34 replies to this topic

[Eddgie]

Hello Eddgie,

thank you for your advice,  I really appreciate your recommendations. I have to say that in this moment I am really unable to take any decision. I got too many good advices from the people here!

My sky is a rural-suburban area, maybe between bortle 3 and 4. So there are better places for sure.

 

Let me add that identifying a scope suitable for binos is not an easy task, also because usually the critical parameters involved (backfocus, drawtube aperture) are not diclosed by the vendors.

 

Renato

If you can see the Milky Way with the naked eye, the achromat will work quite well. This will be when such a scope is at its best. 

[revans]

I would like to buy an achromatic (APO´s are not in the budget) refractor to be paired with a maxbright II bino for backyard observations of wide fields (25x to 60x).

No planets, no moon, no quasars.

 

I was considering 4 possibilities: 120mm vs 150mm aperture, and F/5 vs F/8-10 focal ratio. All combinations will go in focus (even including a diagonal prism) without OCS.

 

Now I am stuck, choosing the best combination that fits my needs.

 

Regarding the opening, I think that the only issues that speak against the 150mm vs the 120mm, apart from the price, should be weight and size. Or there are other tradeoffs to manage about opening?

 

But actually, my doubts are mostly related to the f/ratio. So far I can only see the advantages of the fast one (f5):

 

- can always be turned into a F8-F10 by use of the 2x OCS, while the reverse is not possible, AFAIK.

- wider fields on the same eyepieces,

- brighter views with the same magnification (not really sure of this...is it correct?)

- OTA is shorter

 

Apparently no tradeoff at all come to my mind in this regard. It doesn´t sound good.... As I have no experience with binos I am probably missing something.

 

Can someone in the forum tell me if I am on the right track?

 

Thanks!

Renato

When using achromats for visual my experience has been that higher f ratios were beneficial for the moon and planets while smaller f ratios were best for deep sky objects and extended objects like open clusters.  Sounds like you want a wide FOV.  This implies a longer focal length eyepiece and the longest you can use is (I think) determined by multiplying the focal ratio of your telescope by 7mm (or the maximum pupil size of the eye).  So, an F5 telescope should not be able to use an eyepiece with focal length longer than 5 x 7mm or 35mm without problems like vignetting.  Chromatic aberration will lessen as the f number increases in achromats and an f15 achromat gives a decent view of planets (I used a small one for years). Focal ratio has nothing to do with true FOV.  True FOV depends on telescope focal length and the field stop of the eyepiece used. You can estimate true FOV by dividing the eyepiece field stop by the telescope’s focal length in mm.  For a result in degrees multiply by 57.3.

 

As the focal length of the eyepiece increases, the magnification goes down, and the apparent FOV increases until at some point the field stop becomes the inner wall of the eyepiece barrel.  But long focal length eyepieces can have exit pupils larger than the maximum pupil diameter of the eye which defeats the purpose.  

 

All this is my current understanding anyway.  I have never been and am not likely to ever be very good at really understanding the science of optics.

 

Rick

[kroum]

Just throwing this out there… a 6” f5 reflector with a low profile focuser will give you the same minimum magnification as the achromat with the 1.25x GPC and should come to focus.

It will have no chromatic aberration, and can also be used for lunar and planetary at high magnification.

 

I have just this setup and I can come to focus with a Denkmeier binoviewer with a focal reducer switch chat gives me basically 1x magnification factor without moving the primary mirror.

[RenatoT]

 

Just throwing this out there… a 6” f5 reflector with a low profile focuser will give you the same minimum magnification as the achromat with the 1.25x GPC and should come to focus.

It will have no chromatic aberration, and can also be used for lunar and planetary at high magnification.

I have just this setup and I can come to focus with a Denkmeier binoviewer with a focal reducer switch chat gives me basically 1x magnification factor without moving the primary mirror.

I have to admit that I'm not sure to understand. Do you simply take a 6" f5 newtonian, add just a low profile focuser...and you can focus, even without OCS? What reflector is it? The Orion in your signature?

