Optical Path Recommendations

[Wez (Wesley Lamont)]

Here is my second noob question following on from my Primary Mirror assessing and cleaning. My scope is the Skywatcher 16" Dob.

I currently have a Baader Hyperion Zoom eye piece (as well as the two plossls that came with the scope but aren't really worth mentioning). I am currently trying to keep my light path in 2" as much as I can.
- The Baader zoom + 2 inch extension tube in the Telescope can achieve focus and all is good. Planets are still white shimmering balls so from what I could find going for more zoom would be a benefit.
- I purchased a TV PowerMate 4x. Initially I could not achieve focus with the powermate and the Baader zoom with any combination of Powermate then extension or vice versa.
- I talked to the astronomy store from where I bought the scope and they suggested removing the tube on the Powermate and the extension tube entirely but that was not successful
- By chance I finally did get focus when I was pulling the zoom out of the scope noticed I could get focus floating further out from the actual scope. So that lead me to get some extensions (Baader Locking extensions).
- During that purchase I also got a coma corrector (Baader again) so now I've got the Coma Corrector, 2 extensions, Powermate, regular extension tube and finally the zoom lens.

So that brings me to my question. Is there a way to work out the best focal length? Is there any recommendations on using the combination of the equipment I have in a particular order that is recommended. Every post I found regarding the Powermate just had it sitting in the tube and the users stating it just added magnification. Something I have never been able to achieve.

I'll add the photo of my various bits as well since its likely hard to comprehend just through text.

[sunslayr (David)]

For me I need a 37mm extension inserted in the focuser to reach focus with most eyepieces and I then remove it for taking photos. When I use my 4x powermate I must also use the extension first and then put the powermate in that and then finally the eyepiece in the powermate. Try assembling about 40mm of extension and test it, I think you will find that it will reach focus, even if you have to rack your focuser all the way out. As you build up your eyepiece collection, (naglers are a good choice they come up in the classifieds often) you can use something called 'parfocal rings' to set them all to the same focus point so you can swap them out without refocusing.

[Wez (Wesley Lamont)]

Originally Posted by sunslayr

For me I need a 37mm extension inserted in the focuser to reach focus with most eyepieces and I then remove it for taking photos. When I use my 4x powermate I must also use the extension first and then put the powermate in that and then finally the eyepiece in the powermate. Try assembling about 40mm of extension and test it, I think you will find that it will reach focus, even if you have to rack your focuser all the way out. As you build up your eyepiece collection, (naglers are a good choice they come up in the classifieds often) you can use something called 'parfocal rings' to set them all to the same focus point so you can swap them out without refocusing.

Thanks David, I shall try that and see how I go. I'm hoping to get a 31mm nagler second hand as my next eye piece. Not sure my chances but that's the hope at least.

[Wez (Wesley Lamont)]

Originally Posted by sunslayr

When I use my 4x powermate I must also use the extension first and then put the powermate in that and then finally the eyepiece in the powermate.

Do you use the PowerMate with its own tube or remove that and just use the optical section?


I can screw the Hyperion to the end of the PowerMate but I'm sure the threads aren't correct so that's rather concerning

[astro744]

The Powermate is to be used visually as is with either 2" eyepiece (remove 2"-1.25" adapter that comes with Powermate first) or with 1.25" eyepiece. There is a special Tele Vue T-ring adapter that is screwed to the top of the bottom part of the Powermate when used photographically, (replaces visual top).

Why do you want to use 2" only eyepieces? The is no advantage at higher powers in fact you have the disadvantage of all that extra weight. A 31mm Nagler and 4x Powermate will give you a 235x which is equivalent to using a 7.75mm eyepiece. You will get 82 deg apparent field but you can get the same with a 7 on 9mm Nagler at slightly different powers but a much lighter solution. An 8mm Ethos will also give you a 100deg field and yet is still only 1.25" but has the convenience of a 2" skirt should you not wish to use a 2"-1.25" adapter. If you are using a coma corrector then that is even more weight.

Note going higher power than an eyepiece between 7 to 9mm on your telescope will demand exceptional seeing, good optics, good collimation, good thermal stability of your mirror and no local heat sources in your optical path, e.g. warm driveway, neighbours fireplace etc.

You have a Baader 8-24mm zoom which is an ideal range of powers on your telescope; 228x to 76x. A 31mm Nagler will you 58x and 1.3 deg true field but at 6.9mm exit pupil which requires dark skies. An alternative would be a 26mm Nagler (if you can find one) giving 70x and 5.8mm exit pupil and 1.1deg true field. A 21mm Ethos would give 87x and 4.7mm exit pupil and 1.13deg true field. Note all these calcs are without Paracorr. If using Paracorr divide exit pupil by 1.15 and multiply magnification by 1.15, (or multiply focal length by 1.15 before doing other calcs.).

At the high power end eyepieces between 6 and 10mm would be useful but 6 to 7 only under exceptional seeing. You can use your Baader at the 24mm setting with your 4x Powermate to determine if 6mm is going to be useful. Note given your focal length of 1828mm a 2x Powermate would be more useful more often. However after saying that I once had superb views with a 10.1" f6.4 with 35mm Panoptic and 4x Powermate giving 187x. (It was top heavy though at low altitudes). I now have smaller shorter focal length eyepieces which are more practical.

[Wez (Wesley Lamont)]

Thanks for all the info Astro744,



2" is primarily so I can eventually use a full frame camera as the end. This is also the reason to go limited EP's.


