16 replies to this topic

[AdmiralAckbar]

Hello all,

 

I have a 10" f/4 dob on order. I plan to use it with a Paracorr, and I was also hoping to use a 40mm Pentax XW. Using the FOV calculator, I see that this combination would yield an exit pupil of 8.7mm.

 

If I understand correctly, this will amount to some light being "wasted" since my pupil only accommodates an EP of ~7mm (max). But are there other drawbacks to an EP this large? I would like to have as wide an FOV as possible, so my thinking is that it's ok if a little light is wasted as long as I can get that nice wide FOV.

 

Thanks for your help,
AA

[BradFran]

Not for Admiral Akbar with those big eyes! But check the 46.5mm field stop of the 40XW against the fully illuminated field of your telescope. It should be fine to drop down to 75% illumination for visual use. Depending on the size of your secondary and position of focuser, you could see a bigger drop off in the outer field and even vignetting. The secondary mirror shadow will pop into the center of the view as well, it bothers some more than others. But on an F4 Dob, even with the Paracorr, I wouldn't go past the 30XW.

[markb]

The maximum field is determined by the barrel field stop, and, therefore, by the focuser tube diameter. Or 2-in barrel and a 2-in focuser, this should be somewhere around the 48mm thread diameter for the filters, 47 to 48 mm maximum field

 

You can use the first two calculators at Wilmslow Observatory's site, just Google Wilmslow observatory calculator to fool around with various combinations, but, I did this years ago and again recently.

 

Maximum, or near maximum fields in a 2-in focuser can be had with an APM UFF 30mm, a good choice with a fast refractor and, I would imagine, a good choice with a fast reflector but I don't know for sure, or a 31 Nagler. The APM will be a little shorter the maximum, and the Nagler should hit the maximum field. There are, I believe, 70 degree eyepieces in the 34 to 36 mm range that should provide similar large fields, but I would run them through the Wilmslow calculator to get exact numbers for comparison

 

Many folks like the longer focal lengths and narrower fields of 40 mm eye pieces, but I like maximum fields and maximum field of view, just my preference.

 

On the moon you will almost certainly suffer problems with the secondary shadow. On less intensely bright objects, the secondary shadow may or may not be bothersome as Bradfran mentions.

Edited by markb, 02 December 2020 - 01:40 PM.

[sg6]

Some "might be wasted" as you say, but being practical if what you see is good then so what?

 

Also what makes you say you have a pupil of 7mm max ?

The pupil size discussion went a bit mad here some years ago, people were claiming less and less, and as that was at night iit meant they had to almost have a zero size pupil in daytime. Yes, it was that stupid.

 

One person claimed a probable 5mm to 5.5mm. He also had the chance to ask his optician, who measured it and said 8.3mm.

 

It is almost impossible to measure your own pupil. You need good light to do so and good light means a smaller pupil.

Edited by sg6, 02 December 2020 - 02:22 PM.

[AdmiralAckbar]

Some "might be wasted" as you say, but being practical if what you see is good then so what?

I asked the question to elicit impressions of whether what I would see would be good before I plop down $350 on the Pentax eyepiece.

[Alrakis]

Another issue with using this eyepiece with this telescope is that your eyes can't open to 8.7 mm. Not sure of the exact math, but I am guessing that even if you could use the eyepiece and it didn't cause problems I think that it would in effect reduce the aperture of the scope from 10 inches down to at most 8 inches or even 6.5 inches. You would be better served to keep the focal length of the eyepiece down to 25mm to 30mm. A good example of an eyepiece in your scope to get the maximum TFOV would be a explore scientific 25mm 100 degree eyepiece. 47x magnification, 100 degree apparent field of view, and a true field of view of 2 degrees, 8 arcminutes.

 

Chris

[Tony Flanders]

It is almost impossible to measure your own pupil.

Actually, there are several methods to measure your own pupil accurate to about 0.5 mm. I have done all the methods I can think of, and they agree pretty well in my case.

The simplest is to stand in a dark bathroom in front of a mirror, hold a metric ruler directly below your eye with one hand and snap a flash photo using a digital camera in the other hand.

The second simplest is to get a set of metric Allen wrenches, hold them directy in front of your fully dilated pupil, and find the fattest wrench that does not block out the light from a star.

 

In response to the original question, I sometimes use a 40-mm eyepiece on my f/5 Dob for an 8-mm exit pupil. My own pupils open to about 5.5 mm. The only time I have ever had problems with the shadow of the secondary is when viewing the Moon.

 

Having said that, I wouldn't spend too much on the 40-mm eyepiece. The view in such a situation is dramatically dimmer and less vibrant than the view through an eyepiece that matches my own pupil diameter. So I use the 40-mm eyepiece quite rarely on that particular scope.

[BradFran]

For a 10" f/4 Dob with coma corrector, I would look for a 30mm APM UFF or a 27mm TV Panoptic.

Edited by BradFran, 02 December 2020 - 03:00 PM.

