFOV craze is a little over rated. 55 degrees is the same as a good Plossl and certainly more than an orthoscopic.

I have a M503 and a 42mm Superview will not be "severly" vignetted. Certainly my Vixen LV 50mm EP is not and the Superviewed I tried was no worse (they have the same TFOV). There is vignetting, but it is not very noticable. I can see it more during the day then the night. And you have to look for it.

#3 CHRISTOS

CHRISTOS

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 07 May 2004

Posted 23 November 2005 - 09:50 AM

Thanks very much Will,
do you know what is the largest obtainable TFoV of this scope, using any combination of eyepiece / focal-reducer ?

#4 Gary

Gary

    Messenger

  • *****
  • Posts: 409
  • Joined: 12 Jun 2005

Posted 23 November 2005 - 01:16 PM

Christos,

I have a M603 and the University KONIG MK-70 40mm gives excellent wideviews. It's as if they were made for each other.

Gary

#5 LivingNDixie

LivingNDixie

    TSP Chowhound

  • *****
  • Moderators
  • Posts: 19,275
  • Joined: 23 Apr 2003

Posted 23 November 2005 - 01:33 PM

I have a 40mm MK70 on my 8in SCT and really like it. The new ones come with an eyeguard that can be detached.

#6 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5,746
  • Joined: 30 Sep 2003

Posted 23 November 2005 - 06:17 PM

Thanks very much Will,
do you know what is the largest obtainable TFoV of this scope, using any combination of eyepiece / focal-reducer ?


I would say around 1.8 degrees (perhaps 2 degrees) is the limit to this scope. I would think the TV 55mm Plossl or 31mm Nagler gives the largest TVOF, but a 50mm Vixen, 42mm Superview, or 40mm Pentax are close too. I don't think using a reducer is going to be a good option - more optics and a pain to switch in and out and any gain over 2 degrees will probably lead to obvious vignetting. I would look for an EP in the 40mm to 55mm range. Although I find EPs with long focal length can have too much eye relief and blackout are annoying - I made an eye cup for my Vixen because of this - the Vixen has 30mm eye relief.

But I would not be focused on just TFOV. I bought the 42mm Superview and a replacement for my Vixen, but the contrast in the Superview was noticably less. The Superview is good value for money and a nice EP, but it was only giving me a better AFOV than the Vixen - 55 degrees vs. 45 degrees for the Vixen. I kept the Vixen because of better contrast and a larger exit pupil which gives me about 40% brighter images.

#7 CHRISTOS

CHRISTOS

    Sputnik

  • -----
  • topic starter
  • Posts: 36
  • Joined: 07 May 2004

Posted 24 November 2005 - 11:35 AM

Thanks again Will, I forgot the exit pupil factor, it seems to be important.

#8 Joneil

Joneil

    Vendor (O'Neil Photo)

  • *****
  • Vendors
  • Posts: 218
  • Joined: 28 Sep 2005

Posted 24 November 2005 - 01:20 PM

Antares has just introduced a new focal reducer for 1.25" sized eyepeices. Although it can be used on any telescope (or so I am told) I think it is aimed primarily at the Maks.

How the thing actually works - dunno yet. Haven't got mine yet, but when I do, i plan to test it out on my old C90 mak

joe

#9 JohnH

JohnH

    Gemini

  • ****-
  • Posts: 3,198
  • Joined: 04 Oct 2005

Posted 25 November 2005 - 05:25 PM

I got a Micron M809 recently an got a 26mm Nagler for it. I also tried out a 41mm Panoptic and liked that one very much. My opinion is that while the Nagler has a wider AFOV and slightly better correction, the 41mm Panoptic not really inferior in any meaningful way. It is slightly more affordable though.

I could only afford one at the time, so I got the Nagler. It gives an apparent magnification of around 78X, which shows the entire moon.

Hope this helps you.

#10 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 44,555
  • Joined: 23 Jun 2003

Posted 27 November 2005 - 02:48 AM

My calculations show that the maximum field of view with this scope (with visible vignetting at the edge of the field), will be with an eyepiece combination that yields 1.2-1.3 degrees. Some people are less sensitive to vignetting (these are not lunar viewers, though), so a slightly wider field may be usable.
In general, this scope (and others of the same size) are not designed to be used with 2" eyepieces, regardless of what can be put on the back of the scope.
The widest fields of view in 1-1/4" can be had with a 35mm Parks Gold Series Plossl(or Orion Ultrascopic), a 32mm TeleVue Plossl, a 24mm TeleVue Panoptic, or an 18mm Meade Series 5000 UltraWideAngle.
The situation with a focal reducer is this--it lowers the image scale, producing lower powers, but it does NOT give a wider fully illuminated field, as this is due to the baffles in the scope. In fact, a focal reducer slightly LESSENS the width of the field that is illuminated, making vignetting worse. A focal reducer and a 2" diagonal is an especially bad combination.
Hope that helps.
Don

#11 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5,746
  • Joined: 30 Sep 2003

Posted 27 November 2005 - 03:07 AM

My calculations show that the maximum field of view with this scope (with visible vignetting at the edge of the field), will be with an eyepiece combination that yields 1.2-1.3 degrees. Some people are less sensitive to vignetting (these are not lunar viewers, though), so a slightly wider field may be usable.
In general, this scope (and others of the same size) are not designed to be used with 2" eyepieces, regardless of what can be put on the back of the scope.


Don, how did you determine the FOV and image circle with an M503? And if the M503 is not designed for 2" EPs, why does Intes Macro supply a 2" diagonal with them?