It could be a smart alternative.

[kroum]

I have to admit that I'm not sure to understand. Do you simply take a 6" f5 newtonian, add just a low profile focuser...and you can focus, even without OCS? What reflector is it? The Orion in your signature?

It could be a smart alternative.

Yes, except that you can’t focus without any OCS.  I have a Denkmeier binoviewer that has an OCS, as well as the power switch, and an additional focal reducer switch.  When I use this in the lowest power configuration, it yields about 1x overall magnification factor.

 

I also used this setup with a prism-based Orion binoviewer with a 1.6x OCS and it had lots of extra back focus left.  It can’t quite focus without an OCS, but it’s very close. Probably about 3-5mm, so instinctively, I feel that a 1.25x OCS should work in that configuration.

 

This is indeed the 6” f5 Orion reflector in my signature.  It was my first scope and it’s still my most used and most versatile one.

 

The focuser I use is an older Antares low profile crayford with an extendable draw-tube.  These days they sell the same focuser, but with a rack a pinion, which will provide even more stability with the weight of the binoviewer.  The only issue is that the drawtube is held in place by one brass compression ring, so when it is partially extended, it doesn’t necessarily maintain collimation.  At either extreme it’s not an issue though.

[Jeff B]

I have been watching this thread as the thinking progresses.

 

First regarding  use of a 6" F5 reflector, you will absolutely need a barlow device to reach focus with most viewers, which IMO negates the wide field thing, you're still left coma, the nose of the viewer will intrude into the light path and finally (?) you will applying considerable off axis torque to the side of the tube due to the weight of the bino-viewer system.  This configuration would be my last choice....if I chose it at all.

 

You can actually adapt a Synta 120mm F5 or 152mm F5 to F6.5 achromat to be bino-friendly.   I've done it as you can see.  But I had to be very careful with the designs, including the components chosen, to keep from clipping the aperture.  I based both designs on the Baader T2 BBHS mirror diagonal, my Baader MKV viewer, and Feather Touch focusers to handle the load.  The visual images at low power are bright, coma free and just stunning.   The real big drawback is obvious:  price.  Even assuming you already use the same diagonal and viewer for other OTA, the used FT focuser and adapter (in my case, for the 120mm F5, it's 3D printed) will cost a little more than the basic scope its self. 

 

You might get by with cheaper components but you run the risks of a focuser that struggles with the load and the system clipping the aperture.

 

Visually, the step up in brightness from the 120mm aperture to the 152mm aperture (F6.5) is compelling for me for star fields and open clusters.  However, the 120mm is a very compact and easy to handle package.

 

I hope this helps. 

 

Jeff

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[kroum]

I have been watching this thread as the thinking progresses.

 

First regarding  use of a 6" F5 reflector, you will absolutely need a barlow device to reach focus with most viewers, which IMO negates the wide field thing, you're still left coma, the nose of the viewer will intrude into the light path and finally (?) you will applying considerable off axis torque to the side of the tube due to the weight of the bino-viewer system.  This configuration would be my last choice....if I chose it at all.

 

You can actually adapt a Synta 120mm F5 or 152mm F5 to F6.5 achromat to be bino-friendly.   I've done it as you can see.  But I had to be very careful with the designs, including the components chosen, to keep from clipping the aperture.  I based both designs on the Baader T2 BBHS mirror diagonal, my Baader MKV viewer, and Feather Touch focusers to handle the load.  The visual images at low power are bright, coma free and just stunning.   The real big drawback is obvious:  price.  Even assuming you already use the same diagonal and viewer for other OTA, the used FT focuser and adapter (in my case, for the 120mm F5, it's 3D printed) will cost a little more than the basic scope its self. 

 

You might get by with cheaper components but you run the risks of a focuser that struggles with the load and the system clipping the aperture.

 

Visually, the step up in brightness from the 120mm aperture to the 152mm aperture (F6.5) is compelling for me for star fields and open clusters.  However, the 120mm is a very compact and easy to handle package.

