How much focul length is too long on a dob

[Mike_S]

Seeing as most big dobs appear to be around F4-F5. Have you ever bought one and thought the focul length was too much? Too narrow field of view? If so, what size was it and what did you go back to?

Edited June 29, 2019 by MSammon

[John]

Most of the 10 and 12 inch chinese mass produced dobs are around that focal ratio. The 6 inch ones are usually F/8 and the 8 inchers F/6 though. Orion Optics offer a choice of 2 focal ratios as standard in each aperture but will I believe make one to your own spec if you are willing to wait.

Orion Optics did produce a few 150mm F/11 dobs and some were/are owned by members here:

https://stargazerslounge.com/topic/159354-orion-optics-6-f11-planetary-dobsonian/

Personally I prefer slightly longer focal lengths in a dob but as the aperture increases the scope length can get unwieldy for the dobsonian mount - optically a 12 inch F/8 could be a superb planetary / lunar scope with a minimal secondary obstruction but an 8 foot long tube is going to be a huge challenge to mount steadily.

 

 

[Mike_S]

Thinking more for DSO’s on this one to be honest. I’m having difficulty finding them so I’m using a 30mm eyepiece currently. If I see a used 12 inch F5 solid tube come up I might get it. 

I see some just go all the way from a 10 to a 16 inch but they’re usually 1800mm long. I’m enjoying finding my way about anyway. It was just a thought that popped into my head. 

[John]

My 12 inch is F/5.3 which I find works well on both the deep sky and planetary / lunar. The max true field that I can get with it is 1.6 degrees at 51x using the 31mm Nagler eyepiece. Thats large enough to show the whole of the majority of deep sky objects but the very largest (eg: the full Veil Nebula, N A Nebula, Helix Nebula and M31 I observe in 1.6 degree sections. Or if I want the full view I switch to my ED102SS F/6.5 Vixen refractor that delivers a 3.9 degree true field with the same eyepiece.

Hard to find a scope that does absolutely everything !

 

 

[Mike_S]

Yes. I hear some people bought maks as beginners and regretted it l. I can imagine them being tough to locate dso’s. I understand that shorter focul length may be useful to compliment a longer focul length scope in your collection.

has anyone actually bought a big dob then sold it because the focul length was too long/field of view too narrow? It sounds like focul ratio is the practical limit to aperture size. 

Edited June 30, 2019 by MSammon

[johninderby]

Yes focal lengths of really big dobs tend to be short as otherwise the scopes get a bit impractical to use. Short focal lengths also bring problems in collimation as they’re not very forgiving of being even slightly out of collimation.

Dob insanity??? 😁. Notice the step ladder for reaching the eyepice. 🙀

Edited June 30, 2019 by johninderby

[Stu]

It sounds like your issues are more about getting a decent finding arrangement than anything else, would that be correct? If you fit a Telrad and a RACI finder, both accurately aligned then things get much easier.

I have an Orion Optics 8" f8 scope which is 1600mm focal length. When you think about it, it is no longer than John's 12" f5.3 so the field of view should be similar although  I suspect the 12" will illuminate a 2" field better. The OO 10" f6.3s are similar, as are the 6" f11s.

I bought mine largely just to try out on Planets and perhaps for doubles, it's not the sort of scope I think I would end up star hopping that much with. It is the OTA only but I seem to have found a suitably sized dob mount for it now and will pop it on an EQ Platform for tracking. It is a surprisingly manageable tube; certainly it was alot smaller than I was expecting.

I suspect one reason big dobs are fast is manageability, not having to be up a 10 foot ladder! There are some very fast big dobs out there such as these eg 28" f.27 which has a focal length of 1890mm but because of the huge exit pupils it would generate you still won't be able to get very wide fields out of them, perhaps 0.9 degrees with a 17mm Ethos at 6.3mm exit pupil. Stick a 40mm it it and you get a 15mm exit pupil! Coma is probably a big challenge too at that speed.

[Mike_S]

Yes I have never heard of a Raci Finder. I have a Telrad but prefer the finder scope. Will look into the Raci now. 

[John]

The great thing about the Telrad is that it projects rings of a known angular size against the sky (4, 2 and .5 degrees I recall). By using this in combination with a star chart you can work out where a DSO target will be in relation to a particular star and put the star in the right part of the Telrad finder ring then your target object should be in the field of view of a low power eyepiece in the scope.

You can easily make transparent templates the same size as the Telrad illuminated rings to place on your star chart. Stellarium has an option to switch on Telrad rings so you can get same effect. The Telrad is a powerful tool for finding once you get used to using it and has turned around many folks DSO hunting from frustration to regular success.

 

[Stu]

Have you checked your Telrad alignment? One concern I have is that you should be able to find some of these easier ones just with the Telrad. Just put Eta Herculis towards the edge of the Telrad circles along the line to Zeta and M13 will be fairly centred.

[Mike_S]

Great I will give that a try. I gave up using the Telrad but will get using it again. 

[Mike_S]

14 minutes ago, Stu said:

Have you checked your Telrad alignment? One concern I have is that you should be able to find some of these easier ones just with the Telrad. Just put Eta Herculis towards the edge of the Telrad circles along the line to Zeta and M13 will be fairly centred.

What app is this? 

[AdeKing]

Looks like a version of SkySafari to me. 

[Ricochet]

Judging from the logos I'd guess the pro version but either plus or pro will allow you to enter your equipment to show field of view and/or Telrad overlays. Here's mine (SS5 plus) showing the correlation between the Telrad circles (red) and a standard 9x50 finder (largest blue circle) and some of my eyepieces when used in my dob (smaller blue circles). 

I agree with @Stu that learning to find these brighter objects with your current equipment is the first step. 

[Paz]

My dobsonian is focal length 1600mm and the eyepiece is at around natural eye height for me when it's pointing straight up.

Having an eyepiece height that doesn't need stools/ladders greatly helps with practicality.

After that priority, having a long focal ratio is preferable for me to maximise the quality of the view.

[Stu]

7 hours ago, MSammon said:

What app is this? 

It is Skysafari Pro but, again, as Ricochet says, the Plus version will do the same thing. I set up my kit in it so I can easily see what to expect in the field of view. You can also set the visibilty of stars to match the limiting magnitude of stars you see so the views are much easier to match.

[John]

4 minutes ago, Paz said:

My dobsonian is focal length 1600mm and the eyepiece is at around natural eye height for me when it's pointing straight up.

Having an eyepiece height that doesn't need stools/ladders greatly helps with practicality.

After that priority, having a long focal ratio is preferable for me to maximise the quality of the view.

Just how I like my dob as well

[niallk]

Mine's ~1690mm: no steps required for me (5'10")

My TFoV is limited for say Makarians Chain.  I use wide afov EPs to make the most of it.  The tfov compromise is worth it for me for the image scale achievable - at a given level of brightness to my eye.  I do have a 130p for wide angle fun for camping with the family, but if im going out to observe myself, aperture just wins!

[Mike_S]

Has anyone found a focul length too short and say went from an F4 to an F5 for ease of collimation and eyepiece choice?

[John]

26 minutes ago, MSammon said:

Has anyone found a focul length too short and say went from an F4 to an F5 for ease of collimation and eyepiece choice?

And coma of course. If you use well corrected wide angle eyepieces with a newtonian faster than around F/5 coma starts to show itself and at F/4, to get the best from such eyepieces, a coma corrector is more or less mandatory IMHO.

Another reason that I tended towards slightly slower mirrors was that it's easier to make a good F/6 mirror than an F/4.

Todays production methods might mean that mirror production at F/4 is a more consistent business these days though.