Extension ring for ASI 294pro

Ernesto221

I bought recently an APO 106/700 mm. This TS-optics APO is provided with its focal reducer that should be removed. I use untill now this refractor with focal reducer installed but last week I remove the reducer to increase the focal to reach smal galaxys. After remove the reducer I start to move the EAF focuser IN and OUT to find the focus but without success.
I'm using the same rings and filter drawer lengh adopted in the past (with focal reducer instaleld). This rings composition is 55mm as specified by ZWO for the proper backfocus.
Unfortunatly I don't have additional M48 extender rings to increase the lengh between the camera and the focuser than I would like to understand how this measure can be calculate. For better understanding, these is the link showing dettails of my scope :
https://www.teleskop-express.de/shop/product_info.php/language/en/info/p13290_TS-Optics-APO-Refraktor-106-700-mm---Tripletobjektiv---2-5--0-75x-Fotokorrektor.html

Is there someone that can suggest me the right lengh of rings compsition that i need when I use this scope without the reducer ?

Any suggestion will be very apreciated.

Regards

w7ay

Ernesto221 Is there someone that can suggest me the right lengh of rings compsition that i need when I use this scope without the reducer ?

That telescope looks like a simple (non-Petzval) triplet.

Without a reducer, the focus point should be 700mm from the objective to the sensor. Remember that your 294 camera's sensor has a flange focal length of 17.5 mm, which you need to subtract from the 700 mm.

Estimate where the objective is located, measure 700 mm - 17.5 mm away, and that is where the flange of your camera needs to be. You don't have to be precise, since the rack and pinion focuser will handle small changes.

For daytime terrestial use that 700 mm turns into about 707 mm for a tree that is 100 m away.

Chen

enaiman

I can't tell you the exact length your rings need to be but I would recommend buying an assortment of rings starting at 20mm down to 3mm; you should be able to find some bundles at astro dealers or eBay. You might not need them now but I guarantee that someday you will be happy you bought them.
Then it is just a matter of trial and error - without the flattener/reducer you don't need to worry about backfocus so just add a ring and try to get focus.

Ernesto221

Hello Chen, Today I bought some rings in different lengh, 3x35mm + 1x20mm. So, next clear night I will proceed to add rings to the actual rings I use with focal reducer and find the right focus. The dealer where I bought these rings told me that in case the focal reducer is not used, what I'm doing is not correct and insted of adding rings I need to buy and use a field flattener between the focuser and the camera. In this case, he told me that I can maintain the same rings composition (rings lengh) that normally I use with focal reducer (55mm back focus) and apply small adjustment on that flattener adjustment knob.
Is this field flatterner really indispensable ?

Looking forward to hearing from you soon

w7ay

Ernesto221 insted of adding rings I need to buy and use a field flattener between the focuser and the camera.

If you are going to only image very small galaxies and crop out the rest of the image, you may not need a flattener. The flattener ensures that the entire field has minimal field curvature, but if you only need a small central area, you don't need it.

In fact, field flatteners will increase the size of the spot diagram for stars that are exactly at the optical axis when compared to not using any flattener. Just look at the published spot diagrams for your telescope.

This for example is the spot diagram of my primary telescope without a flattener:

This is with a 1.01x flattener:

This figure provides the actual scales (i.e., the side of the squares is 100 µm):

On a sensor (or a crop) whose diagonal is less than 10 mm, this telescope (a Petzval) works better with no flattener. Even when it is not a Petzval, the on-axis spot will always be better than using a flattener -- it is a matter of at what distance the spot digram of the no-flattener scope is better than the flattener scope.

Just compute what the size the galaxy would be, compare that to the spot diagrams, and you will find out if you really need a flattener.

Don't buy a single thing in the future until you know what they do. The hobby is full of charlatans and shills who make you go out to buy something, with them profiting from your money.

Chen

enaiman

Ernesto221 Really no point in waiting for the night - just try to focus in daytime; it will work much better.
Don't add all the adapters at once, add one and see if you can get focus; if you have gone all the way both directions without getting focus, add another one and try again.

enaiman

w7ay It is a triplet Chen,

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p13758_TS-Optics-APO-Refractor-106-700-mm---FDC100-Triplet-Lens-from-Japan.html

w7ay

enaiman It is a triplet Chen,

I said exactly that in post #3 above.

That is why he could measure the focal length and use it directly, which he cannot with compound optics like a Petzval.

Chen

enaiman

w7ay My bad, didn't look at that post, just saw you continued mentioning Petzvals and I thought you might have had the wrong info.

Ernesto221

Hello to everybody,

I found the focus. In addition to the rings I use to obtain 55mm back focus I add 2x35mm M48 + 1x30mm that enter and clamp on focuser.
One simple question, someone of you know what is the thread pitch of M48 ZWO rings ? 0,50 - 0,75 - 1 ?

Many thanks

Ernesto

w7ay

Ernesto221 thread pitch of M48 ZWO rings ? 0,50 - 0,75 - 1 ?

0.75 mm.

Chen

Ernesto221

Thanks !