 

I hope this helps. 

 

Jeff

Jeff, 

In regards to the 6” reflector, yes you do need some sort of optical path corrector to reach focus without moving the primary mirror up, but it doesn’t need to be a very high magnification factor.  If you use an actual OCS, it will usually have some coma correction built in.

 

In my case, the magnification factor is just about 1x with the Denkmeier and nothing intrudes into the light path.

 

Trading a slight amount of wide field in exchange for being able to use high magnification for planetary observation (when compared to an achromat) is not an unreasonable trade to contemplate. I can still get ~2 degrees TFOV with my setup.

I will attach a photo of my setup at the point of focus with the lowest power combination ( low power arm in on power switch and focal reducer arm in).  While a tiny amount of focuser draw tube protrudes into the OTA, it doesn’t reach the light path, and if you look closely, the actual front element of the OCS is recessed farther in.  I pull it out somewhat so that I can reach focus with the highest magnification combination by extending the extendable part of the draw tube.

 

you are correct that there is a large moment arm that can twist the OTA, but I haven’t noticed any issues.  Maybe my circa 2000 OTA is on the thick side???

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Edited by kroum, 14 April 2023 - 12:55 PM.

[RenatoT]

I think that if one can reach focus with a reflector, without the need for extreme magnification (let's say that, for example a 1.25x OCS will do) then the reflector could be in my opinion a valid option, particularly if one (like me) already has such a scope at home. It would be again a matter of tradeoff: coma vs CA....

 

If focus is attainable only by means of higher magnifications, then I agree with Jeff, reflectors have to be ruled out from the beginning.

 

Mechanical issues worry me not so much. I have a Maxbright and XCel-LX eyepieces, my setup is not so heavy. My reflector has a generous 98 mm backfocus, and I think that if I would only apply a low profle focuser and a 1.25x OCS, focus could be at reach. I think I should try this way first (I have to wait until the end of may though). The reflector is an f/4, so coma will be evident.  On the other side, the 1.25x OCS together with the inherently limited FOV of 1.25" eyepieces could mitigate the impact of coma.

 

If I din't own a reflector, and I had to buy a scope right now,  I think I would go for a refractor, for sure in the range f/5-f/7, but no idea if a big achromat or a (very) small second hand APO...so many good recommendations in this thread made the choice still harder...

Edited by RenatoT, 15 April 2023 - 07:16 AM.

[kroum]

I think that if one can reach focus with a reflector, without the need for extreme magnification (let's say that, for example a 1.25x OCS will do) then the reflector could be in my opinion a valid option, particularly if one (like me) already has such a scope at home. It would be again a matter of tradeoff: coma vs CA....

 

If focus is attainable only by means of higher magnifications, then I agree with Jeff, reflectors have to be ruled out from the beginning.

 

Mechanical issues worry me not so much. I have a Maxbright and XCel-LX eyepieces, my setup is not so heavy. My reflector has a generous 98 mm backfocus, and I think that if I would only apply a low profle focuser and a 1.25x OCS, focus could be at reach. I think I should try this way first (I have to wait until the end of may though). The reflector is an f/4, so coma will be evident.  On the other side, the 1.25x OCS together with the inherently limited FOV of 1.25" eyepieces could mitigate the impact of coma.

 

If I din't own a reflector, and I had to buy a scope right now,  I think I would go for a refractor, for sure in the range f/5-f/7, but no idea if a big achromat or a (very) small second hand APO...so many good recommendations in this thread made the choice still harder...

With a steep f4 light cone and a 1.25” OCS you would be losing some of the peripheral light.  My reflector is f5 and my OCS is 2” (I think 37mm clear aperture).
That is something to keep in mind.

[Spartinix]

I took an angle grinder and a drill to lower the focuser and spider and I get nice wide angle bv'ing without a corrector with my 6" FR5.
I haven't yet measured and calculated any aperture loss but the main goal was wide angle binoviewing for my finderscope.

Edited by Spartinix, 22 April 2023 - 12:46 AM.