The intention with the Nagler was to use it without the PM to get very wide views. I was looking at the Panoptics and Ethos but the consensus I can find online is the 31 Nagler is the ideal for my scope (not that I have enough knowledge to confirm everything I'm reading)


I can certainly see the optic pathway getting huge. I had a play with the Zoom and Varilocks to get extra length and was very impressed (during the day just looking at a tree several hundred meters away)


The Powermate I was thinking would give me extra power with the zoom, double the use of a wide angle lens and then useful for the eventual Camera setup. But for now trying to work out orders and lengths and position of elements is confusing me.

[astro744]

Ok. A 2” focuser to accomodate a full frame camera doesn’t mean you have to stay with 2” eyepieces. Your focuser would have come with a 2”-1.25” adapter, if not your Powermate would have. 1.25” is all you need for high power work.

The lowest power and therefore widest true field though is only achievable with an eyepiece with the largest field stop diameter and of course 2” is going to allow for a larger diameter than 1.25”

Yes the 82 degree Nagler 31mm is a good choice provided your sky background is not too bright. If you use a Paracorr it will give 1.13deg field with 6mm exit pupil which is very nice. Even without a Paracorr will be a good match in a darker surround. Note the reason the 26mm Nagler was invented was because of the ever growing number of f4.5 telescopes in use but it was eventually discontinued when the 21mm Ethos came out. The 31mm has better eye relief than the 26mm and is quite comfortable to use. I’ve not tried the 21 Ethos.

I think you will like the 31mm Nagler if you can get one used. Note there are a few 30mm/80deg lower cost with lesser number of element ‘clones’, some claiming Nagler like performance. These simply do not work well at f4.5 but are OK in f10 or greater although even at f10 I still prefer my 31 Nagler over the 30/80 ‘clone’ that I also have and purchased first thinking it would work well in my f6.4 Newtonian only to find myself eventually getting the Nagler.

[Wez (Wesley Lamont)]

Cheers Astro. You are a wealth of knowledge


my scope is f4.4 so def on the fast side. I have the Baader Coma Corrector which claims being find down to F3.5 I think. That should make the Wide Field eyepieces fine.


You are quite right on the EP not needing to be 2". I guess I was only really thinking of one more and the 31" nagler is suppose to work better at 2 due to the field stop (can't recall measurement)


Do you think there would be anything better for the combo with the Coma Corrector and the Powermate. I'm thinking above and below the Zoom would make sense which the 31" would achieve. The 21 ethos would be too much zoom with the PM and wouldn't get as a wide a view as the Nagler.

[astro744]

May I give you some links to more information.

Tele Vue eyepiece specifications. (discontinued products at bottom of page):

https://www.televue.com/engine/TV3b_page.asp?id=214

Tele Vue Powermate: (click on other related pages on index at the left of page)
https://www.televue.com/engine/TV3b_...d=53&Tab=_back

Tele Vue Paracorr: (click on other related pages on index at the left of page)

https://www.televue.com/engine/TV3b_...d=61&Tab=_back

You've probably seen these links but they are here for others as they are usefull especially the eyepiece specification table.

I believe you are still after a minimalist set of eyepieces to use standalone and with your 4x Powermate and Baader coma corrector. I have zero experience with the Baader coma corrector so cannot comment on it and its suitability with the Powermate. To make your minimalist set work you would also need a 2x Powermate (comes in 2" only) and then complement with all the 2" eyepieces with field stops greater than 27mm. This is an expensive and cumbersome route and I am not encouraging it.


You should buy a low power eyepiece for low power wide true field (and wide apparent field if you desire) work. Then buy some mid and higher power eyepieces for other magnifications.


I recommend you set up a spreadsheet in Excel and list all the large 2" eyepiece focal lengths in the first column. Then in the second column divide each in the first by 2 (shows effect of 2x Powermate). Then in the third column divide each in the first by 4 (shows effect of 4x Powermate).

These will be your effective focal lengths. (Without any amplifying effects of a Paracorr - 1.15x). Not sure if Baader coma corrector is a non-amplifying type.

You could end up buying all the low power options just to get some mid to high power options with the Powermates. If sticking with 4x Powermate only then you jump from low to high power with just about any eyepiece you choose, 21 Ethos, 22 Nagler, 27 Panoptic, 31 Nagler, 35 Panoptic, 41 Panoptic, 55 Plossl, i.e. no mid power options with 4x only.

I do not recommend achieving high power in this way not because of any optical reason, in fact the views are going to be first class, but because overall cost will be greater than if you simply bought a low power, mid power and a couple of high power eyepieces. Also too your optical train sticking out of the focuser will be long and heavy if using the Powermate, coma corrector and large eyepiece together and will likely lead to balance issues depending on viewing angle.

[Wez (Wesley Lamont)]

Thanks again Astro,



I had decided on the Baader coma corrector due to the price difference over a paracor. It is also non magnifying as well.


Minimalist is definitely the aim at this stage. My thinking is 4x only and get a low good low power option.
Zoom + 4x would give 2-6mm (most of which won't really be viable without tracking)
Zoom on own is 8-24mm (covers general use as you said)


Nagler 31 would add, 31mm and 7.75mm
An Ethos 21 would add 21 and 5.25 which isn't ideal at either end
Something like a 26 or 27 as you mentioned would be very nice though it feels as though just a single EP that would get my above and below the zoom would be great for now.
Actually I think I'll add the 26 Nagler to my wanted list as well and see which one I becomes available.



The individual EP at that level would certainly be far superior to the Zoom but that would cost more than the scope and everything added on. Though I may well look back on this post in a decade and laugh at the naivety of a new scope user