Thinking going forward

[TiffsAndAstro]

So I'm quite keen on f4 ish newtownions. 

Not so keen on the horror stories with collimation and an image train hanging off the side, even just osc and maybe filter drawer.

Are there any such scopes that are easier to deal with?

And I realise I'll need a grown up mount and other accutrinontes

£

[John]

Not sure about the image train issue but the challenge with an F/4 newtonian is that the "sweet spot" for accurate collimation has got really small at that focal ratio - around 1.4mm, so collimation has to be that much more precise to achieve diffraction limited performance. 

 

[900SL]

Get an f5. Thank me later. 

[dweller25]

Yes, perhaps the better compromise would be F/5 ?

Edited April 13 by dweller25

[happy-kat]

For me the first question is defraction spikes or not

[Peter Drew]

Learning to collimate at F4 will be the least of your AP problems.      🙂

[Mr Spock]

I have a 12" f5. It takes me a few seconds to collimate and it is perfect.

I 'had' an 8" f4. I spent hours trying to collimate it. Despite my expertise in collimation it was never even close. In six months of ownership I got no observing time out of it at all. All the time was spent collimating...

[900SL]

There is an up side though. You can post dozens of threads on collimation woes, focusing issues, flexure, coma etc here 😆 

f5 is a lot easier

[Elp]

15 hours ago, TiffsAndAstro said:

Are there any such scopes that are easier to deal with?

Yes, theyre called refractors.

[Ags]

15 hours ago, TiffsAndAstro said:

Are there any such scopes that are easier to deal with?

Maybe an RC? (Evil laughter)

[TiffsAndAstro]

2 hours ago, Ags said:

Maybe an RC? (Evil laughter)

Rich cretins have just as much collimation issues as newtownions though maybe?

At least the image train  is somewhere sensible

Edited April 13 by TiffsAndAstro

[TiffsAndAstro]

6 hours ago, Mr Spock said:

I have a 12" f5. It takes me a few seconds to collimate and it is perfect.

I 'had' an 8" f4. I spent hours trying to collimate it. Despite my expertise in collimation it was never even close. In six months of ownership I got no observing time out of it at all. All the time was spent collimating...

Was it a difference of build quality or just a property of a huge mirror?

[TiffsAndAstro]

7 hours ago, happy-kat said:

For me the first question is defraction spikes or not

I quite like them, but not terribly bothered either way.

Plopping a camera in front of the lens I don't like the idea of, even though results from a Rasa look incredible.

[TiffsAndAstro]

9 hours ago, 900SL said:

Get an f5. Thank me later. 

F4 is that much trickier than F5?

I'll have a Google and see ty

[TiffsAndAstro]

17 hours ago, John said:

Not sure about the image train issue but the challenge with an F/4 newtonian is that the "sweet spot" for accurate collimation has got really small at that focal ratio - around 1.4mm, so collimation has to be that much more precise to achieve diffraction limited performance. 

 

Do you know roughly what it would be at F5 for context?

[John]

9 minutes ago, TiffsAndAstro said:

Do you know roughly what it would be at F5 for context?

Well, I believe that it is 4.8mm at F/6 so I guess F/5 will be somewhere in between - say around 3mm ?

There are formulae that can calculate it accurately but I don't recall them offhand.

 

[Mr Spock]

16 minutes ago, TiffsAndAstro said:

Was it a difference of build quality or just a property of a huge mirror?

f4 v f5

13 minutes ago, TiffsAndAstro said:

F4 is that much trickier than F5?

^

[Elp]

Bear in mind collimation difficulty is largely dependent on the construction of the telescope so I wouldn't pay so much attention to f numbers (I mostly image F2). A poor and thinly made secondary will cause a lot of frustration.

 

[TiffsAndAstro]

13 minutes ago, Elp said:

Bear in mind collimation difficulty is largely dependent on the construction of the telescope so I wouldn't pay so much attention to f numbers (I mostly image F2). A poor and thinly made secondary will cause a lot of frustration.

 

Newtownions do vary in price a lot. But so do there types I guess.

Nothing urgent I'm just thinking about future.

[Clarkey]

If you are imaging, base your need on pixel scale. If an F5 scope bin2 gives you the right value, it will save you the headache of collimating at F4. I am looking at getting a 10" F4 newtonian, but only because I have a very small imaging observatory and an F5 won't quite fit. An F4 10" with a 2600 sensor will give me 1.55 arc secs/px at bin 2. This is about as much as my seeing will generally support. An F5 will give me about 1.3 arc secs/px - which would be OK if it fitted. A 200mm F5 will also give 1.55 - but less overall light gathering.

I would add that I spent more time trying to collimate my f4 6" newtonian than my RC8.

Edited April 13 by Clarkey

[Elp]

That's the main thing about the above, if your imaging scale, mount tracking and seeing at the time don't allow you to reap the benefits of a larger aperture there little value going down that route.

The main issues I had with the 130pds were though it's a small Newtonian it was prone to wind vibrations whereas my sct and refractor weren't (and I've got a decent tripod) so imaging when wind speed averages were approaching double digits was near impossible, and for visual it was a pain as you usually had to rotate the OTA to get the eyepiece at a decent usable angle. For imaging you can simply have you camera/focuser train pointing toward the ground (though I'd put something soft underneath just in case, check the focuser mechanics too prior to doing this, use screw connections etc). Getting the scope to prime imaging condition was also a concern though for the price of the scope wasn't so much of an issue. The primary is also centre dotted making collimation easy with simply a cheap collimation cap.

Edited April 13 by Elp

[happy-kat]

When fiddling with processing I prefer no refraction spikes, it's easier to make a star round if there are no spikes. 

[TiffsAndAstro]

53 minutes ago, Clarkey said:

If you are imaging, base your need on pixel scale. If an F5 scope bin2 gives you the right value, it will save you the headache of collimating at F4. I am looking at getting a 10" F4 newtonian, but only because I have a very small imaging observatory and an F5 won't quite fit. An F4 10" with a 2600 sensor will give me 1.55 arc secs/px at bin 2. This is about as much as my seeing will generally support. An F5 will give me about 1.3 arc secs/px - which would be OK if it fitted. A 200mm F5 will also give 1.55 - but less overall light gathering.

I would add that I spent more time trying to collimate my f4 6" newtonian than my RC8.

This is good to know Ty.

I'm here for a good time, not a fiddling time

[TiffsAndAstro]

42 minutes ago, Elp said:

That's the main thing about the above, if your imaging scale, mount tracking and seeing at the time don't allow you to reap the benefits of a larger aperture there little value going down that route.

The main issues I had with the 130pds were though it's a small Newtonian it was prone to wind vibrations whereas my sct and refractor weren't (and I've got a decent tripod) so imaging when wind speed averages were approaching double digits was near impossible, and for visual it was a pain as you usually had to rotate the OTA to get the eyepiece at a decent usable angle. For imaging you can simply have you camera/focuser train pointing toward the ground (though I'd put something soft underneath just in case, check the focuser mechanics too prior to doing this, use screw connections etc). Getting the scope to prime imaging condition was also a concern though for the price of the scope wasn't so much of an issue. The primary is also centre dotted making collimation easy with simply a cheap collimation cap.

Didn't really consider wind at all for imaging.

It's clearing up here, but winds upto 15mph might be more of an issue than I thought.

[Ratlet]

F5 is so easy I can do it.

You can always get more data with an F5 you can collimate than one you can't.

I don't image with my 130pds often (because it's really quite good for visual and I love visual) but when I have a £13 laser collimator works every time.  Every time.

But the flip side of imaging is thus:

"He had acquired good data.  That's when his problems started"