Longer focal length?

[xelasnave]

I know I have yet to master short Focal Length but I can't help looking at the fabulous images coming from the guys with either a Celestron 8 or 9.25...I want one..not both..should I go 8 inch or 9.25...

Any wisdom greatly appreciated.

I know I seem to be rushing but really good or bad I don't have much life left..even if I live to 100 no doubt physical limitations will shelve me long before so why wait?
Alex

[Startrek]

Alex,
For the short time I’ve had my 10” newt (1280mm focal length) the one thing that stands out , at least in my experiences , is that they are not as forgiving as a shorter focal length scope ( like my 6” newt 900mm focal length) with regards to seeing conditions and ultimately resolution and image fidelity.
To get the best out of my 10” , seeing needs to be really good ( stable ) to achieve the type of results you expect from a bigger scope.
So in my experiences, bigger is not always better , it has its limitations like everything else in this hobby, but when conditions are good it shines.
I reckon try to master what you have in your kit and that may save you time as time is precious to all of us.
My 6” , 8” and 10” Newts are my scopes for life ( I don’t want or need anything more )

Best Always
Martin

PS: I take my hat off to folk who image at 2000mm to 3000mm ( I certainly haven’t the time , patience or skill to venture into that territory)

[Merlin66]

If you "must" go for a SCT, then look for the 9.25.
It represents the sweet spot between the C8 and C11.
Usually excellent optics and excellent performance.

I've had a few over the years and always regretted selling them.

[glend]

Alex, I had a Celestron Edge HD8 for about a year. They are f10 native and can be reduced, but the reducer was ridiculously expensive (I could have bought a Newt for the cost of that Edge reducer. ) With SCTs I have a couple of gripes, a big one being their love of condensation, they are magnets for condensation, and thus require good long dew shields, and copious amounts of front end heating to discourage it. They have a reputation for mirror shift as they traverse the sky, and honestly don't try to image across the Meridian with one. Even if you have a supposed mirror locking one, they still shift, it's frustrating. Be prepared for refocusing. They need a good mount, as all longer focal length scopes do, and guiding demands more care.
Still they have a big fan base, especially in the USA.
As we have recently discussed, a focal length of 1000mm is a sweet spot in terms of ease of use, fitness for our Seeing conditions, etc.
I should add I also owned a GSO Cassegrain 8", f12, which i found easier to live with than the Edge 8, but really it was visual only (good for splitting double and triple stars). My deteriorating vision made me give it up.
Which brings me to refractors, my final resting place in astronomy. A good refractor can do it all in my opinion, and does not usually require the collimation rituals of the various mirrored breatheran. I rest my case.:lol:

[xelasnave]

Thanks Gentlemen..but when I said I wanted wisdom that means I want to be told that a 9.25 is perfect and go out and buy one right away.

OK it is back to the ASKAR 130.

Progress is going very well with the additional pier and I have Sortta ordered a ZWO harmonic for it...now that should be a great set as the ASKAR apparently works best without a reducer at 1000 mm ..so back to my original plan..run the eight and settle down..it is just now that the observatory is tidy and I don't need a bed in there there is so much spare room...I can't help planning new things...
Again thanks to everyone for helping me see reason...

Alex

[Tulloch]

You don't say what you want to image - if it's the planets then the 9.25" SCT is better than the 8". If it's nebulae then a Newt/refractor is better. If it's galaxies then there's probably not a lot of difference between the 8 and 9.25.

I love my 9.25" on the planets, it produces great images (https://www.cloudynights.com/gallery/album/10844-solar-system/) with a managable size and weight. It's significantly better than the 8. However, the high focal ratio makes it not ideal on dimmer objects.

But don't just take my word for it, Ed Ting's review.
https://www.youtube.com/watch?v=C6zsxiLvOwQ

Andrew

[xelasnave]

Thank you Andrew I appreciate your input.

Alex

[floyd_2]

I have a C9.25 and love it. It goes just fine on my AZ-EQ6 and works well at my level in astrophotography. I doubt that I'll ever get to the level that others here are at, but that's ok as I don't intend to. I'm happy to look at the amazing shots that others are getting even if I can't produce them myself. I've owned most types of telescope over the years, but am keeping my C9.25. It's of a manageable weight and size for its light gathering capacity too. A good point for those of us who are aging and don't have permanent setups. I use my C9.25 reduced to f/6.3 for narrower field imaging, and my Sharpstar for widefield. My only problem is working out which telescope to use for the evening!

[AnakChan]

I started off shooting at 2500mm F10 (µ250CRS) and I have to say I'm so used to it that I'm very unfamiliar shooting at shorter focal lengths (I also have 2xfracs at 616mm and 455mm respectively).

[xelasnave]

Thank you for your input.
I am not in a rush..yet.
Alex

[xelasnave]

Way to go...I chop and change so I am setting up my eight inch ..I have ordered or on site everything to get it set up.on,y need a hole for the pied..that will keep me occupied.Thanks for sharing your experience.
Alex

[Dilsh]

Hey Alex,

If you have have some short refractors already go long. I have only owned my 925 edge HD for a short amount of time but the challenge is not that crazy. I am doing it from my inner city Melbourne light polluted suburb and if I can do it anyone can. The Celestron T adaptor makes back spacing a breeze and a OAG is a must as is an EAF.

