What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (2025)

What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (1) 2.71

Monty Chandler

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Starting with the square crop sensor is a math problem. I'm at .61 using an aps/c sensor with the RC8 at 1625mm for galaxies and PN - the smaller stuff. I tend to guide at .5" - .6" with an eq6r pro. Much prefer focal length over cropping an image. Just the photographer in me.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (3) 1.20

David Russell

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Tony Gondola:
I think that's an interesting approach and one I've been thinking for trying for while now. Glad to see that someone got good results from the method as it makes prefect sense.

at short focal length I dont think One Direction Guiding is generally necessary.
when I shoot with my Tak at 530mm, I dont necessarily use this method, because things are not as critical.
However at 2000mm or 1500mm I find the advantages of One Direction guiding more obvious.

also I personally feel that if the OTA is small and lightweight, the DEC axis might be ok responding to short 2 or 3 second guide exposures, and constant reversal of direction, but its clear to me that any large heavy imaging set up will never want to be DEC reversing direction every few seconds.

my OTA and guidescope and camera (etc) etc have considerable weight, so for me its an advantage.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (5) 4.02

Tony Gondola

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My main imaging scope runs at 900mm and I typically use very short integration times which makes guiding accuracy a bit more critical, at least on a frame by frame basis.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (7) 1.81

MaksPower

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What’s the point ? Image scale.

There are many interesting targets which on smaller scopes (say under 1000mm) are barely a blip in the frame.

For example the Homunculus (eta Carina) is easily resolved. It’s bright, and tiny yet with exquisite optics and seeing a 10” or larger can produce an interesting result. But few amateurs have tried.

The imaging ive done with my10” at3000mm shows theees a whole universe of targets most of you guys don’t try for.

As I’m also limited to portable, 1-night sessions - and our appalling weather - time is limited so I’m not going to win an APOD, but I am quite happy with what the rig can do in a night.

Edited ...

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (9)

Tim Hawkes

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Tony Gondola:
It's certainly a game of opposing factors. You can cull deeply and improve sharpness or you can cull minimally and improve S/N in the faint areas. On guiding though, I suspect that if RA and DEC errors are balanced, you can get round stars and still have a effect on resolution depending on the total guiding RMS. Meaning that the quality of any given subframe would be indicated by FWHM and roundness.

This is very true. If the criterion is the final image that you end up with then - in practice for a set of say 10s subs of galaxies - I have often found it best not to cull down too many in a quest for lower overall FWHM but rather to go for SNR and rely on BlurXt deconvolution to find the detail. This usually seems to work providing the image scale is small enough, enough electrons per pixel still and the spatial information is in there somewhere.

Just to address the question of the original poster. I would say that there is only a point in having a long focal length for galaxies to the extent that it can help get you down to an image scale of less than about 0.5 arc sec/ pixel but that is no use in itself unless it is combined with a low enough focal ratio that the SNR from a short (semi lucky imaging) sub is sufficient for part of the galaxy that you are imaging (say looking for detail in the bright centre). In practice this all means a big scope -- say 10-12 inch or above at F4

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (11) 1.81

John Stone

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MaksPower:
The imaging ive done with my10” at3000mm shows theees a whole universe of targets most of you guys don’t try for.

So with that focal length and a "regular" 3.76u camera you're at about 0.26"/px and you'd need seeing less than 1" (about 0.83") to see any benefit to such a small sampling size, right? (I've heard seeing / 3.333 is optimal sampling) and then you'd need to guide at least at 1/2 of that, right? So guiding at 0.125" RMS. That's pretty much the limit of even the premium absolute encoder mounts like the Mach2GTO and 10Micron series.

Tim Hawkes:
Just to address the question of the original poster. I would say that there is only a point in having a long focal length for galaxies to the extent that it can help get you down to an image scale of less than about 0.5 arc sec/ pixel but that is no use in itself unless it is combined with a low enough focal ratio that the SNR from a short (semi lucky imaging) sub is sufficient for part of the galaxy that you are imaging (say looking for detail in the bright centre). In practice this all means a big scope -- say 10-12 inch or above at F4

Just so.

I'd like to get a 12"/300mm F3.5 Newt with a Paracorr (1207 mm or 0.64"/px) and put it on a Mach2GTO, but that seems like a scope no-one actually makes so the backup plan would be a 10"/250mm F4 w/ Paracorr (1150 mm or 0.67"/px). This would mean I'd need guiding in 0.35" rms (or below) range.

