Posts Made By: Martin Boys

Hugh, the formula is as follows:

f/ = (telescope f/) X [(projected distance* - eyepiece F.L.)/eyepiece F.L.]

* please note that this distance is from the focal point of the eyepiece to the film plane.

It may be stated using other terms in the books, but this nomenclature is a way for me to understand it. Below is an example for my Meade SCT using a 25mm eyepiece and my variable tele-extender:


f/ = 10 x [(185mm* - 25mm)/ 25mm] = 64 or f/64


with a 9mm eyepiece:


f/ = 10 x [(185mm* - 9mm)/ 9mm] = 195 or f/195


I hope this helps you.

Clear skies,

Martin Boys

David,

I highly recommend having them not mount the slides for you, but rather you do it. I have lost too many good shots by assuming the lab will know where to cut. They should provide you with the mounts as well...

Clear skies,

Martin A. Boys

Jay,

As for the film for planets, I recommend a fine grained, slide film such Ektachrome 100 or 200 to the ones you mentioned. Film grain of the faster films will overpower the detail you want. Also, that SRT-101 you have is big and heavy and the shutter has some mass as well (I use one of these as well). I recommend using the "hat trick" on any shots longer than 1/4 sec. Any shots between 1/60 and 1/15 will most likely be blurred as well. I say this from experience with my 10" Meade. You did not say if you are using a tele-extender or not. If so, the problem is magnified as well.

As for the processing issue...Welcome to Astrophotography where negatives are not printed, beautiful, perfectly guided slides are cut in half, and you wait with great anticipation for the next new moon.... Check you negatives for your images to see what you actually got. I have learned (the hard way I might add) that I now always insist that I get the negatives as well as slides sleeved and uncut. I mount the slides myself as I know what the shots should look like. Also, by getting the negs uncut, you also are able to avoid this with print film as well.

There are fewer quality custom labs with the onset of digital and therefore harder to find a custom printer that really knows how to maximize your image. You will need to find a lab that is willing to listen to your needs. Chances are that they will have to LEARN how to print your astro shots just as you learn to take them.

I own and recommend these three books for learning astrophotography:

Astrophotography for the Amature - Michael Covington

The Backyard Astronomer's Guide - Terence Dickinson & Alan Dyer

Wide Field Astrphotography - Robert Reeves

All three books cover equipment selection as well as guidelines for exposing the heavens. There are other books out there, but these helped me the most. You didn't mention if you have difficulty focusing on the very dim focusing screen of the SRT-101, But I am assuming that you are. The above books will discuss this as well. I really like "The Backyard Astronomer's Guide", as Dickinson & Dyer show examples and discuss the not so great Astrophotos, rather than just showing the "trophies".

Clear skies,

Martin A. Boys

Hugh,

The focal plane of the eyepiece should be at or very close to the shoulder or step of the barrel, (i.e. eyepiece theoretical parafocal point) for whatever eyepiece you use. To keep things simple, use the visual back as your reference. Please note: If you use parafocal rings you will need to take the added distance into account. By normalizing the eyepieces to each other, the focal point of the scope is closer to the film plane to allow for the extra distance required. This will result in lower f/ values and smaller projected images.

For the steps below, I will not include the parafocal ring ring issue.


First, measure from the very back edge of the visual back to the step where the Tele-extender will bottom. Second, measure the length of the Tele-extender. Third, subtract the first dimension from the second dimension. Fourth, add the the T-ring to film plane measurement and you will have your number. Absolute, to the nearest .1mm, accuracy isn't necessary as you will be approximating the f/values typically anyway and bracketing exposure times.

I highly recommend that you get Michael Covington's book Astrophotography for the Amateur. It has the formulas you are looking for, exposure tables worked out for the solar system as well as a wealth of other usefull information.

Let me know how you do.

Martin

Meade old style, pre-eyecup Plossls are 5 element designs. I believe that Meade changed manufacturers at the time they changed their designs to the eyecup version about 1991. I have a bulletin I received from Meade in 1990 that shows the design of the old style 4000 series eyepieces.

There were some issues with the early 4000 series eyepieces that Meade designed out of the updated series. I believe they also designed out the 5th element as well as these newer eyepieces were somewhat cheaper than the original version.

One exampe, the 26mm and 56mm barrels were changed to position the eye at the correct distance....great for those with perfect eyesight, but with my glasses, I cannot see the entire FOV. The older style allows me the entire FOV easily.

Martin

Same issue... cannot read this post without great difficulty.