Do I need to get a larger secondary mirror?

[Uplooker]

First off i hope this is posted in area. Please feel free to move.

I have a SW150P F5 Newt. It came with a 1.25" R&P focuser. I used to attach my DSLR by unscrewing the EP holder that exposed a thread to attach a T-Ring to. I had no problems achieving focus.

I decided to replace the stock with a Revelation Dual Speed Crayford Focuser which takes 1.25 or 2" EPs

Revelation Newtonian Crayford Focuser Dual Speed

Once fitted I needed to use a Baader Finetuning Ring attached to the 1.25" collar to enable me to attain focus for visual.

First Light Optics - Baader Hyperion Finetuning Ring

As I needed to use the fine tuning ring this made me think that the focuser would be low profile enough to achieve focus for AP purposes.

When it came to attaching my DSLR I had to use a nose-piece to go in the 1.25" focuser and attach the T-Ring to that. Once done I was not able to achieve focus as there was not sufficient inward travel. Problem was caused by the additional thickness of the 1.25" adaptor in the 2" focuser. i only need about 5mm more inward focus

Not a problem i thought, just get a 2" MAX DSLR Camera adaptor.

First Light Optics - Max DSLR Camera Adaptor

Does this mean that I need to upgrade the secondary mirror to a larger one?

If so I am going to go mad! I just seem to inch forward in this hobby before realising that I need to buy more bloomin' bits or adaptors to achieve the most simple of things.

What size of secondary do i need to get if this is the case?

regards,

Ian

[Tinker1947]

This is my guess work, the reflection from the primary mirror hasn't changed so the secondary mirror won't need changing, just a case of getting the focal point for the camera right....

[Mike Hawtin]

You very likely will need a larger secondary, if you download the Excel spreadsheet from the following link it will perform the calculations for you. Oldham Optical Telescope Design page It is also likely that you will need to move the primary up the tube a little if you can't reach focus now, plus, using 1.25" fittings in the optical train will probably constrain your fully illuminated field for DSO imaging (the spreadsheets "Max Field" sheet will clarify this for you) and finally recollimation will be necessary. Lastly, as you so rightly said, it is a question of inching forward bit by expensive bit to reach a seemingly simple goal.

Good luck and may the force keep Murphy at bay.

[FrankieValley]

If you are asking if you need a larger secondary mirror, because of the wider 2" nosepiece or the fact that you are getting closer to the mirror, the answer is 'no'.

With an eyepiece, you 'look at' the focal plane and with a camera, the imaging sensor has to 'sit on' the focal plane. That is why cameras always appear to want to get up close to the secondary mirror, whereas eyepieces happily sit away from it.

[Uplooker]

Hi all thanks for the replies,

Mike,

thanks for the link to the Oldham optical telescope design site. The reason I was not able to gain focus is because the new focuser with the 2" to 1.25" adaptor + nosepiece + T-ring means I run out of in-focus.

I am aware that I will suffer vignetting using 1.25" nosepiece.

David / drg35660,

I am asking "do I need a larger secondary mirror because of the wider 2" nosepiece".

I have received the 2" MAX DSLR Camera adaptor and this gives me an extra 10-12mm inward focus. Although i have not tried it in anger I believe this will solve my focus problem.

I will need to do a bit more research regarding getting a larger secondary.

regards,

Uplooker / Ian

[FrankieValley]

Hi Ian,

I'm sure you will be fine, but let us know