8SE on a CGEM II, good for AP?

[JCIdaho]

On forums there's lots about the Nexstar 8SE being a difficult choice for AP.

Is the concern over the 8SE mostly about the mount, or is there some concern about the OTA also? I've a CGEM II, so would the OTA put on this mount be a good setup?

Note, I've a Stellamira 90mm I'm happy with, but I'm looking ahead at another option for the smaller DSOs.

Thanks!

[catburglar]

I don’t have direct experience of this…my guess is that it can probably be made to work, but there are probably better options too.

The specific issues with the scope arise because of the moving mirror focus mechanism- this can cause image shift when you change focus direction which makes focussing tricky, and also leaves you prone to mirror flop- where the mirror moves as the scope points to different areas of the sky (the nexstar OTA doesn’t have a mirror lock). An external focuser would help with the former and using an off axis guider rather than a separate guide scope might mitigate the latter.

On the mount perspective- I guess it mostly comes down to guiding accuracy…if you’re imaging small dso’s e.g. if you want to get nice detail in smaller galaxies- I’m guessing you want to be imaging at about 1-1.2 arcsec per pixel. If you’ve got good enough seeing for that then you need your mounts total RMS guiding accuracy to be about 0.5-0.6 arcsec per pixel.

If you’re getting figures around this with your current setup then the only way to know for sure is to give it a try…

Edited April 1, 2022 by catburglar
Typos

[Same old newbie alert]

5 hours ago, Jon Pither said:

On forums there's lots about the Nexstar 8SE being a difficult choice for AP.

Is the concern over the 8SE mostly about the mount, or is there some concern about the OTA also? I've a CGEM II, so would the OTA put on this mount be a good setup?

Note, I've a Stellamira 90mm I'm happy with, but I'm looking ahead at another option for the smaller DSOs.

Thanks!

It can be made to work, as a suggestion I'd look at using a reducer, a oag..  unless Celestron have changed things on the cgem11 than the original the weak area is the servo motors, backlash in the gears, backlash in the cog to cog setup... It will work but it can be improved with  belt modding and stepper motors using Onstep..

With the scope as I said using a reducer will make things significantly easier sampling wise, you never said what camera you intend to use it with.. might need to search what size chip you can use as you won't be able to use a full frame, APS-C or 4/3rds sized sensor

Using a oag will help with the mirror shift/ flop issues..

Getting it to work can be highly rewarding, especially on those medium to small targets

[Cosmic Geoff]

Just for clarity, the optical tube assembly is a "C8".  The "8SE" or "C8 SE") is the whole outfit including the 6/8SE mount which is essentially a portable visual mount.

In answer to your question, the suitability of the C8 for imaging depends on what you hope to achieve.  I found that mine is good for planetary imaging, but (in the form of a CPC800) was not much good for imaging smal fuzzies until I got the Celestron f6.3 focal reducer, which has transformed it and it now takes nice images of planetary nebulae.

Since you already have the C8 OTA and th CGEMII, I suggest you buy the focal reducer (often available used) and give it a go. It will give larger scale images than the Stellamira 90mm.

[JCIdaho]

Thanks for the advice.

@Cosmic Geoff - noted, I should refer to it as a C8, as without the supplied mount, it's not an 8SE!

@Same old newbie alert - my camera is a asi2600mc-pro. I'll look into a getting a reducer.

@catburglar thanks for explaining the concern with the mirror focus. It does seem a OAG would help here! I'll look into it

I'm broadly happy with my CGEM II, so it feels that with a reducer + OAG, this is something I need to try! Thanks.

[Same old newbie alert]

1 hour ago, Jon Pither said:

Thanks for the advice.

@Cosmic Geoff - noted, I should refer to it as a C8, as without the supplied mount, it's not an 8SE!

@Same old newbie alert - my camera is a asi2600mc-pro. I'll look into a getting a reducer.

@catburglar thanks for explaining the concern with the mirror focus. It does seem a OAG would help here! I'll look into it

I'm broadly happy with my CGEM II, so it feels that with a reducer + OAG, this is something I need to try! Thanks.

The reducer will shrink the image circle also... The edge version can be used with APS-C sized sensors without issue