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Which Diagonal Size


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I am unsure of the best way forward with which size of diagonal to use in my Starfield 102ED. I have been using a Takahashi 1.25”with which I am happy. I have however seen a few folk using a 2” diagonal, be it a prism or a dielectric. Is there any direct benefit to be found in using a 2” as opposed to a 1.25”. I have a spare SW deluxe 2” dielectric I could use. Thanks  

    Ian 

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I have a 2" in mine because I have a 42mm LVW - makes a great finder 😜 No other reason to use 2" really. I do have a 2" Barlow too. If you have all 1.25" eyepieces then you won't need one.

I've been using a Revelation 2" for years, nothing wrong with that. Just (today) bought a Baader 2" though - very nice piece of kit :smile:

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8aNIJL5l.jpg

Could depend on the mount. I can use a 2" prism if I use a Porta II/HAL-130 plus half pillar. The main advantage is that I can use 2" eyepieces. Bear in mind a heavier eyepiece/diagonal can add a kilo or more.

uPFacBWl.jpg

I'm more likely to use 1.25" accessories on the TL-130.

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41 minutes ago, bosun21 said:

I am unsure of the best way forward with which size of diagonal to use in my Starfield 102ED. I have been using a Takahashi 1.25”with which I am happy. I have however seen a few folk using a 2” diagonal, be it a prism or a dielectric. Is there any direct benefit to be found in using a 2” as opposed to a 1.25”. I have a spare SW deluxe 2” dielectric I could use. Thanks  

    Ian 

As far as I'm aware the only benefit is 2" diagonals allow you to use 2" eyepieces... Personally I got rid of my 2" diagonal and eps to keep things simple while also keeping weight down . The less faff in the dark the better.

If you're happy with your current set up, then you're not missing out. Unless of course you want to ring out every last nth degree of widefield observation from your kit...  Although I'm happy with the field a 24mm 65° eyepiece provides with an f7 refractor.

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40 minutes ago, Mr Spock said:

. Just (today) bought a Baader 2" though - very nice piece of kit :smile:

Very nice! I’ve been eyeing one up myself for maybe my Mak. Which one did you buy?

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29 minutes ago, ScouseSpaceCadet said:

As far as I'm aware the only benefit is 2" diagonals allow you to use 2" eyepieces... Personally I got rid of my 2" diagonal and eps to keep things simple while also keeping weight down . The less faff in the dark the better.

If you're happy with your current set up, then you're not missing out. Unless of course you want to ring out every last nth degree of widefield observation from your kit...  Although I'm happy with the field a 24mm 65° eyepiece provides with an f7 refractor.

I am in the same mindset as yourself as I also sold most of my 2” eyepieces to keep it simple. I predominantly prefer using my Morpheus range 4.5-17.5mm, above which I bridged the gap by a ES 24mm 68 degrees. I also find this more than enough in my f7. I also have 32 & 40mm Celestron plossl’s , the later of which is used as a finder. I found the 1.25” Takahashi diagonal prism to be tack sharp so I’m not looking to change it. Thanks 

   Ian 

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Some find that a 2” diagonal has a more solid connection to the scope; the greater circumference that is gripped helps avoid any rotational slipping with heavier 1.25” eyepieces.

The other more obvious benefit, as others have mentioned is the wider field of view achievable with a 2” eyepiece. The widest 1.25” eyepieces are either the 32mm plossl with a 50 degree afov, or a 24mm 68 degree eyepiece. In 2” you can get a 31mm 82 degree or 41mm 68 degree which give much larger fields of view.

These can be used as finder eyepieces but that’s an expensive way of finding things in my book. In your scope a 30mm or 31mm 2” eyepiece with a 68 degree field would give wonderful wide fields of around 3.5 degrees, enough for large open clusters, the whole of the Andromeda galaxy under a dark sky, or the Veil and North America Nebulae with an OIII filter for example. Well worth trying out, just make sure that whatever you use is a decent quality, I’ve heard of some of the stock diagonals being a bit flakey but that’s based on the odd report here and there, no direct experience myself.