#12 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 44,555
  • Joined: 23 Jun 2003

Posted 27 November 2005 - 11:10 AM

I am not sure. I assumed an internal diameter of the primary's internal baffle as 30% of aperture, or 1.5", a focal length of 50". In such circumstances, a 2" aperture will be necessary to achieve the absolutely maximum field of view possible (because there is no 1.5" eyepiece), but eyepieces of long focal length and large field stops will vignette (say, any field stop over 37-38mm). So a judicious use of a 2" eyepiece/diagonal combination will yield the maximum field of view.
And focal reducers do reduce the edge-of-field illumination, more severely the more the reduction.
Since a 24 Panoptic yields 52.9X magnification, and 1.29 degrees of field, it's not really necessary to add the weight, size, and potential vignetting of the 2" star diagonal to this scope in order to get a large field.
Now, if the baffle I.D. is larger than 30%, all bets are off. I would, however, find that a strange design, as the secondary would be unnecessarily large if that large a baffle is required, and inadequately baffled if it isn't.

#13 Bob Pasken

Bob Pasken

    Viking 1

  • -----
  • Posts: 512
  • Joined: 30 May 2003

Posted 27 November 2005 - 03:28 PM

I agree with Don about the choice of eyepieces and the field of view problem. The 32mm Televue Plossl is a good choice. Used in my Q4 (60 inches) I get a TFOV of 1.1 deg and is sharp over the majority of the field.

#14 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5,746
  • Joined: 30 Sep 2003

Posted 27 November 2005 - 06:35 PM

That is strange, I use a 50mm 45 degree AFOV EP with my M503 is there is no significant problem with vignetting and no problem with sharpness at all. That is a TFOV of approximately 1.8 degrees. ITE lists the photographic FOV for the M503 to be 1 degree and 45 minutes and 35mm photographs I have taken with the M503 show no significant vignetting.

But the diameter of the baffle is not the only limit to vignetting, but its length and distance from the focal plane as well - light does not travel in parallel lines, it is focused which is why baffles can be so much smaller the the image circle - measure a baffle, you may be surprised at their dimensions. If I remember correctly, in Modern Optics by Brown, up to 50% vignetting in binoculars is acceptable. Vignetting is acceptable in photographic optics as well even though it is easier to detect in a photograph rather than visually. (I have yet to see a pair of binoculars or a camera lens without vignetting.) So a completely unvignetted image circle is unnecessary. So the question is not how large the unvignetted image circle is, but how large an image circle there is before vignetted becomes obvious. Christos is asking if a 1.8 degree TFOV will be vignetted "severely" with an M503 - he is looking to maximize his TFOV. The answer from my experience using an M503 with that TFOV is no.

#15 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 44,555
  • Joined: 23 Jun 2003

Posted 28 November 2005 - 12:55 AM

Vignetting, like coma, seems to be variably visible based on the individual observer. To wit, I have seen 2" eyepiece modifications to my 5" Orion Mak that yield nearly 2 degree fields of view. The people using this seem to enjoy their scopes and find vignetting not particularly noticeable.
I have the exact same scope, yet notice vignetting with a 1-1/4" eyepiece producing 1.05 degrees. It is completely inscrutable to me that someone could use an eyepiece/diagonal combination that produces such a large field and not notice simply horrible vignetting.
So it is with the Intes scope. It is simply mind-boggling you could use that combination and not notice vignetting. The illumination at the edge of the field must be well over 50% down from the center (not calculated, just off-the-cuff). Yet, I take you at your word that it is not objectionable.
I would merely caution anyone else reading this thread that such a difference describes a vast difference between observers, and you will not know how you will react until you experience the scope with certain diagonal/eyepiece combinations.
Unfortunately, it could be an expensive lesson to learn.
And then again, perhaps not.
Caveat emptor.

#16 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5,746
  • Joined: 30 Sep 2003

Posted 28 November 2005 - 01:22 AM

Don, I am confused? Are you saying the Russian Intes Micro 5" Mak and the Chinese Orion 5" Mak are going to have the same vignetting because they are both 5" Maks? I am not trying to start an argument, but why would a scope designed for 2" EPs be exactly the same as a scope designed for 1.25" EPs. I am just trying to understand where you are making the leap or if your are making this leap and I have missed something?

BTW, I never claimed there was none, just not very noticable.

#17 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 44,555
  • Joined: 23 Jun 2003

Posted 28 November 2005 - 02:15 AM

No, I am not arguing they are the same. The Intes probably has a larger secondary, and larger inside diameter to the primary baffle than the Orion Mak because of the shorter f/ratio.
However, I am not trying to start an argument, merely commenting, in my last post, that the visibility of vignetting varies a lot from person to person. One person may see a 30% decrease in brightness, another may require 50%. It's considered axiom that humans can barely see a 10% difference because of the logarithmic response of the eye. After that, there's a lot of variation.
As for the actuality of vignetting, here is a site that will give you some insight into the vignetting calculations that come from central baffle tubes in SCTs and MCTs: here.
Additionally, this site describes the issues with focal reducers and vignetting.
I would, however, say that it is unlikely a manufacturer would make a 5" catadioptric designed for 2" eyepieces that have large field stops. So there would be, somewhere between the largest field stop in a 1-1/4" eyepiece and the largest field stop in a 2" eyepiece where vignetting would become noticeable.
Where that point is is related to the observer's sensitivity to vignetting. My earlier point was that it may be expensive to experiment with 2" eyepieces to find out where that point is.
That's all.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics