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a couple of questions regarding focusing/focal length and guiding


JR1987

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Im new to the world of observing and imaging, but learning quickly. I have a fairly basic set up of a Skywatcher 130mm with 900mm focal length (not sure of the official model number) with an equatorial mount, and camera-wise I have a Canon EOS 100d. (and a few upgraded eyepieces and what not)

when doing/(learning) my prime focus photography I came across the common problem I already had heard of, which was not being able to focus the camera unless I included a barlow. This works fine.. however when trying some basic lunar stuff the other night, I couldn't even fit the whole moon into the image.. which must because of the barlow. Is there any way around this? 

and a quick question on guiding..i had a go at taking some photos of Orion, they could have been ok for a first time photographer but as best as I could to try and align to polaris i couldn't get exposures of longer that about 10 seconds before the starts started to drag out of focus.. I'm sure its probably down to fine manoeuvres when aligning.. but I've been reading about guiding software on here.. (I'm assuming AstroTortilla is a guiding software?) but I'm confused how this would work.. I have a motor that came with my scope but do you need a special kind of motor drive when using guiding software? 

any feedback is most appreciated!  :smiley:

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I have same issue. Im going to try unscrewing the lens from the barlow and screwing it to my t.mount and adapter next time as i believe that reduces the magnification a little As it brings the sensor a few mm closer to the focal plane.

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Hi there i had the exact same problem. There is a work around i found to get the whole moon in shot.

It was quite risky though sorta...

I removed the barlow from my t ring adaptor, unscrewed the ring on my focuser and rested the dslr on to the threads of the focuser tube. 

Then carefully got focus with the tube screwed right in.

Its not ideal but it works.

Another method i have seen online it to dismantle the focuser and remove a cm off the end to allow further inward travel.

I have the 114mm with 900 focal legnth and the edges of the moon were cut off using the barlow.

i managed this last night with the method im talking about.

post-40820-0-50981500-1422396178_thumb.j

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Im new to the world of observing and imaging, but learning quickly. I have a fairly basic set up of a Skywatcher 130mm with 900mm focal length (not sure of the official model number) with an equatorial mount, and camera-wise I have a Canon EOS 100d. (and a few upgraded eyepieces and what not)

when doing/(learning) my prime focus photography I came across the common problem I already had heard of, which was not being able to focus the camera unless I included a barlow. This works fine.. however when trying some basic lunar stuff the other night, I couldn't even fit the whole moon into the image.. which must because of the barlow. Is there any way around this? 

and a quick question on guiding..i had a go at taking some photos of Orion, they could have been ok for a first time photographer but as best as I could to try and align to polaris i couldn't get exposures of longer that about 10 seconds before the starts started to drag out of focus.. I'm sure its probably down to fine manoeuvres when aligning.. but I've been reading about guiding software on here.. (I'm assuming AstroTortilla is a guiding software?) but I'm confused how this would work.. I have a motor that came with my scope but do you need a special kind . I have of motor drive when using guiding software? 

any feedback is most appreciated!  :smiley:

With regret your scope does not have enough inward travel to accommodate the necessary back focus for a DSLR which for Canon is 44mm from flange to sensor. A barlow lens helps but as you have discovered it doubles the FL so you are imaging with a scope of 1800mm of FL with a DSLR, I have attached a photo of what the Moon would look like with 1800mm of FL and your DSLR so unless you change your scope to something like a 130 PDS or go for an ED type of about 600mm of FL there is not a lot that you can do. The use of a planetary webcam will allow you to focus without a barlow lens but these have typically very small sensors so you would still not get the whole of the Moon in the FOV.

As for your problem with "guiding " I regret to say that you are on the wrong track. Astrototilla is not a guiding software, it is plate solving program to help centre the mount on a specific target using pictures taken of  a particular section of the sky. The most popular guiding software is PHD guiding which works either with an image acquisition software such as APT, BYEOS or Nebulosity amongst others or through a Planetarium software such as Cartes de Ciel or Stellarium using the Ascom platforms and drivers. For guiding you need a combination of the mentined software with the necessary hardware, Guidescope, Guide camera and an interface, GPUSB or EQDIR usb. The other problem that you have is the FL of your present set up. Most seasoned imagers using very expensive mounts, £5000.00+, would shudder to think of guiding at 1800mm of FL as it is impossibly too long. My advice is to read a lot more about this subject before getting any ideas. Hope you don't mind the advice.

Regards,

A.G

post-28808-0-46731100-1422443510.jpg

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I think, but don't quote me on this, as I've never owned one, but the end of the drawtube on the scope unscrews to reveal a T-Thread... attach the camera directly to that, and you may be able to get focus, and it'll be a lot more secure than resting it on the end of the tube. 

You don't mention the mount or how it's being driven, I suspect, the star trailing you're describing is more down to the focal length and the mount, than polar alignment. The polar alignment will show as smearing around the edges, more than trailing. Trailing is more down to the drive mechanisms. 

