Deadlake

The 4" is so much easier to get out for sure, however the 5" is more rewarding and being able grab a one hour session without thermal cooling makes for best use of time. Maybe a 115-120 mm would be a compromise? You've touched on mounting, maybe the weight of the mount should guide you? Any scope over 6 kg starts to rule out a lot of mobile mounts if that's important to you.

Indeed it is all physics, side by side these two scopes from APM, the 5" shows more stars/planetary detail than the 4". I suspect I'll never have the seeing to tell the difference between the Vixen 4" I have and the APM 4", the APM lens is brighter due to the glass used, is more compact if heavier.

You will never know unless you have the Takahashi tested. The point is that unless you are moving to say the Atacama Desert you will never see any benefit from having higher the 0.95 Strehl... I will always take the 5" out due to this, even if the 4" is so much portable. Your eyes will thank you for the extra aperture...

If you are considering a 100 DZ the below scope is within your budget https://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGTEC130-7 This is cheaper marginally then the Tak, however it has a minimum Strehl of 0.95, meaning due to seeing etc I don’t think you will be visually able to tell the difference with a lens with a higher Strehl number, however flourite lens appear brighter to the eye. Also you will see more stars through a 5” compared to the 4”. I have both doublets and triplets and really apart from weight I don’t really see cool-down as an argument to go for the doublet.

Yes, I know, if only the US would move over too. However I suspect unless they have a Mariner type incident (imperial to metric conversion in software) this will never happen. I know the thread pitch argument etc for not moving however outside of US causes no problems...

UNC and Imperial I thought where different.

https://www.telescopengineering.com/wp-content/uploads/2020/01/160-Manual.pdf Sew page 3, other source available: 6.5 Finder Base Screw Holes (Plugged 6.3 with (2), 8-32 Button Head Screws)

The starlight focuser finder scope mounting point has UNC 8-32 button screws. The hex like bolt head is somewhere between a 2-2.5 mm hex bolt head size for an Allen key. Searching an Amazon I cannot find an equivalent set of UNC sized Allen keys, do they even exist?

I thinking the TOE is a little smoother in it's image where the Vixen shows every detail. Both are excellent just more about person preference of presentation of the image. How are you getting with the TOE 2.5 mm? Is it worth having? The HR 3.4 mm and TOE 4 mm are keepers, maybe I should try the TOE 3.3 mm first.

Well here is an AZ100, it handles the weight on both sides quite well.

I've seen a TSA-120 on a Vixen APZ in Alt mode. How realistic would it be to mount a 7 kg scope on the APZ? I'm after a mount which can take a 7-8 kg with slow mo's, this could be it...

I now have a 2" Chroma H-Alpha 3 nm, I'll be able see more with that. But another post...

I can see in the dark😀, however push to on the AZ100 is quite accurate.

Maybe @jetstream will chip in once he has finish playing with his new snowmobile

Astronomik As you can see in the dark easy to get mixed up...

So both the OIII and UHC filter I have are in a UFC holder/draw. Possible I got them mixed up. I rely on where they are stored in a filter pouch.

I have one of those as well, will spend some more time on HH and flame. For Onions nebula I find OIII brings out more details.

One question I have would using a BV help in this case? I used a 30 mm APM UFF eye piece in the above cases. Next I was going to try with a BV and 24 mm UFF's (prefers as more comfortable to use then TV alternatives)?

Optically: 5" LZOS at a dark site -> SQM 21.89 with OIII filter. Borrowed C11 -> SQM 21.09 with OIII filter. NVD works, just allows me to see it with the 5" in the back garden instead of going to the middle of Wales.

Yes and as I said very condition dependent. I actually keep a track of SQM, unless it gets over 21.10 where I live lot worth looking. I was surprised how much SQM varies over the night....

I agree both are key to seeing it. Conditions during the night can vary hugely and it might just be that one night where everything is just right when you can see the flame and HH.

@Highburymark You too?

You certainly need this for NV, I get better views when my eyes have adapted. Putting in a different section also due to cost makes no sense. I can think of a French Sky Vision 16” dob which costs more then a C11 and a NVD put together, however posting in the DSO section using a Dob and cost never comes up. I could also point out the EP sets containing only TeleVue and again cost never comes up. This smells like a red herring… Hypothetically say I did an observing session with a BV scope with an OIII filter on one side of a mount and another scope with a NVD attached on the other side of the mount where would it get reported? I’m visually observing but if I follow the rules I need to report the same observation session in different forums….

I think it's really to the point in that they (OIII and an NVD) are tools which allow an observer to highlight/view different frequencies of light. The NVD allows viewing in the infra-red. Also good for use on gas planets for viewing Methane with the correct filter. I'm not quite sure where the differences in skills is used to make a choice about visual observation, an NVD is really a contrast booster to give you a dark site in your back yard (i.e. to deal with light pollution) it is not to do with using different observation skills. I do disagree NVD is technically difficult. It's more on the AP side of things from an instrument point of view, e.g. astrograph type telescopes and also takes a lot of theory since you can observer into the infra red spectrum not the visible which is one of the main differences. I can understand confusion around DSO, which is why I suggested it was H-Alpha observation.

As an NV newbie I would disagree. NVD based observations really belong in visual DSO section, not in their present location. If you are going to move NVD to a different section out of DSO then observers using OIII filters should not be posting there...