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54 minutes ago, Zeta Reticulan said:

8aNIJL5l.jpg

Could depend on the mount. I can use a 2" prism if I use a Porta II/HAL-130 plus half pillar. The main advantage is that I can use 2" eyepieces. Bear in mind a heavier eyepiece/diagonal can add a kilo or more.

uPFacBWl.jpg

I'm more likely to use 1.25" accessories on the TL-130.

It’s on a new EQ5 pro synscan mount, so the weight of the diagonal and ep is not really a decisive factor.

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4 minutes ago, Stu said:

Some find that a 2” diagonal has a more solid connection to the scope; the greater circumference that is gripped helps avoid any rotational slipping with heavier 1.25” eyepieces.

The other more obvious benefit, as others have mentioned is the wider field of view achievable with a 2” eyepiece. The widest 1.25” eyepieces are either the 32mm plossl with a 50 degree afov, or a 24mm 68 degree eyepiece. In 2” you can get a 31mm 82 degree or 41mm 68 degree which give much larger fields of view.

These can be used as finder eyepieces but that’s an expensive way of finding things in my book. In your scope a 30mm or 31mm 2” eyepiece with a 68 degree field would give wonderful wide fields of around 3.5 degrees, enough for large open clusters, the whole of the Andromeda galaxy under a dark sky, or the Veil and North America Nebulae with an OIII filter for example. Well worth trying out, just make sure that whatever you use is a decent quality, I’ve heard of some of the stock diagonals being a bit flakey but that’s based on the odd report here and there, no direct experience myself.

Which 30 or 31mm eyepieces would you recommend i should be considering? Thanks in advance  

    Ian 

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17 minutes ago, bosun21 said:

Which 30 or 31mm eyepieces would you recommend i should be considering? Thanks in advance  

    Ian 

The two that immediately spring to mind are the Explore Scientific 30mm 82 degree or the one I have which is the 31mm Nagler type 5. The ES is very good and cheaper, to me the Nag is just a smidge ahead, gives me a flatter, slightly wider fov in my Genesis.

See if anyone else has any suggestions too 👍

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Both have their uses. 2" for 2" eyepieces but also for a more solid connection when using dual size eyepieces like my LVWs, which can be used either way, just like the Baader Hyperions and others. These multi-element jobs are quite heavy and the shorter focal lengths quite tall so they get held better with a 2" diagonal. Having said that, it does add weight so balance issues and mount capacity have to be taken into consideration. For light weight set ups using simple eyepieces then 1.25" is fine. Your Tak 1.25" is a great diagonal and the SW deluxe 2" dielectric is very capable too.

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I always use a 2" diagonal,  except on very rare occasions when I need to travel light. In my C8, the extra FOV offered is too big to ignore,  in my little APM 80 mm F/6 triplet the huge FOV offered is amazing fun. In the case of dielectric diagonals, there is an added bonus with 1.25" EPs of slightly better image quality due to a larger truly optically flat area. 

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1 hour ago, bosun21 said:

Which one did you buy?

https://www.firstlightoptics.com/diagonals/baader-clicklock-2-dielectric-star-diagonal.html

1 hour ago, Stu said:

These can be used as finder eyepieces but that’s an expensive way of finding things in my book.

I see you don't live in light polluted skies. For me, a 9x50 finder is next to useless. With my 102mm the 42mm LVW gives me nearly 4°. I point the Baader red dot finder at the nearest bright star, put the 42 in, and star hop to where I want to be.
Same with the 305. I use the Telrad, then 9x50 if there are some star patterns recognisable, then the 42mm at 1.8°.

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2 hours ago, bosun21 said:

It’s on a new EQ5 pro synscan mount, so the weight of the diagonal and ep is not really a decisive factor.

I think for low power sweeping or rich field a 2" diagonal/eyepiece takes some beating. I like to use my BHZ with its 2" skirt as it sits lower in the diagonal itself. Although I prefer a 1.25" diagonal for higher magnifications. I take a 2" Baader dielectric and a Tak' prism out usually with my  80 ED Evostar. The Tak's so light it helps keep down vibration. 

EG6bAxll.jpg

I bought this TV Everbrite for my 60 EDF, unfortunately the nosepiece undercut didn't play well with the visual back compression ring.

