8" F4 or 10" F5

[bomberbaz]

I am doing some forward thinking as at some point if things progress, I may upgrade from my fairly basic setup to something more advanced. Putting aside slit spectroscopy for now, I am more concerned with the merits of reflecting OTA's as per the thread title.

They are similar priced, they 10" will have a better resolving power but the F4 helps gathering data and should give a brighter image but that's where I lose it. I do not know if the F5 10" will have a significant increase in resolving power over an 8" OTA at F4 with regards spectroscopy.

I don't think I have ever got my head fully around how F ratio works and I have simply learned to accept the merits of slow/fast and how best to exploit them.

[Peter Drew]

If weight and general larger mass are not a serious consideration I would favour the 10" F5 every time, better resolution, light grasp and a more forgiving focal ratio.   🙂

[bomberbaz]

2 minutes ago, Peter Drew said:

If weight and general larger mass are not a serious consideration I would favour the 10" F5 every time, better resolution, light grasp and a more forgiving focal ratio.   🙂

actually you have thrown a new consideration  into the mix Peter, physical capability!  Having just turned 60 and suffering from carpel tunnel in both arms (which has led to muscle wastage) maybe the 10" is less of a good idea. 

However I would still like to hear other comments if there are any regarding the merits as per initial query. 

cheers, Steve

[paulastro]

I had an 8inch f6 for over a year up to July when I had the opportunity to part-ex it for a 10inch f5 version of the same make Dob.  I used the 8 at every opportunity and became very familiar with its performance while I owned it.  A wonderful scope.

Roll on about six weeks, and I have to say I am surprised how much the 10 surpasses the performance of the 8 .  Both on deep sky and planetary, it clearly gives a better performance unless the seeing is particularly bad

I was surprised as I've owned 10 inch and up to 14 inch scopes at various times over the years but couldn't recall the difference being so much..  Though, I hadn't owned a scope greater than 6 inches for some time before buying the 8 inch. (I had used other people's larger than this though in the odd occassions).  

So for me, on performance it is a no brainer.

However, Peter makes a good point on size.   I should have had a hip replacement in July, posponed as during the pre-op I was diagnosed with a heart condition so the op is off for some time.  I also have a back problem.

I only have to carry the 10 (in two pieces)  a few yards to use it, but it can be very painful - despite doses of codeine.  I don't regret the swap of scopes at all, but I do have three smaller scopes I can use instead if I'm having a particularly bad day.  The weight between the 8 and 10 isn't great, but it depends on how your condition affects your lifting ability, which will be different to mine of course.  But it is important you take it into account when deciding what to do.

PS I should have said, moving the 8 around hadn't given me any real problems.

 

Edited August 24, 2023 by paulastro
PS

[bomberbaz]

12 hours ago, paulastro said:

I had an 8inch f6 for over a year up to July when I had the opportunity to part-ex it for a 10inch f5 version of the same make Dob.  I used the 8 at every opportunity and became very familiar with its performance while I owned it.  A wonderful scope.

Roll on about six weeks, and I have to say I am surprised how much the 10 surpasses the performance of the 8 .  Both on deep sky and planetary, it clearly gives a better performance unless the seeing is particularly bad

I was surprised as I've owned 10 inch and up to 14 inch scopes at various times over the years but couldn't recall the difference being so much..  Though, I hadn't owned a scope greater than 6 inches for some time before buying the 8 inch. (I had used other people's larger than this though in the odd occassions).  

So for me, on performance it is a no brainer.

However, Peter makes a good point on size.   I should have had a hip replacement in July, posponed as during the pre-op I was diagnosed with a heart condition so the op is off for some time.  I also have a back problem.

I only have to carry the 10 (in two pieces)  a few yards to use it, but it can be very painful - despite doses of codeine.  I don't regret the swap of scopes at all, but I do have three smaller scopes I can use instead if I'm having a particularly bad day.  The weight between the 8 and 10 isn't great, but it depends on how your condition affects your lifting ability, which will be different to mine of course.  But it is important you take it into account when deciding what to do.

PS I should have said, moving the 8 around hadn't given me any real problems.

 

Some good points made there Paul although what you said doesn't surprise me. Visually I use a 14", moving up from a 10 from three years since. My difference was immense and immediately apparent although like you I would struggle to actually say what those differences are right now. (It's an age thing)

I think I have already decided that an F4 by 8" will end up being my scope should I go ahead. It is the balance between weight and aperture, ease of set up, matching with a decent mount and having thought about it, the quattro (which is what I was looking at)  seems to meet all the requirements.