10 replies to this topic

[sshamsi]

Hello all! Long time lurker here. I'm a bit new to designing setups and end up asking a couple of questions. If not appropriate I can definitely move the thread.

 

For some context, I am currently making potential observational/imaging setups for my college, with the aim of introducing entry-level undergraduate labs (doing things like cluster photometry, variable star light-curves etc.), and some public visual observation. I'm really hoping to get good equipment to be used for some time, and a setup where I can add things over time. As of yet, I feel I've arrived at a 9.25" - 12" SCT for these functions, and at a CCD like the Atik 383+L (22.5mm diagonal) or Starlight Xpress Trius Pro 964 (16mm diagonal), or a CMOS like the ZWO/QHY 533M (16mm diagonal) or ZWO/QHY 294M (23.1mm diagonal).

 

With budget constraints, we might have to decide among a Celestron 9.25" EdgeHD, a 10" Meade ACF,  and a regular Celestron C-11 SCT. Now, I know that field curvature comparisons for all these scopes have been discussed on various threads. The EdgeHD providing an almost completely flat field and coma correction, the ACF providing some field curvature correction (enough to be almost flat for an APS-C size sensor or smaller) and coma correction, and the C-11 not providing any correction. So we have that aperture vs correction consideration. Other questions that pop in in making the decision are:

• Considering the SCTs' F/10 ratio and the smaller pixel sizes, I think we'd get a reducer at any rate. The 11" EdgeHD OTA retails for about $4400 on OPT right now, while the C-11 OTA retails for about $3200. If I were to purchase the widely praised Starizona 0.7x LF reducer/flattener/coma-corrector for $500, could I have an EdgeHD equivalent for $3700? I realise this topic has been discussed before (here, here, or here), and the general answer is yes. However, does this consensus hold up when we're talking about potential scientific uses?
• Adding to the question above, the cameras we're probably gonna buy are smaller so a full 42mm flat field imaging circle isn't necessary, but my instinct is to future-proof for when we get a larger sensor and accommodate APS sized DSLRs also. In this case, would an EdgeHD 9.25 be better than a C-11 + Starizona x0.7 reducer?
• Alternatively, maybe I am overestimating curvature's effect in doing scientific imaging. In this case, would a regular Meade ACF provide enough curvature correction for this to not matter (for my use case at least)?
• I think we'd get a Celestron Focus Motor (or equivalent primary knob focuser), focusing via the primary mirror. My question is, does the back-focus requirements for SCT with a focal reducer change when focused with the primary mirror (I imagine that since the focal length changes with the primary mirror)? Would that mean I should be prepared to do a lot of back-focus adjusting with some extra spacers? To remove this potential complication, should I forget about primary mirror focusing and stick to image train focusing?

My questions come down to the experienced members here giving their opinion for my use case, and I hope I haven't just facilitated a debate which has been had ad nauseum! At any rate, TIA!

 

Clear skies,

Shoaib

[12BH7]

I understand that your setup needs are not for hobby level amateur AP. Can you contact other universities and ask what setups they went with? The reason I'm mentioning this is because if you're using equipment to detect IR, temperature, chemical compositions, etc...  You're not just hooking up a camera to take a nice nebula picture.  Some of this equipment may need specific fittings, mounts, scope design types, etc... 

 

Otherwise for a more mundane reply, there really isn't much of a difference between the 9.25" and 10". Especially if you're going AP. If you have the dedicated space the 11" is more flexible for future upgrades and needs.

[jgraham]

Wow, you've got a lot of moving parts here. All three of these offer some interesting options and it may depend on the level of the students. I make a living in research (44 years in basic an applied research) and I do quite a mix of recreational and scientific work with my astronomy gear, mostly photometry and spectroscopy. On the scientific side of the house there's a lot to be said for the smaller 9.25" EdgeHD. For most research that I'd be looking at bigger isn't always better and I'll take simpler and bullet proof every time. I have an older pre-Edge C9.25 and it is one of my favorite scopes. I also have a 8", 10" and 12" ACFs and they are fantastic. I also have a pre-Edge C11 and you are correct, the common practice is to add a focal reducer/field flattener primarily for the field correction. Focal length-wise I prefer to leave my gear in the native focal lengths. For my areas of interest (primarily photometry with a smidge of spectroscopy) I tend to use smaller scopes so that I can get my target and reference stars in the same field of view. My larger scopes are used for high resolution work. Pondering your options I'd be tempted to go with the C11 with a good quality focal reducer/field flattener, though it may depend on your mount, the C11 can be a bit of a beast and needs a good mount to work well. In contrast, the C9.25 is pretty light and is very forgiving.