Q96AOZFl.jpg

I had no problems with the Baader. These days I use the 60 EDF with 1.25" accessories predominantly because of weight and balance issues. I do kind of miss the 2" stuff with it for rich field. 

Edited by Zeta Reticulan
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As well as the above points, other, perhaps less well appreciated factors are that a 2" diagonal often/usually has a longer light path and of course weighs more. This could be a plus or a negative affecting balance and extension to reach focus depending on the scope etc. 

Personally I like the Baader 1.25" prisms as the connectors available  make their uses almost infinite. I use mine with my binoviewers and also for my pst mod and cyclops mode for night use. I have one 2" eyepiece (26mm Nagler) and there's very minimal issues that I can detect. For that I have a 2" adapter. It's mainly in my case for newts though. 

Edited by Moonshane
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I have an Altair Astro 102 f7 refractor and have gone down the 2” route. I only have two budget Opticstar eyepieces, a 32mm and 26mm. For some strange reason, I like to observe certain targets, especially open clusters, framed’ in space if that makes sense. The above 2” eyepieces give me wonderful ‘framed’ views of the larger clusters such as the Pleiades, Beehive, and the Double Cluster etc. I guess it’s a personal thing, also as has already been mentioned, when clamped the 2” diagonal does feel really, really solid.

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I have just two refractors now and they have always been fitted with 2" Diagonals.  As a long time user of binoviewers I want the safety of a 2" nosepiece and the secure clamping that goes with it.

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42 minutes ago, DeanCJ said:

I have an Altair Astro 102 f7 refractor and have gone down the 2” route. I only have two budget Opticstar eyepieces, a 32mm and 26mm. For some strange reason, I like to observe certain targets, especially open clusters, framed’ in space if that makes sense. The above 2” eyepieces give me wonderful ‘framed’ views of the larger clusters such as the Pleiades, Beehive, and the Double Cluster etc. I guess it’s a personal thing, also as has already been mentioned, when clamped the 2” diagonal does feel really, really solid.

I think that’s quite a common preference Dean. Seeing those open clusters framed with space around them allows you to see them in context and appreciate their overall shape. Can’t beat the views through a nice frac with a widefield eyepiece 👍 

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5 hours ago, bosun21 said:

Which 30 or 31mm eyepieces would you recommend i should be considering? Thanks in advance  

    Ian 

I prefer the 30mm APM UFF to my 30mm ES-82 for both eye relief reasons and for image quality.  The latter has a bit bloated stars on axis and severe CAEP at the edge.  The APM has none of this.  My 27mm Panoptic might be a tad sharper on axis, but the APM trounces it for eye relief, field flatness, and edge sharpness.

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6 hours ago, bosun21 said:

Is there any direct benefit to be found in using a 2” as opposed to a 1.25”.

I couldn't use my ES-92 eyepieces in my refractors or Mak without one, so there is that.  However, for binoviewing, I use a 1.25" diagonal to keep the optical path shorter than in a 2" diagonal.

Using my 24mm APM UFF or 32mm GSO Plossl as a widest field eyepiece instead of one of my 30mm to 40mm widest field 2" eyepieces seems unnecessarily restrictive just to keep an eyepiece collection simple or to avoid balance issues.  Sure, my A-team eyepiece case weighs in at over 20 pounds, but there's a lot of observing joy packed in there.  I know someday I won't be able to hoist that weight, so I may have to split it across multiple cases.

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2 minutes ago, Louis D said:

  I know someday I won't be able to hoist that weight, so I may have to split it across multiple cases.

Let’s hope that time is far away.

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8 hours ago, Mr Spock said:

Just (today) bought a Baader 2" though - very nice piece of kit :smile:

This is contagious! I have just ordered the same Baader click lock diagonal from FLO. It was the last one they had.🤫

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Mine came last week. It feels good and solid. Makes the stock one that came with my 8” evo looks very feeble. I chose the clicklock because I am clumsy and if it can happen it will happen to me. Tried it out on the scope today while I sorting out my new case and it looks reassuringly hefty. 

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