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Ah wow, I had no idea the longer focal length was such an issue with guiding. hmm I guess its back to the drawing board. I do have slight regrets at the scope I have as I didn't want to spend too much in the first place as I was unaware of how hooked id get! 

thanks for all the advice guys. ill be sure to study the links posted. 

So.. what is my gear capable of currently then? are exposures that are say, 2 mins onwards completely out of the window if i have just a basic equatorial mount and 'clock drive' that attaches to the mount and the focal length of 1800? 

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Ah wow, I had no idea the longer focal length was such an issue with guiding. hmm I guess its back to the drawing board. I do have slight regrets at the scope I have as I didn't want to spend too much in the first place as I was unaware of how hooked id get! 

thanks for all the advice guys. ill be sure to study the links posted. 

So.. what is my gear capable of currently then? are exposures that are say, 2 mins onwards completely out of the window if i have just a basic equatorial mount and 'clock drive' that attaches to the mount and the focal length of 1800? 

Have a look at the " imaging Deep Sky " section and look at the FL of the equipment used you'd find that most guys including myself use scopes with FLs of between 350mm~650mm. There is the occasional long FL job but these are rare and mostly observatory based using top grade mounts with encoders and extremely accurate motors and mechanics and so on. A 10 Micron or  an ASA direct drive mount will not leave you much change out of £7000.00. Budget equipment was not designed for that kind of imaging. It is doable for fun once you gain experience but in the beginning I'd stay away from long FL imaging.  Value your sanity over attempts to image at 1800mm of FL.

This however does not apply to planetary or the Moon imaging using a fast webcam, I am talking about long exposure DSO imaging.

A.G

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Im glad I'm finally starting to understand imaging more in terms of gear. Its a battle of the forum truth tellers against the salesmen! 

Well I'm sure I wouldn't be the first person to find they have bought something that doesn't work well for there uses in this field of hobby!

Truth is right now is the time i have the most disposable income that I will have for some time... So quite honestly, I've been searching the forums for good recommendations of a new scope!

Lensman57, you mentioned the SW 130PDS, which I had a browse of. looks like a decent upgrade..and its around what id say is the ceiling of my budget (around the £500-£600 bracket) I also did a search for it on the forums and found a guy had gotten some fantastic shots with it (no doubt, after a lot of practice and processing skills!).

Im finding myself searching for questions regarding focal length and aperture...from what I seem to be noticing is a smaller scope is almost more preferable to imaging and the opposite is suited to just observing. surely if bigger scopes are better for observing then they are still good for imaging? 

Are there scopes that would suit both needs to a decent level that do not cost 2 kidneys on the black market?

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Ah now... Guiding... that's a whole other ballgame. The challenge is, that the longer the focal length, the more any tracking errors are going to become visible. Thus, imaging with a 50mm camera lens, is much easier on the tracking, than imaging with a 1400mm RC (I've both). With the 50mm, I do not need to use guiding, and can easily shoot 5 minute exposures (mounted on an HEQ5). With my 80ED (600mm), I can do 2 to 3 minutes. I've not tried with my RC (1400mm) unguided, it just doesn't seem worth it to me. Thus the longer the focal length, the more accurate everything needs to be. I'd expect with my HEQ5, with a 900mm focal length, that I'd be limited to around 1 to 2 minutes per exposure before trailing became a problem. 

Before you spend any money, if you haven't already got it, you need to purchase and read http://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html.

For you're last question, you can use bigger scopes for both visual and imaging, but the mounting requirements become rather expensive, and you will need to add guiding into the equation. Frankly, something like the ED80 is a great workhorse scope, and can be pushed far harder than you'd think would be sensible. The small size makes it relatively easy to mount, and it doesn't act like a huge sail in the slightest breeze :D.

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Im glad I'm finally starting to understand imaging more in terms of gear. Its a battle of the forum truth tellers against the salesmen! 

Well I'm sure I wouldn't be the first person to find they have bought something that doesn't work well for there uses in this field of hobby!

Truth is right now is the time i have the most disposable income that I will have for some time... So quite honestly, I've been searching the forums for good recommendations of a new scope!

Lensman57, you mentioned the SW 130PDS, which I had a browse of. looks like a decent upgrade..and its around what id say is the ceiling of my budget (around the £500-£600 bracket) I also did a search for it on the forums and found a guy had gotten some fantastic shots with it (no doubt, after a lot of practice and processing skills!).

Im finding myself searching for questions regarding focal length and aperture...from what I seem to be noticing is a smaller scope is almost more preferable to imaging and the opposite is suited to just observing. surely if bigger scopes are better for observing then they are still good for imaging? 

Are there scopes that would suit both needs to a decent level that do not cost 2 kidneys on the black market?

Aperture is everything if you are in the observing game, how fast the scope is in it's native form ( without reducers and so on ) is important for imaging alongside the FL which will affect the FOV. Take your time and ask a lot of questions before buying anymore equipment.

Regards,

A.G

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