Are some 2" Eyepieces really 1.25" in "disguise" ?

[barnstorm]

My apologies if this topic is already covered.

I did some searching of threads and learned some great things about exit pupil size and that 2" eyepieces really just give you FOV and a big piece of glass as an advantage.

My questions are:

Question 1:

When hunting cheap eyepieces I see 2" eyepieces for sale in the range I am looking for 10-25mm, that appear in the photos, to have the same glass size as my 1.25" 25mm plossl.

Are they just 1.25 eyepieces lurking in a 2" holder waiting to pounce on the scope-naive?

Question 2:

My scope is a 16" Dob Meade Starfinder f4.5

I have a new 2" 56mm Meade Super Plossl and a pile of 1.25" eyepieces. (28,25,20,10mm).

I also have some used eyepieces that never really did much for my ETX90 as the lens seemed terribly small and the image pale. If the clouds would pass sometime soon I might get to see if they work in the DOB. (photo attached) Should I sell these off for 2" eyepieces or would I just end up with glass-grenades that do the same thing?

I would like to get nice SHARP reasonably wide FOV deep-sky and planet images pasted on the back of my 45year old eye.

Question 3:

Though normally a staunch skeptic...

My new telescope seems to be attracting clouds at night.

Especially, now that my colluminator and 2" plossl have arrived.

Has anyone done a peer reviewed study on this phenomena?

.

.

[Don Pensack]

Q1. The size of the fieldstop and the lenses in the body determine whether or not an eyepiece must be 2". In general, eyepieces under 18mm, regardless of field size, can be 1.25". However, if all your eyepieces are 2" except a couple shorter focal lengths, there is an advantage of making them 2" so all will fit in your focuser equally well. I add 1.25" to 2" adapters on any 1.25" eyepiece to convert it to 2" for that reason. I just think it's more stable for big eyepieces to be 2".

Q2. With an f/4.5 scope and a 4" secondary mirror, you really shouldn't be using eyepieces with longer focal lengths than 4.5*6=27mm. Why? Because the secondary mirror's shadow in the image will be 25% as wide as the image produced at the exit pupil and will start becoming obtrusive above a certain exit pupil. Also, the pupil of your eye doesn't get as big as the exit pupil produced by that 56mm eyepiece (56/4.5=12.4mm!) so your eye is essentially stopping down the scope, making your low power views a lot dimmer than they need to be. It is rare for someone with such a short f/ratio scope to use an eyepiece longer than the 31 Nagler, which is often the low power eyepiece of choice. Add a Paracorr, which corrects coma, and your f/ratio will be 4.5*1.15=f/5.18, and a 31 Nagler would be ideal. That 56mm Plossl was and is intended for long f/ratio scopes like the Schmidt-Cassegrains.

Q3. The theory goes that telescopes emit moiston particles that cause clouds to condense in the atmosphere. Only the Full Moon can burn through the moiston-produced clouds. This theory explains why it's always cloudy around the New Moon period and clear as a bell on the Full Moon. LOL.

By the way, what tool(s) are you using for collimating?

[Saleratus]

There's no advantage to a 2" ep intrinsically. But some low-power and wide field eps need to be 2". Otherwise they could not take in the whole field of view they need to.

An example is a panoptic 24, with a 68 degree field of view. This is the largest field of view that can be encompassed by a 68 degree ep. Other low powered pan optics, like the 35 and the 41 are 2" eps. They HAVE to be to take in the 68 degree FOV at these low powers.

So the answer to your question is maybe. Maybe you have a 1 1/4 ep in a higher focal ratio--like a 30mm Plossl, that you'd like to replace with a 2" ep so that you can have a better 30mm ep with a wider FOV. If that's the case, do it. But don't get a 2" ep just for the sake of it.

In lower f/ratios (like 10mm and below), you will rarely see 2" eps, unless you are using super wide eps like Ethos. There is no reason for them.

[Northern Soul man]

Hi Ya Barnstorm, your dob at F4.5 would need a good quality ep to show its worth. Plossls, by design, make good planetary ep's, with the smaller focal lengths 912mm and lower) but not so good if you wear glasses - your looking through a tiny hole when you get down below about 8mm - this is where the 2" eps work better especially for people who wear glasses, I use the Hyperions, but at F4.5 the FOV edges would be very poor - as with the cheaper ep's. I used them with my 12" F5 dob - not too bad but noticeable.

The plossls are good planetaries (I have the Meade 4000's) and work well - but as you say you need sharp and relatively widefield - your moving into Nagler territory - I don't need to tell you how pricey they are with, the Plossls being relatively cheap and "fairly" widefield - you can take your pick. Just my experience, using both types at F10 - really good, but at F4.5 you would see the difference immediately - as for clouds tell me about it - been hunting for Comet Garradd - no luck in binos, cloudy, hazy and murky even when the clouds move a little.

Wait for a few more comments, possibly from gazers using different types of ep at different focal lengths - just my two penneth - Clear Skies (not!!!) and regards Paul.

[barnstorm]

Oops. forgot to attach the photo..

If anyone has any opinions on which of these I should keep, replace with 2" or sell to pay for cloud-removal services, I am all ears...

(16" dob, f4.5, planets, DSO, 45yr eyes)

[John]

The 56mm Meade plossl won't be any use in an F/4.5 scope as the exit pupil that it produces will be far to big and you will probably be able to see the secondary shadow while you are using it. So that could go.

The Speers-Waler 12mm is a gem, if you can get it to focus, so that could be a "keeper". The Meade research grade ortho and erfle's are very nice apparently although an erfle is going to show distorted stars in the outer parts of the field of view in an F/4.5 scope.

The Meade and Spective plossls should be some use and are competent enough eyepieces. The Kellner won't work well in an F/4.5 scope though.

Don't know about the others I'm afraid.

Your 16" F/4.5 scope will really shine with a high quality 2" 30mm wide angle or ultra wide angle eyepiece. Examples include the Nagler 31mm, UWAN / Nirvana 28mm, Explore Scientific 30mm and the Meade UWA 30mm. These are expensive treats though but the views through one of them and a big dobsonian are to die for

[Rapallokid]

John,

Interesting information thanks for sharing.

Would the SW Aero swa 2" eypieces be any good in this type of scope?

RK

[barnstorm]

Thank you so much! I don't know where to start.

Don,

Thank you. I bought a Hotech cross-hair for collimating. That is what first attracted the clouds. I found it was WAY off and since adjusting it I have not had so much as a 5min patch of open sky.

Will, thank you!

Paul,

I have contacts with a slight stig in the right eye.

John,

WOW, Thanks!

I am not even sure which EP's you are referring to so I started a new thread to try to ID the ones that are not marked.

Thanks!