Light path lengths & Problems with focus

[Mac]

I spent about two hours today setting everything up in daylight and having a play with my new F3.3 reducer.

My F3.3 reducer consists of the following

F3.3 Lens and 3 separate length tubes, which when connected together make up the F3.3,

with just the lens and no extension tubes it is F5, plus various combinations inbetween.

To start with I set the scope up as follows.

Scope - F3.3 reducer - H9.

had a nice play and got to grips with focusing, ect, no problems, light path was square, ect,ect, and quite a nicce field of view.

I then wanted to add the focuser and filter wheel.

Scope - F3.3 reducer - Focuser - filter wheel - H9.

As the focuser and filter wheel were aditional lengths, i removed the extra extension tubes.

The problem is the length of the focuser and filter wheel, is about 1cm longer then the extension tubes

I now have the following problems.

The H9 is now behind the focus point of the reducer, and i am unable to focus the H9 at all.

From full travel end to end of the scope, i am unable to achieve focus.

Idealy i want to keep the focuser as it allows me to focus the image without mirror shift.

Which set up would be better?

1) scope - Focuser - F3.3 - Filter wheel - H9.

2) scope - Focuser - Filter wheel - F3.3 - H9

3) scope - Filter wheel - F3.3 - Focuser - H9

Assuming i have the various adapters.

My only worry is the filter wheel, as the amount of thread is tiny, and dosent look like it would support a huge amount of weight.

I was thinking of 1 as my best bet, any thoughts or comments, apart from getting another scope

[Rocket Pooch]

Hi,

If its on the 200R take the focusor out all together and test it with the normal focus, it will work then.  Then try the focuser, one small point though, again as with another posting here, I don;t think the light cone will cover the chip evenly.

If you need a SCT to 2" adaptor you can morrow mine to give is a go, also are you using the normal nose for the H9 or have you got a T-T mount male to male to get it bang up against the filter wheel?

Lastly check the focus point of the reducer, its 4" in my AP from the front element of the reducer to the chip, you should be able to find this somewere, this will give you the measurement guide you will need.

Ramble ramble...

Chris

[Mac]

I've got the male - male Mount so its bang against the filter wheel..

If i remove the focuser and filter wheel and replace it with the extension tubes. I can get it to focus without any problems.

The focuser and filter wheel is about 1cm longer then the Extension tubes, so im assuming its pushing the H9 past the focus point.

I can only asume the light paths are something like this.



The top one allowing me to move the H9 in to focus, ect.

The bottom one, because the H9 is now further behind the point of focus, i am unable to bring the H9 close enough, to achieve focus.

I was just wondering if by placing the F3.3 after the focuser would it cause any problems? as the reducer itself is now moving?

so although the light would be able to be brought to focus, would the image size shift?

I supose the only real way to find out is to have a play during daylight and try it.

[RobertM]

Mac, If this is a 200R then the 3.3 reducer will overcorrect the corrected optics.  The main advantages of the AP reducer that Chris mentioned are that it is a pure reducer (no correction) and you can screw it directly (via spacers) to the filter wheel.  Since it's designed to fit inside a 2" focuser it travels in and out with the image train.  I found the SCT thread Celestron reducer corrector was causing vignetting with the H9 which will be worse with the 3.3.   If I wanted to change the reducer before I had to dismantle the image train and the JMI focuser to get the reducer off, now I just release the 2" clap screws and unscrew it from the nose piece.

Moonlight do make a special focuser for doing what you want and it places the RC inside the focus assembly where it moves with the drawtube (only fits the larger sct thread I think).

[JohnP]

Mac - when did you get an H9?

[Rocket Pooch]

get with it john

[JohnP]

OK - I've just looked back at the old threads... He never actually said he had got one.... Geeezzzz...!!!!

Look mate I'm getting old the memory starts to go...

John

[Mac]

Look mate I'm getting old the memory starts to go...

Your memorys fine.

I got my Starlight Xpress SXVF-H9 CCD Camera last week.

Spent all night effing and blinding, so decided to set it up during the day,
dam site easier, i must say.

My scope is just the old LX200 GPS, so the F3.3 wont over corect the optics, in  fact it should give me a nice flat field.

BUT. If i use the focuser to move the F3.3 and eveything else, then the size of the image will change, as the reducer is actually being moved as well.

So i have a choice of either not using the filter wheel, or not using the electric focuser, and rely on using the main mirror focus.

By the way, your memorys fine 

[JohnP]

Did u get it new or used? I think i'm at the stage where I need something with a larger chip.. What are you using for a guide camera?

John

[Mac]

I got it new, as its the F version,
As for guiding, err, nothing at the moment,

I did get the 300mm lens to use as a guide scope, but im still looking for my toucam.

But i've ordered an adapter for my nikon lens's to H9
so hopefully i will be able to do some super wide field.

I also got a set of narrow band filters.
Ha
Hb
Oiii
Sii
Clear
and light pol.

So when its all up and running i hope to get some good images.

Fingers crossed.

[RobertM]

Sounds like a good combo Mac, looking forward to some great images.  Which Filter wheel did you get ?

[Mac]

Which Filter wheel did you get ?

I bought the True Tech super slim wheel.
http://www.iankingimaging.com/show_product.php?id=171

With the 7 slot filter wheel, including dark position.

So i can use the clear (ir uv) filter for quick focus,
and then more selectively focus in each of the Ha Hb ect.
plus Loight pol for the luminance.

Although unless im doing 10 min + subs, I wont actually need a dark frame, as the H9F has such a low noise factor.

The SXV-H9 is a 1392 x 1040 pixel high resolution CCD camera that provides truly world class performance.
It features a Sony ExView CCD that has such low noise levels dark frames are rarely required.

The download speed from the chip is fantastic, Less then 0.6 secs full frame, and a binned 4*4 is <.1 secs. so focusing should be a lot quicker

Mac.

[Mike]

The download speed from the chip is fantastic, Less then 0.6 secs full frame, and a binned 4*4 is <.1 secs. so focusing should be a lot quicker

You're lucky, you've got the newer version. It takes a few seconds on my version of the H9. 

[RobertM]

Mac,  That's great, mine takes about 4-5 seconds for a full frame download.  Also I've found that darks are unnecessary even with 20 min subs but bias do take out a fair amount of the residual background noise (not that there is much).  I find the difference between an average bias and a 10 minute dark is around 50-70ADU, if you're a purest or have really dark skies then fine but for the effort it doesn't really seem worth it around here.  The SupaSlim is one I've been looking at too, the main advantage is for a quick change of filters, the SBIG I have is like the Atik - held together by Allen screws.

[JohnP]

Blimey Mac all I can say is you must be in your mrs's good books if you've been allowed to spend that much on toys....

I can't wait to see the results you get - Looks like you have some great kit. I look forward to welcoming you to the 'club' of autoguiding trial & tribulations....

John