It's critical that the mechanics of this scope be absolutely rigid or you'd never get that guiding performance out of the SkyModeling those encoder mounts require to get such precise guiding. That's hard to do with a Newtonian.

The company Artesky has a line of premium newts in the ARTEC line and I've spoken to an owner of a 250 who tells me she "treats it like a refractor, no collimation needed even after driving over bumpy dirt roads".https://artesky.it/it/135-astrografi-artec

They seem to be even a step above the ONTC line with the motorized focuser and 3" Paracorr integrated with their custom designed mechanics. (They even have figured out a way to run the Pegasus Prodigy with an ASIAIR!!)

I wish they made the 300mm in a F3.5 and had the mechanics to hold collimation at that focal ratio. I'd buy it today!

Edited ...

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (13) 4.02

Tony Gondola

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I think the point is lost that average seeing is just that.....average.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (15) 1.81

MaksPower

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Something to consider is Active Optic units. Starlight Expresshttps://www.sxccd.com/active-optics/
makes them. It will iron out most bad mount behaviours and provide better overall results.

After a thread on CN banging about the small AO products I had a look at them briefly - really just a tip/tilt plate driven by coils, able to respond at 0.1s.

As many would know, often it is suggested the mount guiding be slowed down so the mount does NOT "chase the seeing". Which left me wondering - if you have a mount that CAN respond at 0.1s, what happens if it does chase the seeing ? Can it do that fast enough - i.e chase the seeing fast enough to reduce the FWHM of the star ?

The answer to that is actually yes, it is possible with the CQ350; using my ASI2600DUO on my 10" f/12 I did some experiments with bright guide stars - and it is possible to run the guider at 0.1s and the mount does respond fast enough that the jiggling of the guide star essentially stopped, and the FWHM was indeed improved. But the effects of seeing on poor star shape remain, obviously.

But the proviso is having guide star bright enough to do that, which often is not the case, and a rig where the lowest frequency for vibration is well above 1Hz.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (17) 1.81

MaksPower

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John Stone:
MaksPower:
The imaging ive done with my10” at3000mm shows theees a whole universe of targets most of you guys don’t try for.

So with that focal length and a "regular" 3.76u camera you're at about 0.26"/px and you'd need seeing less than 1" (about 0.83") to see any benefit to such a small sampling size, right? (I've heard seeing / 3.333 is optimal sampling) and then you'd need to guide at least at 1/2 of that, right? So guiding at 0.125" RMS.


Yes. And sure, it's oversampled, so no need to dither or drizzle.

It's more a case of (1) its the scope I happen to have and (2) optically it is very nice, and (3) I have tried binning and 2-3 reducers but concluded the results are invariably best at 1x1 and no reducers.

So many people seem to settle on a rig that is suited to work with "average" seeing 1.5...2.0 arcsec. But thats only "average". I set outto have a rig that can always take full advantage of whatever the seeing offers when it is better than "average". I have two sites where it does get better than 1.0 arcsec often enough that it's worth having the capability to use it. And last year I had a night between it went rock steady 1...2am - when the guiding dropped to 0.1 RMS the whole time and the subs were just beautiful.

I have images from a similar scope that show what is really possible.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (19) 9.42

Jerry Yesavage

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Hi,

Have to put in my two cents as I faced a similar decision a couple years ago and decided to go with a C11 and an ASI 6200, which is a heavily oversampled combination. This runs on an old Mach 1 under Bortle 7 skys. The thing that convinced me to get this equipment was that I saw Gary Imm use this combination and get excellent resolution at 1x1 binning... theoretically not too possible, but you have to look at the image, and remember it all gets processed with AI.

So my suggestion is theory is one thing, but empirical data another and we have lots of data on Astrobin where you can check out what people actual get from their equipment.

CS

Jerry Y

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (21) 7.16

Bill McLaughlin

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MaksPower:
After a thread on CN banging about the small AO products I had a look at them briefly - really just a tip/tilt plate driven by coils, able to respond at 0.1s.

