Newtonians, Focal Reducers & Focusers

[deejayry]

Hi all

I've heard a bit about using Focal Reducers on newtonians and that in many cases focus can not be reached. Is this a problem of having too much "racked down" height in the focuser assembly and not being able to get the EP/CCD close enough to the secondary?

In the next month i hope to have an Atik 16IC-S & filter wheel, and plan on getting down to imaging as much as possible, i would like to be able to have a wider FOV at times so i'd like to know to what extent if any i can reduce the focal length? anyone know what Focal Rducers work on a 6" f/8 newtonian?

If it is a case of not being able to wind the focuser in far enough, would i be able to solve or improve this by fitting a low profile focuser such as a moonlite without any spacer plates and use a short drawtube?

Going by the guide on Moonlites web site, it looks as if i should have 2.25" of spacers and either a 2.35" or 2.75" draw tube, if i dont use the spacers and use a shorter 1.75"-2" drawtube i may be able bring the racked height down by 1.5"-2". For "normal "aplications where i need the focuser to be racked out further i can use an extender.

Does this sound like it would work?

[RickJ]

In a Newtonian, if you can't focus in enough just move the main mirror forward and slide the OTA back on the mount until it balances. If you can't move it back, add weight to counterbalance but not so close to the mirror that it increases the time needed to come to temperature.

Or you can change the focuser to a lower unit. You can find the current focal point using the moon and a piece of wax paper. Where the moon is clear is the focus. Compare that to where camera and filter wheel is when focused fully in and you know how much you have to move or shorten things.

Focus for CCD work is very critical and most single speed focusers aren't up to it. At f/8 the range of good focus is only .0055" so if you can't focus that accurately with the present focuser you may need a new one and thus pick one that's short enough to reach focus without moving the mirror if that is possible. As focal length gets shorter the range of good focus gets smaller and does so by the square of the focal ratio. So going from f/8 to f/4 means your focusing must now be 4 times better. I had to go to electric focus to achieve this accuracy with my 6" f/4. No way I could do it manually. If seeing is lousy then the focus isn't as critical as seeing will ruin it anyway.

If you focus manually you'll want a two speed focuser but electric focus works fine single speed as you can set the steps so small that speed isn't a problem. My SCT is so temperature sensitive I quickly discovered the joy of temperature compensation even if it did cost a bit more. I use Robo-Focus controller for my temperature compensation. It interfaces seamlessly with my imaging software, CCDSoft.

Most telecompressors are designed for either refractors or SCT scopes. One won't work well on the other. Newtonians have yet another set of corrections needed. I know of no compressor designed for them. I haven't looked very hard however. With a newtonian you need to correct for coma thats much stronger than in the other two designs and since an f/8 Newtonian has so little curvature I doubt your small CCD would see it and correcting for the curve of a SCT or refractor might be way too much. You'd need one designed for about f/8 newtonians for best results. Google didn't turn up one when I tried. I've heard of home made ones made from a binocular objective but don't know where to find the plans.

I started out imaging with a 6" f/4 but no compressor needed. My ST-7 chip is very slightly larger than yours, not enough difference to matter. It was fine at f/4 but would be a bit hard to use at f/8 as you've figured out. What are you using for a guider?

Rick

[deejayry]

I guess for now i'm stuck with the small FOV.

I have seen the Atik .5x focal reducer;

ATIK - .5x Focal Reducer

This focal reducer allows shorter exposures, by reducing the focal ratio of the telescope to half. This is especially useful for SCT and MCT owners, since this types of telescopes normally have a focal ratio above f/10. By reducing it to half, exposure duration is 4 times lower, and the field of view is 2 times larger. Refractor owners will also find it useful. However, Newtonian owners will not be able to achieve focus with it.

I was hoping that by fitting a focuser that alows the camera closer to the secondary i could acheive focus.

As for the focuser, If i chose the moonlite which seems to be the only decent option, i will get the servo motor driven option, i know how sensitive things are with small FOV and want to keep all interference with the scope to a minimum.

Initially I will be relying on the tracking of my HEQ5 synscan, I'm going to do the PEC training soon to make it as close as possible.