[AdmiralAckbar]

For a 10" f/4 Dob with coma corrector, I would look for a 30mm APM UFF or a 27mm TV Panoptic.

Well then guess who's a lucky duck

 

[Alrakis]

Wonderful. You should be able to squeeze the Pleiades into the 30mm eyepiece. 

 

Chris 

[spaceoddity]

f/4 or f/4.7? I'm not aware of any dob makers that make an f/4 10" or why they would want to. The 30 apm and 27 pan will be good. You don't want an 8.7 exit pupil I would say so wider field options would be ES82 30mm, Nagler 31mm or Meade PWA 28mm. 

[BradFran]

If you're looking to beat those, your only option is a 31mm Nagler, but it weighs more than a can of baked beans... probably a bit much for a 10" Dob!

[AdmiralAckbar]

Ok! Thanks! 

 

Edited by AdmiralAckbar, 02 December 2020 - 06:51 PM.

[Redbetter]

8.7mm is getting pretty large when paired with what will likely be a substantial obstruction on a 10" f/4.   Unless you know your pupil reaches 7mm already, this combination trends toward the risky side as far as possibly seeing secondary shadow dimming in the center of the field.   In bright sky conditions it could be an issue, particularly if your pupil dilates to 5 or 6mm. 

 

My guess is that the secondary mirror is 88mm since GSO sells the 10" f/4 (actually 250mm) and 88mm secondary as a set.  That is a secondary obstruction of  35.2% (about the same as an 8" SCT.)  If the eyepiece exit pupil is 8.7mm, then the secondary will be 3.1mm of that.  With a 7mm dilated pupil the effective obstruction to the eye will be 44%, with 6mm it is 52%, and with 5mm it is 62%.  

 

A 31 Nagler would get you nearly the maximum possible field of view (42mm field stop) with a 6.7mm exit pupil.  A 35 Pan could be used to "overshoot" a little on the 7mm guideline with less concern, but it will provide only about the same field of view as the 30 UFF.

 

Since you already have the 30, I recommend using it first to find out how well it does for you and how clean the edge is with the Paracorr.   If you don't see any dimming issues compared to the 27 Pan, but you are not as happy with the edge performance of the 30 by comparison, then my recommendation would be to sell the 30 UFF and 27 Pan and upgrade to the 31 Nagler.  I don't see what the 27 Pan will do for you with this scope, assuming that the 30 UFF performs well...and if the 27 Pan is producing a better visual image than the 30 UFF, then it would make sense to expand the field with the 31 Nagler as long as weight/balance/inward focus travel with the Paracorr don't preclude it.     

 

I never really understood 27 Panoptics, at least not when the 26 Nagler was still available.  The difference in field size is considerable:  30.5mm field stop vs. 35mm.  For everything the 27 Pan could do I would rather have either the 31 Nagler or the 26 Nagler (the 26 has been discontinued.)  I picked up a 26 to use some day with a sub f/4 scope.  I use it occasionally with various scopes while trying to find the best balance of image scale and exit pupil.    

[Jon Isaacs]

My 2 cents:

 

I've had my 12.5 inch F/4.06 for 20 years. I have a Paracorr. I've had a 41 mm Panoptic for 5 or 6 years.. my pupil is (or was) larger than 7 mm..

 

An eyepiece like the 40 mm XW or the 41 mm Panoptic in an F/4 Dob is one of things:

 

If you have one, try it. If you don't have one, don't bother buying one. An eyepiece like the 31 mm Nagler offers a 6.8 mm exit pupil, plenty large enough.

 

Jon

[phillip]

Just experience with a 40mm plossel huge field of view but flaws on far edge and background loses contrast as brightens up, but still grab for searches.

 

Best is 30mm ranges, my Baader 32mm has nice contrast and sharp to edge on the XT10. Very reasonable cost eyepiece consideration. 

 

Clear Sky

[maroubra_boy]

I have 2 8" f/4 Newts. One as a dedicated truss dob, the other a solid tube that i can put on a dob mount or on to an eq mount if I want to do imaging with it.

A 30mm 82° gives me close enough to the widest TFOV i can get with this focal length (3°!). I know the exit pupil is larger than my pupil. I could either use a 100° 20mm to get a similar TFOV, but I struggle to enjoy 100° eyepieces. I live what this 30mm eyepiece & scope combo gives me. So i lose a bit of light, but big deal really!

A 40mm EP won't give you anything more TFOV wise, but will seriously challenge the image quality if this Pentax 40mm EP shows astigmatism in such a fast Newt.

The only thing with such a scope is I don't use this 30mm eyepiece with it from my home in Sydney as the brighter sky means the shadow of the secondary mirror is just too prominent and way too distracting. 25mm is also too long from home.

From home i would use a 25mm plossl solely as a finder EP, and use EPs 13mm & shorter with this scope. But you will find our own set of EP/scope combos that work best for you.

Alex.

Edited by maroubra_boy, 03 December 2020 - 06:04 AM.