Spacer guessing game !!

[halli]

Hi All

I have recently decided to get a coma corrector for my 130pds and chose the standard skywatcher CC.   The problem is fitting this into my imaging train.

Atik 383l+,  EFW2, and Skywatcher CC .   Please see the diagram below !

The question is where do I get the spacer from ? !

To maintain the 55mm spacing between the CCD chip and CC lens I  need a 6mm M54 male to M48 spacer !

Has anyone solved this problem or can suggest a solution ?

I would appreciate if there are  any leads.  

I can get a low pofile M54 to M48 convertor ring and then I guess I need to add an M48 to M48 extender.  

For example  

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4041_TS-Adapter-M54-lens-thread-to-M48x0-75--2---filter-thread.html

http://www.telescopehouse.com/accessories/eyepiece-adaptors-and-extension-tubes/revelation-m48-2-spacer-extension-ring-5mm.html

This is quite complex and also expensive for a couple of glorified washers ! 

Any views would be greatly appreciated

Ta

Ian

   

[Oddsocks]

Ian,

Which make of filter are you using?

For accurate calculation of the total back focus distance needed one third of the filter thickness is added to the BFD

For Astronomic add 0.33mm, Baader add 0.70mm, Astrodon add 1.00mm.

The parts you link to total 2.3mm for the M54 male to M48 female and 5.0mm for the M48 extension = 7.3mm.

The gap you need to fill is 6.0mm plus the filter thickness so...

6.0mm + Astronomic 0.33mm = 6.33mm

6.0mm + Baader 0.70mm = 6.77mm

6.0mm + Astrodon 1.00mm = 7.00mm

If you have Astronomic filters with 6.3mm BFD required then a closer match would be obtained using a 3mm M48 extension and either 1mm M48 spacer ring or 1mm M54 spacer ring (2.3mm + 3.0mm + 1.00mm = 6.33mm).

Using the TS parts that would be:

#TSM54a-M48 = 2.3mm

#TSVF203 = 3.0mm

#M54Abstimm1 or #M48Abstimm1 = 1.0mm

Total BFD = 6.30mm

If you have Baader filters with 6.77 BFD required then a closer match would be obtained using a 3mm M48 extension and a 1mm M54 spacer ring plus 0.5mm M48 spacer ring (2.3mm + 3.0mm + 1.0mm + 0.5mm = 6.8mm total BFD)

Using the TS parts that would be:

#TSM54a-M48 = 2.3mm

#TSVF203 = 3.0mm

#M54Abstimm1 = 1.0mm

#M48Abstimm05 = 0.5mm

Total BFD = 6.80mm

If you have Astrodon filters with 7.0mm BFD required then the same spacer stack used for the Baader filters giving 6.80mm would be closer than your original choice giving 7.3mm, or, you could possibly add an extra M48 TS spacer ring #M48Abstimm03, 0.3mm, to the #M48Abstimm05, 0.5mm, giving you a total BFD of 7.10 which is closer still but the concern is whether this would leave enough male thread on the M48 extension tube to securely screw into the M54-M48 adaptor once the two spacers rings were added. A quick call or email to TS should clear that up if you ask how long is the male threaded section of the #TSVF203 3.0mm M48 extension.

I have one of the TS 5.0mm M48 extensions and the male thread depth on that is 4.8mm so adding a total of 0.8mm in spacer rings still leaves enough thread to screw into the filter wheel adaptor but I don't know if the male thread depth of the M48 3.0mm extension is the same TS will be able to confirm that for you.

As usual, I can not guarantee the above part numbers, dimensions and calculations are correct, you would need to double check before ordering any of the items and note that there is some latitude in the sensor to flattener distance, the specified BFD distance becomes more critical as you increase in sensor size because coma increases the further away from the optical axis you look, the KAF8300 is a big chip so it is important to get as close as possible to the correct BFD.

You are right about it being expensive though, I use a Finger Lakes Atlas focuser and wanted to buy telescope side adaptors to fit five different telescopes, Precise Parts quoted $460 each, needless to say I bought a model engineers lathe and made my own, just had to pay £2 each to have them anodised.

H.T.H.

[halli]

Hi

Thanks very much for your detailed response - its really  appreciated.  I didn't realize I need to add extra length due to the filters !  

I have Baader filters - so I will research the parts you suggest  - I agree that the depth of threads on these things can 'screw' up the calcs pardon the pun !

Anyhow thanks again for the excellent information - I can more confidently progress this now.

Cheers

ian

[Pompey Monkey]

Ian, I will post more details of my very similar set-up after work today.

You have  the right approach to calculating the spacers required. However, there is an error in your diagram: The 55 mm should be measured from the back face (i.e. away from the focuser) of the coma corrector. So, by your diagram, you would need 55-(22+17) = 16 mm of spacers, plus a the correction for path length through the filters.

I found that Ian King and Telescop-Express are good sources of spacers. And, yes, it is a money pit

Edit: here is a link to the thread where other members kindly helped me get my head around the calculations: http://stargazerslounge.com/topic/255270-skywatcher-coma-corrector-with-ccd-and-filterwheel/

BTW, what is the "10 mm" in your diagram?

[Pompey Monkey]

I can't really think of anything eles to add to my post above, apart from the fact that I ended up shortening my OTA (150 PDS) at the primary end by 15mm. This was because the focuser extends a long way into the tube when the coma corrector is used and that creates odd shaped stars. I'm not totally sure if the same happens on the 130 PDS, but I believe it does. 

[halli]

Thanks for your responses Paul.

Just to share my understanding of the 55mm to see if its correct - I thought it was the distance from the coma corrector lens to the camera chip ?  This is what I tried to show on my diagram ie 17mm from chip to CCD mating face , 22mm for filter wheel - the 10mm is the internal distance from the CC shoulder to its lens which means around 6mm left over.  The external 48mm cc thread length which is about 5mm will hopefully be consumed by he spacer female thread. 

I think the 16mm in your calcs includes the 10mm that I have given for the CC internal distance between lens and shoulder resulting in a spacer of 6mm .  Do I just ignore this ?  I guess this is fundamental to the outcome as it is a large distance !  Its a shame that there is no explanation of the 55mm in the cc literature!

Yes my 130pds drawtube was already protruding into the scope and giving a bite out of the stars, however that was with a DSLR - with the CCD it now sits more outside the tube so it will be interesting to see what difference the CC makes !

[Pompey Monkey]

Thanks for your responses Paul.

Just to share my understanding of the 55mm to see if its correct - I thought it was the distance from the coma corrector lens to the camera chip ?  This is what I tried to show on my diagram ie 17mm from chip to CCD mating face , 22mm for filter wheel - the 10mm is the internal distance from the CC shoulder to its lens which means around 6mm left over.  The external 48mm cc thread length which is about 5mm will hopefully be consumed by he spacer female thread. 

I think the 16mm in your calcs includes the 10mm that I have given for the CC internal distance between lens and shoulder resulting in a spacer of 6mm .  Do I just ignore this ?  I guess this is fundamental to the outcome as it is a large distance !  Its a shame that there is no explanation of the 55mm in the cc literature!

Yes my 130pds drawtube was already protruding into the scope and giving a bite out of the stars, however that was with a DSLR - with the CCD it now sits more outside the tube so it will be interesting to see what difference the CC makes !

The 55 mm is definitely measured from the back face of the flange on the CC to the chip.

It took me over a week to source the right combination spacers and then two separate orders from two different companies delivered the wrong spacers! I got it sorted in the end though.  

[halli]

OK thanks for the clarification Paul.

I guess that gives me more spacer distance to play with as I can forget about the 10mm internal CC distance I included