Posts Made By: Lorne Johnson

Dan Ritts said:

Hello all,
Could someone please direct me to a good explanation of proper secondary sizing, illuminated field diameter, etc?

I ask because I've long suspected my secondary is undersized. If memory serves me, I followed the advice of Tom Clark of Tectron Telescopes back in the early '90s. I used a 2.14" secondary for my 14.5" f/5.2 primary. If nothing else, it makes collimation harder for me as I can not see the periphery of the primary in the reflection of the secondary. I also suspect some vignetting. What do you all think?

Forgot to say that this is a Newtonian we're talking about... a typical Dobsonian.

Thanks,
Dan

Newt and Sec are excellent programs, and you should use them. But, keep in mind that secondary size is a compromise between field of full illumination and percentage of central obstruction (no perfect answer but some very good answers). I started to get a feel for using the programs by running the numbers on some telescopes I liked. The MN 56 for example had about 50% of full illumination at the edge of the 2 field--sounds awful, looks great (that's probably a little tighter that your dob, depending on the focuser height). Doesn't work for 35mm immaging though. (

Sanjay Veerkar said:

Hello

I recently bought an 8" Meade LX200 and procured basic 1.25" eyepiece equipment to go with it. I have 2 questions before I build out my eyepiece collection:

1) How much difference is there between a 2" and 1.25" eyepiece? Intuition tells me that for faint deep sky objects the 1.25" would be desirable as less light would be dispersed into a smaller area that is coincident with the eye pupil. How about wide-angle views - does a 2" outperform a 1.25"? For reference, I am a poor guy with a budget to buy only UltraWide Knight Owl quality eyepieces - no Naglers in my near future.

2) My eyepiece setup currently has only a 1.25" adapter. How do I convert my SCT to also use a 2" diagonal? How about camera adapters?

Though I have read quite a few forums I am missing all the basic knowledge and am having trouble putting diverse pieces of info and jargon together. For instance, is that plastic tube the comes out of the back of the mirror cell of my SCT an adapter or is it called something else? Please assume that I have little knowledge.

Sanjay,

The advise to avoid running out and spending a lot of money right away is very good rule of thumb advise. Finding a club or someone who observes (most observers will happily go out of their way to help someone starting) it is the best thing possible.

Try using the eyepiece that came with the scope, they are good eyepieces and at f10 the scope does not require especially expensive eyepieces to get quality visual images.

However you may find that you have a hard time locating objects in that narrow field (see below), or that larger objects such as the Orion Nebula, the Double cluster, the Andromeda galaxy just don't fit in the field of view. The formula for true field of view is

Diameter of field stop/focal length X 57.3 = True Field

The maximum field stop for a 1.25 eyepiece is 27 so the maximum True Field of view is .76 degrees. That will fit the full moon, any planet, most of the Orion Nebula, Andromeda will look better in the finder scope, one cluster of the double cluster, forget the North American Nebula or the veil (at least until you are very experienced). For those you really start to want one, or possibly two, 2 inch eyepieces. Of course that's not a lot of objects, but they are nice ones.

Sanjay Veerkar said:

Hello

I recently bought an 8" Meade LX200 and procured basic 1.25" eyepiece equipment to go with it. I have 2 questions before I build out my eyepiece collection:

1) How much difference is there between a 2" and 1.25" eyepiece? Intuition tells me that for faint deep sky objects the 1.25" would be desirable as less light would be dispersed into a smaller area that is coincident with the eye pupil. How about wide-angle views - does a 2" outperform a 1.25"? For reference, I am a poor guy with a budget to buy only UltraWide Knight Owl quality eyepieces - no Naglers in my near future.

2) My eyepiece setup currently has only a 1.25" adapter. How do I convert my SCT to also use a 2" diagonal? How about camera adapters?

Though I have read quite a few forums I am missing all the basic knowledge and am having trouble putting diverse pieces of info and jargon together. For instance, is that plastic tube the comes out of the back of the mirror cell of my SCT an adapter or is it called something else? Please assume that I have little knowledge.

Sanjay,

The advise to avoid running out and spending a lot of money right away is very good rule of thumb advise. Finding a club or someone who observes (most observers will happily go out of their way to help someone starting) it is the best thing possible.

Try using the eyepiece that came with the scope, they are good eyepieces and at f10 the scope does not require especially expensive eyepieces to get quality visual images.