Also shooting at native F10 is not bad. I think a reducer adds complications.

Go for it!!!!

Cheers,

Dilshan

[xelasnave]

Thank you Dilshan umongst the recent big order of everything except a scope I picked up a OAG and suitable camera to at least learn to add that to my latest set up...it just started thinking putting in an additional pier was a good idea ...but got out of hand after I bought the mount...like camera, filters, a wheel..EAF...Asiair..it is surprising what you really must have..
Thanks for the encouragement but I will see how good guiding gets with the ZWO Harmonic mount and I will run my eight inch which is higher than anything I have been using for a long time...I really like the look of the ASKAR 130 which would be a good choice...And I don't know about it this " if I can do it anyone can"...You are probably just being too modest with your ability.

Thanks again.

Alex

[Paul Haese]

Just some thoughts here on long focal length imaging.

Guiding and PA require greater precision with longer focal length imaging. Errors related to that become very obvious at longer focal lengths.

Focus often is the motto. I generally have my systems refocus (automated) every 60 minutes, no matter what size scope they are. Focus with steel tubes changes rapidly during the first few hours of imaging even if you give it an hour or so before commencing imaging. Even carbon systems will contract a little and on critical focus zoned instruments you get noticeable focus shift.

SCT's are best used for planetary imaging but can produce very good results imaging deep sky objects but I would rather use an RC, CDK or Newtonian for the task.

Collimation on an SCT is more forgiving but still needs to be controlled well to get excellent results.

Using a dew heater on an SCT can and does cause all sorts of issues with tube currents. All the planetary imaging I have done over the years, I have never used a dew heater, I use a hair dryer. That is not practical when doing DSO work.

Would I get a 9.25 or 8" SCT for DSO imaging? Hmm, nope. There are better tools to be obtained. A quality Newtonian with reasonable focal length will give you a good compromise between long and short focal length imaging. However if you really want to do galaxy work I would recommend an RC or a CDK. Using off axis guiding is a necessity too. You don't get any issues that come with SCT's. Dedicated tools for the job.

Best of luck with your hunt.

[xelasnave]

Hi Paul...Sorry for such a late reply but I thought I had replied...one of those posts that I must have hit "previous thread" ...

Thank you very much for your post I greatly value your opinion.
I am setting up my eight inch with its own camera, filters etc and to hear your comments makes me very happy as it appears I have made a good choice.

Alex

[AnakChan]

That's a very interesting guideline on focusing. With my FL @ 2500mm/F10 & camera 3.76µm pixel pitch, my CFZ varies 232µm to 317µm depending on colour wavelength which seems quite generous.

Having said that, I do set my triggers to refocus every 5th shot, 2 deg temp changes, or if HFR changes by 10% (whichever comes first) and I find that it refocuses quite often - definitely much more frequently than every 60 mins Paul has recommended. I probably could tone that down a little and not refocus so frequently.

Even though I image almost exclusively at such long focal lengths, with suburban Perth's rather "flat" terrain and highly susceptible jet streams, shorter focal lengths probably makes more sense, as with those conditions, it's almost like lottery to get a night where I maximise the capability of focal lengths. i.e. aside from the right scope for the right target, IMO, it's also the environmental conditions with the right scope.

[xelasnave]

Hi Sean

Thank you for your comments.

Your frequent focusing has got me thinking.

We bang out subs like sausages really somewhat taking away the importantce of a good photo graph..like if you were taking a shot, say a portrait, you would be very careful and also for following shots...the need for many subs has at least for me caused me to firget that finally it is about individual captures.... and so having read your approach I am inclided to tell the EAF to focus more often. Certainly I review subs and throw out bad ones and I guess although one must do that it may promote the sausage turn out approach....

I will run at 1000 mm (200 mm ap f5) with my additional new set up and avoid the long F/L experience ..at least until I get it all together..I mean I still don't do dark and flat frames so that's where I need to concentrate...I have been rushing forward and need to tidy up my game.

Thanks again for your input I greatly appreciate you taking your time to do so.

Alex

[The_bluester]

I do not use HFR change as a focus trigger as I have found it to often be a poor trigger as such. One night in particular I did see the HFR going up over about 10 minutes (Taking 2 minute RGB subs) and manually injected a focus run. The result was more or less exactly the same (Enlarged) HFR as before the run, about 20 minutes later the HFR settled down again so I put it down to a period of poor seeing and discarded those subs.

[AdamJL]

Good thread! At some point I want to start shooting southern hemisphere galaxies a bit more, so the knowledge here is great so far.

[Startrek]

Adam,
Don’t be intimidated by imaging galaxies, you don’t need a whopping big scope and super dark skies ( but it does help ) !!!
Here’s a few of my Galaxy pics using my $300 Bintel 6” newt, focal length 900mm under Bortle 7/8 Sydney skies

These are native size ( only slightly cropped ) and crappy res under 200KB IIS reduced size

Cheers
Martin