 

Soooo... I'd suggest that the C9.25 is the safe choice, the C11 would push the edge a bit and I'd be a little concerned about it's size the the associated fiddle factor, the 10" is the proverbial middle child. Probably the biggest issue with the ACF and Edge scopes is that they require a focal reducer that is designed for use with corrected optics. For my work I tend to avoid using focal reducers and instead opt for a smaller scope like an MN6.

 

Food for thought.

 

Enjoy shopping around, that's the fun part.

[Ljubo]

Hi there Shoaib!

 

You didn't mentioned the mount. Please note that for C9.25 SCT you're Ok with EQ6R (or similar, but EQ6 is the least expensive new at the market), and for C11 SCT you'll need something stronger, CEM70 for instance (again, not expensive one, but OK).

 

Othervise, I support that your choice would be regular C11 with the new Starizona IV reducer (and for regular star photometry you even might not need the reducer). Also note that Starizona reducer is  very focus sensitive, so it's best to obtain new focuser (which will help you with mirror flop too!) and that should be the short one (preferably JMI Event Horizon) with which you could reach back focus with your dedicated astro camera, considering you'll have an reducer in imaging train.

[sshamsi]

I understand that your setup needs are not for hobby level amateur AP. Can you contact other universities and ask what setups they went with? The reason I'm mentioning this is because if you're using equipment to detect IR, temperature, chemical compositions, etc...  You're not just hooking up a camera to take a nice nebula picture.  Some of this equipment may need specific fittings, mounts, scope design types, etc... 

 

Otherwise for a more mundane reply, there really isn't much of a difference between the 9.25" and 10". Especially if you're going AP. If you have the dedicated space the 11" is more flexible for future upgrades and needs.

Hi! I agree, the first priority should be to replicate what other universities are doing because they'd focus on things other than taking excellent pictures. From my (not too extensive) sleuthing, many smaller university observatories seem to use the Meade LX200 SCTs. Even my alma mater had a 16" LX200 ACF, and I remember taking some very oversampled, but working, images for photometry . I've heard they even use dual Meades for visible light interferometry at some places. Perhaps curvature isn't such a big deal when outside of astrophotography?

 

Either way, I deeply appreciate your input!

[carolinaskies]

Hi! I agree, the first priority should be to replicate what other universities are doing because they'd focus on things other than taking excellent pictures. From my (not too extensive) sleuthing, many smaller university observatories seem to use the Meade LX200 SCTs. Even my alma mater had a 16" LX200 ACF, and I remember taking some very oversampled, but working, images for photometry . I've heard they even use dual Meades for visible light interferometry at some places. Perhaps curvature isn't such a big deal when outside of astrophotography?

 

Either way, I deeply appreciate your input!

BINGO!  Though the pro-Celestron people on CN are loathe to admit, there is little reason to worry about field curvature from an institutional level for curriculum that is taught in Astronomy & Astrophysics coursework.  Lets remember that stellar separation isn't based on a wide-angle sample but by parallax motion of a star between it's apparent neighbors.  For prettiest pictures the thought that individual images of the full FOV is even being utilized through the back of the telescope is laughable.  Hyperstar at F/2 presents the closest facsimile of the full FOV.  The following images shows relevant FOV with a 294 on HD, HD+.7x reducer, HD+Hyperstar

https://astronomy.to...1|1|0&messier=1

 

There are many people who instead of worrying about field curvature of images are stitching multiple images together via mosaics.  
 

[sshamsi]

Wow, you've got a lot of moving parts here. All three of these offer some interesting options and it may depend on the level of the students. I make a living in research (44 years in basic an applied research) and I do quite a mix of recreational and scientific work with my astronomy gear, mostly photometry and spectroscopy. On the scientific side of the house there's a lot to be said for the smaller 9.25" EdgeHD. For most research that I'd be looking at bigger isn't always better and I'll take simpler and bullet proof every time. I have an older pre-Edge C9.25 and it is one of my favorite scopes. I also have a 8", 10" and 12" ACFs and they are fantastic. I also have a pre-Edge C11 and you are correct, the common practice is to add a focal reducer/field flattener primarily for the field correction. Focal length-wise I prefer to leave my gear in the native focal lengths. For my areas of interest (primarily photometry with a smidge of spectroscopy) I tend to use smaller scopes so that I can get my target and reference stars in the same field of view. My larger scopes are used for high resolution work. Pondering your options I'd be tempted to go with the C11 with a good quality focal reducer/field flattener, though it may depend on your mount, the C11 can be a bit of a beast and needs a good mount to work well. In contrast, the C9.25 is pretty light and is very forgiving.