As many would know, often it is suggested the mount guiding be slowed down so the mount does NOT "chase the seeing". Which left me wondering - if you have a mount that CAN respond at 0.1s, what happens if it does chase the seeing ? Can it do that fast enough - i.e chase the seeing fast enough to reduce the FWHM of the star ?

I have not seen the CN thread since I seldom visit CN but I do have 8 + years of experience with the Starlight Express AO,the most common non-SBIG system and more common recently since SBIG is becoming "yesterday's news".

I have used the units (I have two, main and backup) with several mounts and OTAs including Paramount ME, Planewave L-350, TOA-130, AGO 10 inch iDK and most recently my 14 inch CDK (on the L-350).

To be blunt, my experience is that the idea of significant image improvement using high guide rates is mostly total BS. It simply does not work very well as it can only help with one of many aspects (aka "orders") of less than ideal seeing and it also needs a super bright guide star which is likely to not be present, especially in galactic areas. Multiple guide stars are even less likely to be available, further compounding the problem.

What the units are excellent at is making guiding more reliable and straightforward simply because you only move the mount during dithers, not during the image, so moves are much faster and often more precise (especially with less than ideal mounts).

Since I have the encoder equipped direct drive L-350, which is capable of fast and precise guide moves, I have briefly tried that alone and it is not better and usually worse than slower rate guiding as it does indeed chase seeing.

I typically use quite long guide rates with my L-350/CDK 14 - around 7 seconds. That allows PhD to do multi-star guiding with quite a few stars which is much more beneficial than high rate ever is.

I am at a site that often has periods of arcsec level seeing and my sampling of .3 arcsec/pixel is nearly ideal for those "best of times" and works great with BXT.

So the short version is that AO units are often worthwhile but not for the reason most people think.

Edited ...

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (23) 1.81

John Stone

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Bill,

i’m a little confused. As an 8-year veteran of AO usage (you must like them) are your saying you use it with your L-350/CDK-14 at 7-second guiding intervals?

that seems to be exactly opposite of it’s advertised use? What benefits do you see by moving the glass in the AO unit every 7 seconds vs just moving the mount?

it seems to me that with a mount with high precision encoders you’d have no trouble just pulse guiding the mount?

thanks for any clarification you can provide.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (25) 13.22

Arun H

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MaksPower:
CAN respond at 0.1s, what happens if it does chase the seeing ? Can it do that fast enough - i.e chase the seeing fast enough to reduce the FWHM of the star ?

While the mount itself, without load, may respond quite fast, how well does the system respond? Meaning the mount with a large inertial moment from the scope and counterweights? I'd love to see the time constants of a fully loaded system to a step or pulse input to see if this actually works. A small plate is much easier to move, obviously.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (27) 1.81

MaksPower

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Arun, as I was using an ASI2600 DUO with ASIAir the result is easily seen in the guider display - the movement of the guide star from frame to frame is significantly reduced - as the cycle time was reduced from 1s to 0.1s the guide star basically stood still, with only the effects on star shape from frame to frame being visible (the scope aperture being 10"). On a smaller scope (eg 4" refractor the star shape will be less affected.

I'd also suggest that for this to work the guide camera has to be working at a pretty extreme focal length to reliably sense the seeing - in my case 3000mm -so the resolution in the guide camera is 0.26 arcsec/pixel, and diffraction is also evident.

With the guide camera at the focus of the main scope, this is indeed the sum total of all the mechanical effects - the net mass and rotational inertia of the rig, flexure and elasticity of all the bits from the ground up to the camera. And since the guide camera is in the same body as the main camera, it does result in tighter stars from the main camera.

One of the issues I suspect many have with guiding is that from a mechanical perspective the response of the complete rig to vibration is that of a low-pass filter with a fundamental frequency of a few Hz. if you are guiding at a cycle time close to the fundamental frequency of the rig there may very well be a resonance between the guide signals and mechanical oscillations. And the resonance of most amateur rigs would indeed be in the range 0.5...5 Hz - and most are guiding at 0.3...1 Hz.

While I am using. CQ350 and its dec blacklash is so low that ASIAir reports it as "negligible", in the next few months I hope to see whether an EQ6 class mount can also manage this, as the electronics and drive train are very similar, with a choice of 4 payloads from a 70mm APO to 10" RC that will fit it.

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What's point of more than 1300mm - 1500mm focal length for Galaxies and PN? (2025)

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