Celestron C90 Visual back

[Alien 13]

I am looking to replace the visual back on my C90 with something of better quality, there is no requirement for eyepiece fitting so a straight ?????? to T thread would be ideal.  The problem is that I cant find any info on the thread size of the original and there seems to be a number of different sizes depending on the age of the scope (mine is a new one), a quick measurement across the threads reads a 45.4mm. Has anyone got any thoughts?

Alan

[vlaiv]

I wondered that as well for SW102 Mak (not that I have one, but wondered if I got myself one what would it take to change visual back to nicer one).

I believe there are non standard threads. Depending how you measured the thread size (like using calipers and measuring thread diameter) - that could be M45 thread - meaning 45mm diameter and 1mm or 0.75mm pitch size - so its either M45x1 or M45x0.75 - you can use calipers to see what the pitch is.

On the other hand, it could indeed be 45.4mm for some reason as I've found adapter for large SW/Orion Maks at TS website that says:

Telescope-sided thread: 66,5mm Maksutov female thread
Eyepiece-sided thread: SC Male thread

So it is some crazy 66.5mm diameter. I would not be surprised if you indeed have some weird thread like 45.4 mm (but pitch is measured from the middle of grooves not outer diameter as per:

at least I think so - not sure).

Don't know if there are adapters to other threads for that.

[johninderby]

Get an adapter ring then fit an SCT visual back.

https://www.firstlightoptics.com/adapters/lynxastro_makcas-sct_adptr.html

Edited September 28, 2019 by johninderby

[vlaiv]

20 minutes ago, johninderby said:

Get an adapter ring then fit an SCT visual back.

https://www.firstlightoptics.com/adapters/lynxastro_makcas-sct_adptr.html

Oh great, I did not know these existed.

Btw, do you know if they are compatible with Mak102?

[RT65CB-SWL]

13 minutes ago, vlaiv said:

Oh great, I did not know these existed.

Btw, do you know if they are compatible with Mak102?

@vlaiv ...according to the FLO page,  https://www.firstlightoptics.com/maksutov/skywatcher-skymax-102-ota.html and lower down, go to 'Related Products'; it says it is compatible.

Edited September 28, 2019 by Philip R

[Alien 13]

Thanks folks, looks like it is 45.4mm and as far as I know from looking online its the same for all the small Maks up to the 127mm.

The reason I am looking for a swap is that the internal diameter of the original is 1.25 inch and is fairly long so wondered if it contributed to vignetting with a DSLR fitted....

I cant think why they chose such a daft size on these instruments?

Alan

[happy-kat]

The C70 the threads are m3 for the bolts that hold the back on and do collimation.

Now I've said that I don't feel as confident I'll have to read my project thread.

Edit: the two bolts that hold the C70 back on (under the outer coating) are M3

Edited September 28, 2019 by happy-kat

[Alien 13]

I do have another question regarding alternative visual backs, the Mak has a huge range of back-focus so is it better to use a short adapter meaning the main mirror is further down the tube or a much longer one that puts the sensor position at a similar point to that obtained if you were using a diagonal and eyepiece?

Alan

[RT65CB-SWL]

On 28/09/2019 at 14:32, Alien 13 said:

Thanks folks, looks like it is 45.4mm and as far as I know from looking online its the same for all the small Maks up to the 127mm.

The reason I am looking for a swap is that the internal diameter of the original is 1.25 inch and is fairly long so wondered if it contributed to vignetting with a DSLR fitted....

I cant think why they chose such a daft size on these instruments?

Alan

You think that is a daft size... the 'original' Meade ETX90 / 105 / 125 port is smaller at 34.5mm.

 

My 're-modded' ETX105 'before' and 'after' I added the Baader Planetarium/ETX to SCT adaptor ring. 

Edited September 29, 2019 by Philip R

[vlaiv]

5 minutes ago, Alien 13 said:

I do have another question regarding alternative visual backs, the Mak has a huge range of back-focus so is it better to use a short adapter meaning the main mirror is further down the tube or a much longer one that puts the sensor position at a similar point to that obtained if you were using a diagonal and eyepiece?

Alan

Good question. I know that SCTs have issue with spherical correction and back focus, or rather there is spherical aberration if you focus far away from "ideal" focus point. Not sure if it applies to MCTs as well.

Another important point is if focus position is optimized for stock 1.25" diagonal and whether you should calculate things from that point.

[Alien 13]

9 minutes ago, vlaiv said:

Good question. I know that SCTs have issue with spherical correction and back focus, or rather there is spherical aberration if you focus far away from "ideal" focus point. Not sure if it applies to MCTs as well.

Another important point is if focus position is optimized for stock 1.25" diagonal and whether you should calculate things from that point.

I actually bought an SCT to T adapter by mistake thinking it might fit a year ago and have noticed it is a good 50mm longer than the original so that would put the focus point at a similar position to that if you used a diagonal. I guess the Mak might also be optimized for a certain focus point but not realy tried any comparison tests.

Alan

Edited September 28, 2019 by Alien 13