Once I have finished my webcam mod to SC4.2 I will be using that on a 300mm SLR lens with 2x teleconvertor. Eventually I plan to buy a William Optics ZS80II ED APO as my secondary scope. I guess I could use the ZS80 with its wider FOV for the larger objects, using the newtonian as a guidescope.

[RickJ]

I think that Williams refractor would make an excellent wide field scope for that camera. Probably better than modifying the newtonian to try that compressor. So would using that 300mm lens. Get a T-thread adapter for it and you'll be in business. Tracking errors are much reduced with this as well. The shorter the focal length you start with the easier it is to climb the learning curve. Once short focal lengths are routine then you can move up to longer ones with a successful outcome.

Even with PEC I doubt you'll get much more than 1 minute tracking on that mount at 1200mm, 300mm might go 5 or longer however. The higher the declination the longer it will track successfully. The Sony chip is rather low noise on readout so you can stack a lot of short shots to make up for the individual exposures.

When using a second scope to guide be sure there's no flex in the system or else guiding will be a mess. I did it that way for years with film, sometimes it worked and sometimes something moved. When off axis guiders became available it eliminated that problem for me.

Contrary to what some newcomers assume, guiding doesn't make up for polar alignment errors. You still need accurate polar alignment. Without it the field of view will rotate around the guide star. The more it is off the faster the rotation. Also the farther the guide star is from the camera's field of view the worse the rotation problem as well. Something I learned the hard way when I was starting out over 50 years ago.

Rick

[deejayry]

Cool, thats got it settled then... WO APO or camera lens for the widefield stuff, 6" f/8 newtonian for small field.

In the meantime i'm about to receive a nice range of old m42 camera lenses, after a recent ebay frenzy, i've bought;
a 200mm f/3.5
a 300mm f/5.5
a 400mm f/6.3
& 500mm f/8.8 Makustov type lens

Less than £70 for the lot. as soon as they start ariving i will get an M42-T-Mount adaptor for fitment to the cameras.

I'm still going to "modify" the newetonian to some extent. the existing focuser is almost useless when i use my 10mm EP & barlow, and its even worse when i use the webcam and barlow. I've found K3CCD tools is usefull for focusing when using the webcam, but the main problem is the accuracy, as you say, the critical focus point is .0055" which is a pretty small distance, IIRC about the thickness of a sheet of paper, even with really carefull movements i think that is too fine to manage by hand. Even then you have the vibrations of your hand disturbing the image making it even harder to judge focus when your subject is dancing all over the FOV. What ever i do it will certainly be a motor driven crayford focuser.

So far i have had quite good luck using only the tracking on the mount (no PEC) allthough that was on mars, and exposures arent very long, i also keep an eye on K3CCD tools drift monitor, depending on how good my polar alignment has been each session makes a lot of difference, but when i get it right there is very little drift. I am aware how important good polar alignment is and understand problems with field rotation. I'm soon going to use "drift polar alignment" to get it spot on.

[RickJ]

Sounds like you are well on your way. Short term tracking is easy, long term gets more difficult, especially if somewhat low in the sky as objects down there move slower than sideral rate due to atmospheric refraction. Sticking near the meridian helps a lot in this respect.

Many years ago 400mm f/6.3 lenses flooded the market. They were really cheap, mine was about 7 pounds at the exchange rate back when I bought it. Center of the field wasn't bad but half way out it got really nasty. Your chip is small enough it should be in the good region if indeed yours is one of those. One nice thing about small CCDs, they often work well with even inexpensive lenses due to their limited fov.

A shot I took with my 400mm 6.3 lens taken of M31 using a home made double hinge barn door drive (not really a mount) powered by a wind up alarm clock guided with a 60x spotting scope (manually) is at:
http://www.usenet-replayer.com/cgi/content/framebanner_3?http://www.usenet-replayer.com/9/3/6/6/1195716639.5.jpeg
Due to problems with the tripod the drive was on and finding a guide star in the spotting scope M31 was out of the sweet spot of the lens so M32 is hard to tell from a nearby star. This was taken decades before CCD on Tri-X (30 minutes) on a very cold, below zero, night which greatly increased the long term sensitivity of Tri-X film. Close inspection shows it picked up several globular clusters in M31. You should get a lot better result with CCD.

Rick