 

Soooo... I'd suggest that the C9.25 is the safe choice, the C11 would push the edge a bit and I'd be a little concerned about it's size the the associated fiddle factor, the 10" is the proverbial middle child. Probably the biggest issue with the ACF and Edge scopes is that they require a focal reducer that is designed for use with corrected optics. For my work I tend to avoid using focal reducers and instead opt for a smaller scope like an MN6.

 

Food for thought.

 

Enjoy shopping around, that's the fun part.

Hi John! Thanks so much sharing your sharing your experience, it sounds really cool! It's good to see that all these scopes could be useful-so I can't go terribly wrong whichever way. On the mount's side, we're going with an appropriate Skywatcher/Celestron mount according to the end-result's weight, so hopefully that won't be an issue.

 

Additionally, because of the wonderful members here at CN I feel like I know the appropriate reducers for each scope (Starizona x0.7 LF for C-11, Celestron x0.7 925 for EdgeHD, Lepus/AP for Meade ACF), so that's good.

 

I'm definitely tempted by the C-11 + reducer/corrector but am wary about any extra quirks/complexities the reducer may introduce (which I'm assuming the EdgeHD 9.25 won't have). I keep thinking about if there would be aby problems with the reducer if I change primary mirror focus! Due to the nature of how I'm obtaining the equipment, returning and/or switching in case something doesn't work is quite difficult, so there's that pressure to get it right the first time. Anyways, I appreciate the food for thought!

[sshamsi]

Hi there Shoaib!

 

You didn't mentioned the mount. Please note that for C9.25 SCT you're Ok with EQ6R (or similar, but EQ6 is the least expensive new at the market), and for C11 SCT you'll need something stronger, CEM70 for instance (again, not expensive one, but OK).

 

Othervise, I support that your choice would be regular C11 with the new Starizona IV reducer (and for regular star photometry you even might not need the reducer). Also note that Starizona reducer is  very focus sensitive, so it's best to obtain new focuser (which will help you with mirror flop too!) and that should be the short one (preferably JMI Event Horizon) with which you could reach back focus with your dedicated astro camera, considering you'll have an reducer in imaging train.

 

Hi Ljubo! Thanks for pointing out the EQ6-R and CEM70. I plan to add up total weights, leave a 5-10 lb overhead, and then choose a mount accordingly so hopefully that won't be a problem.

 

Thanks for sharing your recommendation! I'm curious to see if you know if the Starizona reducer when added with a Celestron SCT produces any complexities/quirks in operation compared with an SCT without the reducer?

 

Also thank you for the focuser recommendation! I'm curious as to why an additional short focuser is recommenced when there is a reducer in the imaging train (besides helping with mirror flop)? I'd love to know!

[sshamsi]

BINGO!  Though the pro-Celestron people on CN are loathe to admit, there is little reason to worry about field curvature from an institutional level for curriculum that is taught in Astronomy & Astrophysics coursework.  Lets remember that stellar separation isn't based on a wide-angle sample but by parallax motion of a star between it's apparent neighbors.  For prettiest pictures the thought that individual images of the full FOV is even being utilized through the back of the telescope is laughable.  Hyperstar at F/2 presents the closest facsimile of the full FOV.  The following images shows relevant FOV with a 294 on HD, HD+.7x reducer, HD+Hyperstar

https://astronomy.to...1|1|0&messier=1

 

There are many people who instead of worrying about field curvature of images are stitching multiple images together via mosaics.  
 

Well it's great to keep in mind that if curvature does be come a problem, taking mosaic's are always an option.

 

"For prettiest pictures the thought that individual images of the full FOV is even being utilized through the back of the telescope is laughable." Just to confirm, by that you mean that only at F/2 with Hyperstar does the projected image by the telescope fully fit the ASI294 sensor?

 

Thank you,

[gnowellsct]

I think you should have a prolonged conversation with starizona. If you want to specialize in photographing or doing photometry of Galaxy clusters that's highly specialized.

Greg N

You might want to get one of the RASA telescopes from Celestron it might be more appropriate to your needs. But bear in mind that the rasa telescopes can't be used for visual observing.

Edited by gnowellsct, 16 June 2022 - 11:03 PM.

[sshamsi]

I think you should have a prolonged conversation with starizona. If you want to specialize in photographing or doing photometry of Galaxy clusters that's highly specialized.

Greg N

You might want to get one of the RASA telescopes from Celestron it might be more appropriate to your needs. But bear in mind that the rasa telescopes can't be used for visual observing.

Hi Greg! Thank you for the suggestion, I'll definitely email them to see if there could be any problems. The RASA is definitely a great device, but like you said, it unfortunately won't allow for visual use.