However you may find that you have a hard time locating objects in that narrow field (see below), or that larger objects such as the Orion Nebula, the Double cluster, the Andromeda galaxy just don't fit in the field of view. The formula for true field of view is

Diameter of field stop/focal length X 57.3 = True Field

The maximum field stop for a 1.25 eyepiece is 27 so the maximum True Field of view is .76 degrees. That will fit the full moon, any planet, most of the Orion Nebula, Andromeda will look better in the finder scope, one cluster of the double cluster, forget the North American Nebula or the veil (at least until you are very experienced). For those you really start to want one, or possibly two, 2 inch eyepieces. Of course that's not a lot of objects, but they are nice ones.

In that scope you don't need the Naglers, a 50mm plossl will work great, at 40 x, and give you over 1.25 degrees True field.

Mike Alexander said:

In your opinion, what would the optimum exit pupil range and magnification be for viewing nebulas?

I use a 12 inch f/10 Meade LX90.

Thanks.......Mike

I find a 1.5 mm exit pupil to be about the best foir maximum detail--that's not "scientific", it just works best for me.

John Meiners said:

I have tried several Neb filters and found them too dark.. I end up removing them so I can actually see the objects. There is no light pollution where I live so that is not an issue.. I have a 11" SCT. Using a Nag 31T5 and a 22mm T4 Nag. thought the 31 was using too much light... thanks for any suggestions.. JM

Jon Isaacs certainly offers good advice. I prefer the slightly broader band UHC to the O III, but would add that a plenty of people prefer the O III. I am using an 8" f 5.7.

I also find that I am somewhat conflicted about whether or not I can actually see more with or without the filter.

As Jon says, it's a compromise.

tim wetherell said:

Hi Everyone. I'm building an 8" f9 refractor and want a wide field eyepiece option. I'm happy to go with larger than the standard barrel (and the physics dictates I must 8O ) so have been looking at options. Has anyone her ever seen or used one of Harry Siebert's 2.7" or 3" EPs?

I've had some of Harry's other EPs and they've been good but never seen one of these - I guess they're rare beasts!

all the best

Tim

First, I confess that I have never used a Siebert 2.7 or 3 inch. The closest I've been is the 36mm 2 inch (which is available in the larger format, and appears to be of that series). I really like my 36, more than the TV 35 Panoptic, or at least as much as. So, in your position I would consider one of the Sieberts pretty seriously.

Your getting plenty of advice here, most of it very good.

How you buy eyepieces is often as important as what you buy. Most of us don't get it just right the first time! So if you buy used premium eyepieces (such as TV's), you can generally sell them without losing much, mostly shipping and insurance. This allows you to change your mind. It keeps you from ordering 5 new eyepieces and then losing on the three you decide to trade later. And, it builds up some feedback ratings on Astromart, which is very helpful when you decide to sell something.

I'll just shoot my mouth off here! Personally, I believe that optical differences between types of telescopes are often exaggerated (covnienence, weight, personal preference are a different matter).

I did a fairly careful side-by-side comparison of 3 five-inch scopes as follows. My intes-micro MN 56, my friend's D&G f12 achromat, the school's Takahashi 130 reflector. Right off the Tak got killed on contrast. This was not subtle. Then, we took a piece of black construction paper and made a dew shield for the Tak, and differences in contrast became subtle, very subtle, possibly observable. Over time we reached a consensus that the D&G was definitely brighter, and the MN 56 definitely dimmer (Ed Ting made a similar observation in a similar side by side comparison that I came across later). We also reached a consensus that execution of the design is the more important factor.

Flame on!

Hands on Optics in Damascus Maryland, is the closest large scale dealer that I know of.

There is a store in Northampton Mass (A to Z Science) that has some.

There was a store in NH, but I don't know what they stock.

NEAF is a great source in late April, and Stellafane (Springfield VT) is coming up fast, and many dealers go there, even though it is intended for amateur scope builders.

Todd Gross and Sky and Telescope are also local, as is Bob Royce.

I would suggest looking for star parties (Stellafane, Hartford area)and see what people own.

A lot of observers will go out of their way to be helpful.

Okay . . . so the advice you have gotten so far is very good. It is excellent.

But . . . Dobsonians aren't just for virgins--they are also for sluts!!!!!!!!!!!!!!!!

So . . . wait six months, as advised, before becoming a slut: